Starting Smart - Home Power Magazine

Starting Smart - Home Power Magazine
Starting
Smart
Calculating Your
Energy Appetite
Scott Russell
©2004
©2004 Scott
Scott Russell
Russell
F
rom solar to microhydro, in
Barbados or Barrow, for a hen
house or a townhouse, every renewable
energy system should begin with a load
analysis. This analysis is an assessment
of your site’s electrical use—your
electrical “load profile.” You’ll need
to ponder and juggle a lot of numbers
in the process of selecting, sizing, and
installing a solar-electric system. A
reliable load analysis is essential to get
your calculations off on the right foot.
While a load analysis is a necessity for an off-grid
system, it’s also an excellent idea for a grid-intertied system.
For most grid-tied systems, your old electricity bills are an
excellent record of how much energy your new RE system
will need to produce. But only a thorough load analysis can
enable you to target efficiency opportunities and ultimately
minimize your system costs. Even if you plan to have a
professional installer handle the entire project, your help
with this critical task will ensure the highest possible value
for your money.
As any RE veteran will tell you, for every dollar you
spend on efficiency measures, such as replacing old,
energy-hogging appliances or lighting, you’ll save US$3 to
$5 on the final cost of your system. Note that we’re talking
about increased efficiency, not necessarily conservation.
While conservation is a wonderful thing, you don’t need
to be a puritan to use less electricity and “buy down” the
cost of your system. Most important—you don’t need to
70
sacrifice the conveniences that you enjoy to afford an REbased electrical system!
Where Does My Electricity Go?
The load analysis process can take a little time, but
it’s easy. A form like the sample featured in this article is
available in the Promised Files section on our Web site (see
Access) in Microsoft Excel format. This spreadsheet can
make the necessary calculations for you. Or you can just
grab a calculator and a blank sheet of paper.
The idea is to itemize everything in your house that uses
electricity, and then estimate how much each item uses in
“watt-hours per day.” All the information you need is already
either in your house or in your head. Just write it down.
Many system articles in Home Power include an abbreviated
load table, so good examples are readily available.
A complete load analysis collects and calculates several
bits of data. What follows is a column-by-column breakdown
of the form, describing what each piece of information
means and how to get it.
Load
The term “load” refers to an electricity-consuming
item—a toaster, DVD player, water pump, alarm clock,
lightbulb, or power drill. List everything in your house
that uses electricity, no matter how insignificant you think
it is. The more complete this list, the more accurate your
load profile will be. For multiple, identical loads that are
on for the same length of time—for example, ten, 60 watt
lightbulbs—list the item once and indicate the quantity in
the next column. Multiplication will take it from there.
Load Voltage & Run Watts
Time for a little legwork. For each item, you’ll need to
specify both its voltage and wattage ratings. No cause
for panic—every electrical load is required to have this
home power 102 / august & september 2004
load analysis
information printed directly on it. All you need to do
is march around with your clipboard and jot down the
numbers.
Voltage, amperage, and run wattage data is usually
located on a sticker or plate found on the bottom or back of
the appliance. There is no universal standard for how the
information appears. Voltage can be listed in a number of
forms: 120 volts, 120 V, 120 volts AC, or 120 VAC. Sometimes
an appliance nameplate will just list voltage and current, and
leave off the watts (W). Current is expressed as amperage, and
appears in a number of forms: 0.5 amps, 0.5 A, or 500 mA. To
figure out the run wattage, just multiply the volts and amps
(V x A = W).
Nearly all of the standard electrical loads found in North
America run at 120 volts AC (alternating current). Larger
appliances, such as electric stoves, clothes dryers, and electric
water heaters usually run at 240 volts AC.
Although increasingly rare, if you happen to have any DC
(direct current) loads in your off-grid home, they’ll probably
operate at 12, 24 or 48 volts DC. Battery operated appliances,
such as cordless drills, cordless phones, or (unplugged)
laptop computers, operate on DC. But for your load analysis,
use the information on their battery recharging units, rather
than on the appliances themselves, unless you’re running
them directly off of DC.
For each load, indicate whether its voltage is AC or DC in
the next column of the spreadsheet. Although voltage type
Helpful Tools & Aids
• Load profile chart
• List of approximate wattage for common loads
• Clipboard and pencil
• Watt-hour meter
• Calculator
• Willing assistant (must have opposable thumbs)
• Flashlight
• Stepladder
isn’t terribly important to your load analysis, it’s critical for
off-grid system design purposes. As long as you’re collecting
data, better to do it now.
Run wattage is usually the maximum an appliance will
draw during operation. The watt rating on the appliance
typically represents a “worst case” estimate, but since you
rarely watch your television at full volume or use your
jigsaw to cut granite, feel free to reduce this number by
about 25 percent for “variable wattage” items such as these.
For the most accurate readings on these and all of your
loads, consider getting a handy watt-hour meter to breeze
through the task with digital precision.
Hours & Days
Now comes the sitting-and-thinking part of the exercise.
It may involve some collaboration with others in your
household to get the most accurate estimates possible. The
task simply requires that you approximate how many hours
Two typical name plates are shown here.
The sticker on the left lists the running watts as 18 W.
The sticker on the right reports the voltage as 120 V and
current draw as 9 A. From this information, we can estimate
that this vacuum draws about 1,080 watts.
www.homepower.com
71
load analysis
What’s a Watt-Hour
Meter?
Watt-hour meters are great tools for anyone
interested in collecting and analyzing electrical
energy consumption data. Although effective on
any 120 VAC electrical load, they’re particularly
useful for variably cycling appliances, such as
washing machines, that are difficult to measure
based solely on their run time. Most watt-hour
meters can tell you the instantaneous power
(watts) and the total energy used (watt-hours or
kilowatt-hours) by an appliance. They take the
guesswork out of your load analysis by providing
actual numbers instead of estimates.
Common models include the Kill A Watt by
P3 International, several models by Brand
Electronics, and the Watt’s Up? meters by
Electronic Educational Devices. Meters from all
three of these companies have been reviewed
in past issues of Home Power (see Access). All
of these meters are easy-to-use, plug-and-play
models. Retail prices range from US$40 to $350,
and features vary accordingly.
or “interrupted” using a plug strip. Always-on loads include
answering machines, fax machines, VCRs that you don’t
want to reprogram, smoke detectors, and others. Some
of these loads can be eliminated, for example by using a
voicemail service instead of an answering machine.
Unless you plan to get rid of your phantom and alwayson loads, they should all be listed in your load profile as 24
hour, 7 day loads. Most phantom loads draw less than 15
watts, but that adds up to a whole lot of energy over a span
of weeks or months. Use a watt-hour meter for a precise
measurement of phantom loads. Sometimes you will need to
list a load twice—once for its phantom load and once for its
full, “on” load. The two together should add up to 24 hours.
Before you accept your hours-per-day and days-per-week
numbers as final, it might be a good idea to compare them
to a few weeks of real life. Pay attention to your electricity
habits for two or three weeks and then revise your estimates
as needed. You can also check your estimate against your
monthly utility bill. It’s also important to consider seasonal
variations in your electricity use. For instance, you may use
your lights much more in winter and fans more in summer.
Ultimately, for most grid-connected installations, you want a
load profile that represents a year-round daily average.
Average Watt-hours per Day
Light math, anyone? With the essential data now in hand,
use the formula below to calculate “Average watt-hours per
day” for each item. This is the average amount of electrical
energy that each load consumes in a day.
Quantity x run watts x hours per day x
days per week ÷ 7 days = average watt-hours per day
Three common watt-hour meters (from left to right):
the Kill A Watt, Brand Electronics, and Watt’s Up?
(or fractions of hours) per day and days per week each of the
items you’ve listed is used or may be used down the road.
In most cases, this is perfectly straightforward, but a
couple of notable exceptions will apply. Appliances that turn
themselves on and off automatically based on need have
what are called “duty cycles.” Refrigerators, water pumps,
and any thermostatically controlled electrical devices fit
this description. You can try estimating the percentage of
time that they run by observing how often they turn on and
for how long they stay on. But a watt-hour meter is the only
way to obtain accurate consumption information for such
loads (see sidebar).
The second exception is with “phantom loads” and
always-on loads. Phantom loads are electrical loads that use
energy even when turned “off.” Instant-on TVs, microwave
ovens, computer printers and modems, and many other
devices consume electricity 24 hours a day unless unplugged
72
Once completed, the sum of this column in your load
profile will represent an estimate of the total amount of
electricity you use on an average day. This is the consumption
rate that your renewable energy system must support if you
plan to produce 100 percent of your energy. When you get
around to system sizing and component selection, you’ll
adjust this number to account for a number of seasonal and
technological variables.
Lightening Your Load
At this point, it’s helpful to add a column for calculating
the percentage of your total load that each individually
itemized load represents.
Individual load average watt-hours per day ÷
the sum of all items’ average watt-hours per day =
percentage of average daily load
This information will help you target specific, high
consumption loads when taking efficiency measures—your
next step following a load analysis. One of the best examples
of the potential impact of such measures is described in John
Robbins’ article, “Recipe for a Solar Office: 1 Part Solar, 5
Parts Load Reduction” (see HP97). John reduced his homeoffice loads by more than 85 percent at a cost of US$1,500,
saving him US$5,000 on the cost of his solar-electric system.
That’s real money.
home power 102 / august & september 2004
load analysis
Home Load Profile
AC /
DC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
Run
Watts
400
850
170
60
90
21
240
500
60
800
840
3
1,050
40
600
150
600
15
Totals Before Efficiency Measures
6,489
Loads (Before)
Refrigerator, 18 ft.3 (old)
Well pump 1/3 hp
Television, 24 in. color
Incandescent bulbs
Computer monitor
Combined phantom loads
Light fixture (4 incandecent bulbs)
Washing machine (old)
Mac G3 computer
Microwave
Vacuum cleaner
Alarm clock
Toaster
VCR
Food processor
Coffee grinder
Power drill, 1/2 inch
Printer
Loads (After Efficiency Measures)
Refrigerator, 20 ft.3 (Energy Star)
Well pump 1/3 hp
Television, 24 in. color
iMac G4 computer w/ LCD display
Light fixture (4 fluorescent bulbs)
Compact fluorescent lights
Microwave
Vacuum cleaner
Washing machine (Energy Star)
Alarm clock
Toaster
VCR
Food processor
Coffee grinder
Power drill, 1/2 inch
Printer
Qty.
1
1
1
12
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
12
1
1
1
1
1
1
1
1
1
1
Volts
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
175
850
170
45
80
13
800
840
120
3
1,050
40
600
150
600
15
Totals After Efficiency Measures
5,551
If John could save that much money on RE equipment by
making his office efficient, think of the potential for a whole
house. Compare the tables above for the electrical loads of
a modest home before and after efficiency measures. By
replacing incandescent bulbs with compact fluorescents,
replacing the old refrigerator and washing machine with
modern Energy Star appliances, replacing the desktop
computer and separate CRT monitor with a model that has
an LCD screen, and switching off phantom loads, the home’s
energy use was reduced by nearly 50 percent.
Hours /
Day
7.00
1.25
5.00
1.00
8.00
24.00
2.00
0.75
8.00
0.16
0.50
24.00
0.06
3.00
0.05
0.05
0.05
0.30
Days /
Week
7
7
6
7
5
7
7
7
5
7
2
7
5
2
3
7
1
5
Avg. WH /
Day
2,800.0
1,062.5
728.6
720.0
514.3
504.0
480.0
375.0
342.9
128.0
120.0
72.0
45.0
34.3
12.9
7.5
4.3
3.2
% of Total WH
/ Day
35.20%
13.36%
9.16%
9.05%
6.47%
6.34%
6.03%
4.71%
4.31%
1.61%
1.51%
0.91%
0.57%
0.43%
0.16%
0.09%
0.05%
0.04%
7,954.4
7.00
1.25
5.00
8.00
2.00
1.00
0.16
0.50
1.00
24.00
0.06
3.00
0.05
0.05
0.05
0.30
7
7
6
5
7
7
7
2
7
7
5
2
3
7
1
5
1,225.0
1,062.5
728.6
257.1
160.0
156.0
128.0
120.0
120.0
72.0
45.0
34.3
12.9
7.5
4.3
3.2
29.62%
25.69%
17.61%
6.22%
3.87%
3.77%
3.09%
2.90%
2.90%
1.74%
1.09%
0.83%
0.31%
0.18%
0.10%
0.08%
4,136.4
A few of the ideas in Zeke Yewdall’s article in HP101 focus
on home electricity efficiency, and many more solutions can
be found. The U.S. government’s Energy Star and energy
efficiency and renewable energy Web sites are great places
to start (see Access).
An Essential Cornerstone
Without a load analysis, designing a renewable energy
system is a shot in the dark. It’s like trying to plan your
weekly food shopping trip without knowing how many
www.homepower.com
73
load analysis
Load Calculation Excel spreadsheet •
www.homepower.com/magazine/downloads.cfm
“Watts Up? Pro KWH Meter” by AJ Rossman & Joe
Schwartz, HP95
“Things that Work: P3 International’s Kill A Watt WattHour Meter” by Joe Schwartz, HP90
“Things that Work: Brand Electronics’ Digital Power
Meter,” by Richard Perez, HP67
“Doing a Load Analysis: The First Step in System Design,”
by Ben Root, HP58
This plug strip is used to control multiple phantom loads
with the flip of one switch.
guests you’ll have and how much they’ll eat. It’s also where
you’ll save the most energy and money. Many people get
excited about making their own electricity, and lose sight
of the fact that analyzing energy usage and increasing
efficiency is where you get the most bang for your buck.
Don’t skip this step!
It’s easy to make the case for a comprehensive load
analysis. So take the time to do a good job and then reap the
rewards. Not only will it tune you in to how and where you
use the electricity you pay for, but it enables you to construct
a lean, green foundation on which to build your renewable
energy system.
Access
Scott Russell, Home Power, PO Box 520, Ashland, OR 97520 •
scott.russell@homepower.com
Brand Electronics, 421 Hilton Rd., Whitefield, ME 04353 •
888-433-6600 or 207-549-3401 • Fax: 207-549-4568 •
info@brandelectronics.com • www.brandelectronics.com •
Brand watt-hour meters
Electronic Educational Devices, 2345 South Lincoln St.,
Denver, CO 80210 • 877-928-8701 or 303-282-6410 • Fax:
303-282-6411 • info@doubleed.com • www.doubleed.com •
Watts Up? meters
P3 International Corp., 132 Nassau St., New York, NY
10038 • 888-895-6282 or 212-741-7289 • Fax: 212-741-2288 •
sales@p3international.com • www.p3international.com •
Kill A Watt meter
U.S. DOE energy efficiency and renewable energy info •
www.eere.energy.gov/consumerinfo
Energy Star • www.energystar.gov • Info on energy
efficient products & tips for home energy efficiency
74
home power 102 / august & september 2004
Load Analysis
Doing a Load Analysis:
The First Step in System Design
Benjamin Root
©1997 Home Power
I
t’s not that we really care about
electricity. We don’t even care
about the appliances that the
electricity powers. Our wants and needs
are even more basic than that. We want
to read after dark, hear good music, and
learn about what is happening in the
world. We want water on demand and
unspoiled food. We don’t need the
electricity like we don’t need the drill.
What we need is the hole.
Electricity is merely a tool used to meet our needs and
wants. When planning a renewable energy (RE) system
it is important not to lose sight of what our needs
actually are. Only once our needs are defined can we
then begin to design an RE system to meet them. We
must analyze each need and determine how much
energy it takes to meet that need. Long before we start
comparing prices on photovoltaic modules we must first
create a list of needs called a “load profile.” This article
will first discuss some important considerations in
choosing appliances to meet certain needs. Then we
will go through a step by step discussion of the various
elements in a load profile.
Why Do a Load Profile?
RE systems are expensive. Costs to produce one’s own
electricity from renewable sources average between
$0.25 and $1.15 per kilowatt hour (kWh). This is many
times the price of buying power from the electric utility.
Off grid, it is a waste of money to use more energy than
we need to and a waste of money to produce energy
that is not used.
If done correctly, your load profile’s average daily kWh
figure can be quite accurate. Careful load analysis can
assure that we size our RE system appropriately.
Which Loads are Appropriate Uses for Electricity?
Most of us need to eek out as much functionality from
as little energy as possible. For example, electricity is
an expensive way to produce thermal energy. The
electricity needed to provide space heating is generally
38
Home Power #58 • April / May 1997
cost prohibitive. Passive solar, wood heat, and propane
furnaces are all much more practical. Domestic hot
water heaters and cookstoves are also best powered by
passive solar, wood, or gas.
Certain loads can be powered by electricity or by other
sources. Refrigeration is a good example. Propane
refrigerators are available but have their own set of pros
and cons. In an energy efficient home the electric
refrigerator (even the energy efficient kind) is usually
the largest single load. Many RE systems use electric
well pumps, but wind-powered mechanical pumps have
effectively provided domestic water for generations.
These choices are ours. Do we need a 1,200 watt hair
dryer or will a towel do just as well? Is using candles or
kerosene for light really a smart (or safe) alternative to
compact fluorescents?
Some needs are surprisingly appropriate for use with
renewable energy systems. Power tools, microwave
ovens, toasters, and other kitchen appliances can draw
a lot of power and are often mistakenly considered to
be too much for an RE system. Actually, these
appliances are used for short periods of time and the
energy consumed is rather small.
Why Pay Extra for Efficiency?
It might sound like we must do without certain luxuries
in order to live with a renewable energy system. This is
not the case! RE systems can provide the same
amenities that our city cousins enjoy. The trick is to
carefully choose how these luxuries are implemented.
The most cost effective way to produce one’s own
energy is to first reduce one’s needs for that energy.
Richard Perez has a saying that sums it up quite well,
“Every watt not used is a watt that doesn’t have to be
produced, processed, or stored.” When buying grid
power we can dip into a limitless supply and pay as we
go. But with RE systems the cost of the energy is the
up front cost of expensive system components.
Choosing energy efficient appliances is cheaper than
renewable energy system components.
For example, compact fluorescent light bulbs have
improved immensely. The light is natural colored, flicker
free, and very efficient. A 15 watt compact fluorescent
produces the same amount of light as a 60 watt
incandescent bulb—at one fourth of the power
consumption. They cost about $22 but last 10,000
Load Analysis
hours, about ten times longer than a standard
incandescent bulb. More important is the money saved
by power that doesn’t have to be produced. Saving 450
kWh of electricity, at $0.65 per kWh (a hypothetical
middle ground cost for RE based on a well designed
photovoltaic system with generator back-up), over the
bulb’s lifetime translates to about $292 dollars. More
than enough savings to cover the $7 price difference
between one compact fluorescent and ten
incandescents!
Refrigeration is another good example of energy
efficiency paying for itself. It is often the largest load in a
RE-powered home. A sixteen cubic foot Sun Frost
fridge may cost $2,500 but uses only about 540 watt
hours each day. A typical major brand, non-efficient
fridge may cost only $600 but will use 1,500 watt hours
“Every watt not used“
is a watt that doesn’t
have to be produced,
”processed, or stored.”
per day. Assuming $0.65 per kWh for an RE system,
the electricity to operate the non-efficient fridge for ten
years costs about $3,558. The electricity to operate the
Sun Frost for ten years costs about $1,281. The
difference is $2,277 worth of renewable energy system
components that never need to be purchased, and
more than covers the $1,900 difference in price.
A good rule of thumb says that for every extra dollar
spent on energy efficient appliances, three dollars will
be saved in energy system components. It becomes
obvious that before one dollar is spent on photovoltaic
panels, wind generators, or hydro turbines we must
streamline our electrical demands.
Are Phantom Loads Really a Big Deal?
If you read many Home Power articles then you know
phantom loads are one of our biggest pet peeves.
Phantom loads use electricity while providing nothing in
return. A phantom load is any appliance that consumes
power even when it is turned off. While they may seem
small they use power twenty-four hours a day. A 4 watt
phantom load can cost about $22 a year on an RE
system, a lot for an appliance that is supposed to be off.
Any appliance with an electronic clock or timer is a
phantom load. If we want a clock we should use one
that is mechanically wound, battery powered, or even
electrical. But a clock in an appliance keeps the
appliance’s entire power supply “alive” just to tell us the
time. Very inefficient.
Appliances with remote controls remain alive while
waiting for the “on” signal from the remote. Any
appliance with a wall cube is also a phantom load. A
wall cube is a small box that plugs in to an AC outlet to
power appliances. Wall cubes consume 20 to 50% of
the appliance’s rated power even when the appliance is
off.
“One human“
”One Light”
Most modern TVs, VCRs, stereos, computers, Fax
machines, and other electronics are phantom loads.
They may contain a transformer, much like a wall cube,
that stays alive even when the appliance is off and
consumes between 50 and 200 watt-hours per day.
They may also contain a filter or line conditioner, to
clean up incoming power for the sensitive electronics
inside, consuming 8 to 40 watt-hours per day.
Modern televisions have an “instant on” feature so we
don’t have to wait for the picture tube to warm up. We
might as well call these TV’s “always on.”
The most direct way to overcome phantom loads is to
unplug the appliance when it’s not in use. A more
convenient technique is to use a switched plug strip.
These short extension cords with multiple receptacles
allow us to cut all power to multiple appliances with one
flip of a switch.
Use care when shopping for appliances that will run on
a renewable energy systems. Models that are not
phantom loads often have the fewest bells and whistles
but are the least expensive.
For more information on detecting and avoiding
phantom loads see HP 55, page 36.
How to Do a Load Analysis
On page 41 is a load profile form. It is available as a
Microsoft Excel spreadsheet on the Home Power web
site (http://www.homepower.com). Every appliance in
your household that receives regular use should be
logged onto this form. When completed you will have
an accurate estimate of your average daily kWhs used.
This is the foundation on which to build an RE system.
You may be planning for a future RE system at a home
that is not yet completed or fully inhabited. Is is
important to estimate your future loads as accurately as
possible. Try to be realistic about your lifestyle and
energy usage habits (Americans watch twice as much
TV as they think they do). Be aware of possible
appliance purchases in the future, like for growing
families. Remember obscure loads such as well pump,
satellite dish, garage door opener, etc. The accuracy of
Home Power #58 • April / May 1997
39
Load Analysis
the final estimate is dependent on the accuracy of your
initial data.
In a load analysis we evaluate a variety of parameters
for each appliance. By combining this data we will be
able to see this appliance’s impact on your energy
needs as a whole, and in comparison with other
appliances. What follows is a discussion of each
parameter (vertical column on the form) and how to
obtain the data.
Column A: Appliance
Simply, what appliance are you testing?
Column B: Number
How many of these appliances? An example of multiple
identical appliances is lights. There is no need to list
every light bulb in the house separately. Richard Perez
has a super analogy of one light for every member of
the household. Imagine each person has a light that
follows them around the house as they move. This is
just an analogy, and until technology improves, it is up
to each person to throw the switches to get their light to
“Every dollar spent for“
an efficient appliance
saves three dollars in
renewable energy system
”components.”
follow them. Ideally then, a three person family should
be able to enter 3 in this column for personal lighting.
Lights of different wattages should get separate entries.
A light on a timer in the driveway should get its own
entry, as should a night-light that stays on all night in
the hall.
Column C: Load Voltage
At what voltage does this appliance operate? RE
systems are moving away from 12 Volt systems.
Modern RE-powered homes often run on 24 or even 48
Volt systems. Some DC appliances are available for 12
Volt, less so for 24 Volt. Most inverter-powered AC
appliances run at 110 Volts (117 Volts rms) but we must
not forget about the indispensable 220 volt power tool.
Column D: AC or DC
Does this appliance operate on inverter power or
directly from battery power? Inverters consume power
by just being on. However, many renewable energy
system users are finding that the advantages of
constant ac power easily offset inverter losses. Here at
Home Power we run all our communications equipment
directly on DC for emergency reliability reasons.
40
Home Power #58 • April / May 1997
Column E: Inverter Priority
Does this appliance spend a large amount of time on?
The purpose of this column is to get a feel for the
normal operating wattage of the inverter. If an appliance
spends a good deal of time on or if we want to be sure
that this appliance will always have access to inverter
power, then we consider it to be an inverter priority
load.
Any appliance that turns itself on and off must be an
inverter priority load because we cannot control its
access to the inverter. Some loads are operated
infrequently and we can decide what other appliance
we will allow to operate at the same time. These loads
are not inverter priorities.
Later, when we are designing our RE system, this
column will help us choose the size of our inverter. It
will also help determine the inverter’s average operating
efficiency.
Column F: Run Watts
How much power does the appliance consume when in
use? The most accurate way to determine this is to
measure current through the appliance then multiply by
117 volts if it’s an ac appliance. If the appliance is DC,
multiply the measured Amps by the system voltage to
determine Watts. Measuring Amps involves getting an
ammeter in series with the load. HP 33 page 82
illustrates an effective little gismo for breaking into ac
wiring to measure amperage.
Another technique for measuring amps, if your meter
has limited amp capability, is to use a shunt. A shunt is
a small resistor of known value. It, like an ammeter,
must be placed in series with the load being tested.
Once in place, measure voltage across the shunt, then
use Ohm’s law to determine the amperage. If you don’t
want to buy a shunt then make one out of #10 wire.
One foot of #10 copper wire has a resistance of 0.001
ohms. Set your voltmeter to the millivolt scale and
measure the voltage drop across the makeshift shunt.
For more information on using wire as a shunt see HP 6
page 35. To review Ohms law see HP 52 page 64.
Electrical appliances display their power use data on a
plate or sticker. The noted watt value represents a
worst case scenario, the most power that the appliance
will ever draw. We generally don’t listen to the stereo
with the volume all the way up (punk rockers aside), or
juice marbles in the blender. If you want accurate
numbers you should measure actual watts. If you can’t
measure then derate the sticker wattage by about
25%.
Column G: Hours per Day
How much is the appliance used each day? In some
ways this information is easy to figure: The radio plays
A
Appliance
B C D E F
Qty. Volts
G H
I
J
K
L
AC P Run Hours Days W-hours Percent Surge Ph-L
DC Y/N Watts /Day /Week
/Day
of Total Watts Y/N
Espresso Maker (example) 1 117 AC N 1350 0.20 7
270.0 6.8% 1350 N
Total Daily Average Watt-hrs
Inverter Priority Wattage
Max. ac Wattage
Max. ac Surge Wattage
Home Power #58 • April / May 1997
41
Load Analysis
every morning for forty-five minutes while you get ready
for work. The washing machine takes twenty minutes to
complete a cycle. Other appliances are more tricky, for
example the three light bulbs for your three person
family. You need to guess how much time each day that
each light is on.
Some appliances turn themselves on and off
automatically. Refrigerators start up when the
temperature inside gets too warm. They run until they
are cooled down to certain temperature when they turn
themselves off. This is called a “duty cycle” and can be
estimated by direct observation. Just pay attention to
how often that fridge comes on and how long it stays
on.
When determining energy use, the time element of
column G is interconnected with the power element of
column F. We can ignore duty cycle by using a
recording ammeter and a stopwatch. Simply divide total
amp-hours consumed by the number of hours tested to
“If you want a clock,”
“then buy a clock.”
obtain a constant amps rating. Multiply amps times
appliance voltage (column E) to get watts (column F).
Then use 24 hours per day in column G.
Column H: Days per Week
Do you do wash every day? Do you only watch TV on
Saturday mornings? This helps determine average
energy use per day.
Column I: Average Watt-hours per Day
Number (Column B) x Watts (Column F) x hours
(Column G) x days (Column H) ÷ 7 days per week =
average watt-hours per day for this appliance.
BxFxGxH÷7=I
This amount tells us, on average, how much electricity
is consumed each day by this appliance. The total at
the bottom of this column tells us how much electricity
we use on an average day.
Column J: Percentage of Total Electricity Use
Just for your information, what percentage of total
electrical use does this appliance represent? Column I
÷ the total sum of column I for all appliances.
Column K: Starting Surge in Watts
Does this appliance have a starting surge? How much?
Any appliance with a motor has a starting surge. This
means that before the motor is up to operating speed it
is drawing more than its rated operating power. This is
especially true if the motor is starting under load.
Refrigerators, well pumps, and most power tools have
42
Home Power #58 • April / May 1997
starting surges. Motors surge between three and seven
times their rated wattage.
Other appliances that may have starting surges are
TVs, computer monitors, and any appliance with an
internal power supply. These loads have large
capacitors that charge themselves when the appliance
is first turned on. They can surge up to three times their
rated wattage.
Because they are relatively short—in the millisecond
range—starting surges don’t make much of a difference
in the amount of energy that an appliance consumes.
Starting surges are important, however. Inverters must
be sized to handle the starting surge of ac appliances.
Battery banks must also be sized to handle the voltage
depression caused by a high amp surge. Voltage
depression can cause an inverter to shut down even if
the inverter itself is large enough to handle the surge.
Measuring the starting surge of an appliance requires a
meter with a peak hold (maximum) capability.
Column L: Phantom Load
Does this appliance consume power even when turned
off? Home Power is ruthless with phantom loads! Our
offices are totally controlled by plug strips. No phantom
load is allowed to haunt the system. Column L will do
three things. First, it reminds us to check each
appliance while doing our load profile. Second, it
reminds us later that this appliance is a phantom load
and must be dealt with as such. Third, if for some
reason this appliance is allowed to operate as a
phantom load, we will remember that a separate entry
must be made in the load table to reflect its energy
usage (whenever the appliance is not in use).
The Completed Load Survey
You have combed your house testing loads. You have
estimated future loads and maybe even made purchase
decisions based on this load survey. But what does the
table really tell you? The total at the bottom of column
I.is most important. This number represents the
average daily electricity that your household uses. This
is also the amount of power that your RE system must
generate daily.
Some days you do wash and some you don’t. Some
days you run a lot of power tools. Some days the sun
shines and some it doesn’t. There are inefficiencies in
batteries and inverters. There are a lot of other
variables involved in system design. However, average
daily kWh is the basic need that must be met. All
system design starts here!
Other information in this table (inverter priority wattage,
max ac wattage, and max ac surge wattage) will
become useful during system design. Do you install 220
Load Analysis
Examples
Here are load tables for two
example households. Both of
these homes provide the same
functionality, meeting the
same needs and luxuries for
their inhabitants. The only
difference between these two
homes is the efficiency of the
electricity use.
Home 1 represents the use of
some inefficient appliances: a
name brand refrigerator and
incandescent lights are used.
Also, the inhabitants of this home
ignore the phantom loads,
allowing them to run constantly.
Notice that each phantom load
has its own entry on the load
table representing the power
used by that appliance when
turned off.
Home 1 uses an average of
almost 7.4 kWh of electricity
each day. At 65¢ per kWh this
adds up to about $4.80 per day
for electricity!
Home 2 represents a more
efficient use of electricity:
Compact fluorescent lights and
an efficient refrigerator. Also,
phantom loads are completely
eliminated by the use of switched
plug strips. These are the only
differences between Home 1 and
Home 2. However, Home 2 only
uses an about 4 kWh per day of
electricity. At 65¢ per kWh this is
about $2.53 per day for
electricity.
The $2.27 daily difference
between Homes 1 and 2 is
substantial. Over $828 dollars
saved each year can easily pay
for the expense of efficient
appliances.
Remember, the accuracy of the
final energy use estimate is only
as accurate as the data within
the load analysis table.
Home 1 (inefficient)
Appliance
Incandescent Lights
Refrigerator RCA 16 cu. ft.
Blender
Microwave Oven
Phantom Load-Microwave
Food Processor
Espresso Maker
Coffee Grinder
21" Color Television
Ph/L-TV
Video Cassette Recorder
Ph/L-VCR
Satellite TV System
Ph/L-Satellite Sys.
Stereo System
Ph/L-Stereo
Computer
Ph/L-Computer
Computer Printer
Ph/L-Printer
Power Tool
Radio Telephone (receive)
Radio Telephone (transmit)
Phone Answering Machine
Washing Machine
Ph/L-Washer Timer
Clothes Dryer (motor only)
Ph/L-Dryer Timer
Sewing Machine
Vacuum Cleaner
Hair Dryer
Ni-Cd Battery Charger
Ph/L-Batt Charger
Qty. Volts
4
117
1
12
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
12
1
12
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
Inverter Priority Wattage 599
Home 2 (efficient)
Appliance
Fluorescent Lights
Fridge Sun Frost 16 cu. ft.
Blender
Microwave Oven
Food Processor
Espresso Maker
Coffee Grinder
21" Color Television
Video Cassette Recorder
Satellite TV System
Stereo System
Computer
Computer Printer
Power Tool
Radio Telephone (receive)
Radio Telephone (transmit)
Phone Answering Machine
Washing Machine
Clothes Dryer (motor only)
Sewing Machine
Vacuum Cleaner
Hair Dryer
Ni-Cd Battery Charger
Inverter Priority Wattage 325
AC
P
Run Hours Days
/Day /Week
DC Y/N Watts
AC Y
60
5.0
7
AC Y
141 10.0
7
AC N
350
0.1
2
AC N
900
0.3
7
AC Y
4 23.8
7
AC N
400
0.1
5
AC N
1350
0.1
7
AC N
150
0.1
7
AC Y
125
5.0
7
AC Y
20 19.0
7
AC Y
40
2.5
7
AC Y
15 21.5
7
AC Y
60
2.5
7
AC Y
22 21.5
7
AC Y
30
8.0
7
AC Y
3 16.0
7
AC Y
45
6.0
3
AC Y
3 21.4
7
AC N
120
0.3
3
AC Y
3 23.9
7
AC N
750
0.5
3
6 24.0
7
DC N
20
1.0
7
DC N
AC Y
6 24.0
7
AC N
800
0.5
4
AC Y
8 23.7
1
AC N
500
1.0
4
AC Y
8 23.4
7
AC N
80
2.0
1
AC N
650
0.5
4
AC N
1000
0.2
7
AC Y
4 15.0
2
AC Y
2 19.7
7
Total Daily Average Watt-hrs
Max ac Wattage 1350
W-hours Percent
Surge Ph-L
/Day
of Total Watts Y/N
1200.0 16.3%
0 N
1410.0 19.1%
1300 N
10.0 0.1%
1050 N
225.0 3.1%
1200 Y
95.0 1.3%
0
28.6 0.4%
1200 N
135.0 1.8%
1350 N
7.5 0.1%
200 N
625.0 8.5%
570 Y
380.0 5.2%
0
100.0 1.4%
80 Y
322.5 4.4%
0
150.0 2.0%
1600 Y
473.0 6.4%
0
240.0 3.3%
60 Y
48.0 0.7%
0
115.7 1.6%
135 Y
64.3 0.9%
0
12.9 0.2%
360 Y
71.7 1.0%
0
160.7 2.2%
2250 N
144.0 2.0%
0 N
20.0 0.3%
0 N
144.0 2.0%
0 N
228.6 3.1%
100 Y
27.1 0.4%
0
285.7 3.9%
1500 Y
187.4 2.5%
0
22.9 0.3%
400 N
185.7 2.5%
1950 N
200.0 2.7%
1500 N
17.1 0.2%
25 Y
39.4 0.5%
0
7376.8
Max. ac Surge Wattage
2250
AC P
Run Hours Days
W-hours Percent
Surge Ph-L
DC Y/N Watts
/Day /Week
/Day
of Total Watts Y/N
AC Y
15
5.0
7
300.0 7.7%
0
N
DC N
48 11.3
7
540.0 13.9%
1300 N
DC N
350
0.1
2
10.0 0.3%
1050 N
AC N
900
0.3
7
225.0 5.8%
1200 Y
AC N
400
0.1
5
28.6 0.7%
1200 N
AC N
1350
0.1
7
135.0 3.5%
1350 N
AC N
150
0.1
7
7.5 0.2%
200 N
AC Y
125
5.0
7
625.0 16.0%
570 Y
AC Y
40
2.5
7
100.0 2.6%
80 Y
AC Y
60
2.5
7
150.0 3.8%
1600 Y
AC Y
30
8.0
7
240.0 6.2%
60 Y
AC Y
45
6.0
3
115.7 3.0%
135 Y
AC N
120
0.3
3
12.9 0.3%
360 Y
AC N
750
0.5
3
160.7 4.1%
2250 N
DC N
6 24.0
7
144.0 3.7%
0 N
DC N
20
1.0
7
20.0 0.5%
0 N
AC Y
6 24.0
7
144.0 3.7%
0 N
AC N
800
0.5
4
228.6 5.9%
100 Y
AC N
500
1.0
4
285.7 7.3%
1500 Y
AC N
80
2.0
1
22.9 0.6%
400 N
AC N
650
0.5
4
185.7 4.8%
1950 N
AC N
1000
0.2
7
200.0 5.1%
1500 N
AC Y
4 15.0
2
17.1 0.4%
25 Y
3898.4
Total Daily Average Watt-hrs
Max. ac Wattage 1350
Max. ac Surge Wattage
2250
Qty. Volts
4
117
1
12
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
117
1
12
1
12
1
117
1
117
1
117
1
117
1
117
1
117
1
117
Home Power #58 • April / May 1997
43
Load Analysis
volts worth of inverters or do you run your single 220
vac load on your generator? Do you want an inverter
that can run your ac well pump at the same time as the
washing machine? What happens when someone turns
the microwave oven on too? If you run the fridge and
the well pump on DC, can you get away with a smaller
inverter? These kinds of questions will come up during
system design. Being able to refer back to a complete
and detailed load profile will help with the answers.
Access:
Ben Root is still trying to remember to turn off his stereo
at night while writing and doing graphics for Home
Power at Agate Flat.
c/o Home Power or E-Mail: ben.root@homepower.org
44
Home Power #58 • April / May 1997
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