get started sizing your solar electric system
Will Solar
WORK
for You?
Here’s how to calculate
the size of a grid-tied
photovoltaic system
by assessing your
electrical use, your roof
and your yard.
By DAN CHIRAS
Before you calculate what a solar sys- tem will cost to install, and whether it’s a good economic investment, you need to know how much electricity ou consume and how much electricity a solar electric system can provide.
Household electrical needs vary. My own
house, designed for efficiency, consumes 75
to 80 percent less electricity than a standard
home of similar size, so we get all the electricity we need from a modest 1.1-kilowatt (k W)
photovoltaic (PV) array.
In a typical home using natural gas for
cooking, water heating and space heat, electrical consumption may run about 500 kilowatt-hours (k Wh) per month. A 3- to 4-k W PV
system might cover most household needs in
a good sunny region. A larger system might be
required in a cloudier area.
In an all-electric home equipped with the
usual assortment of appliances — especially
Your household electrical needs will help determine the size of your solar electric system.
dAn Bihn
40 April 2010 SOLAR TODA Y solartoday.org
Dan Chiras ( danchiras@evergreeninstitute.org) is
president of Sustainable Systems Design Inc. and
director of The Evergreen Institute’s Center for
Renewable Energy and Green Building. This article
is adapted from the book Power from the Sun, with
technical advisors Robert Aram and Kurt Nelson.
heavy hitters like a central air conditioner, a
water heater, a stove and space heaters — average monthly electrical consumption can run
between 2,000 and 3,000 k Wh per month. It
might take a system 10 k W or larger to meet
that demand.
Assessing the electrical needs in an existing house is different from planning the needs
for a new building. For your existing house,
start by gathering your electric utility bills for
the past two or three years. If you don’t have
them, you can usually obtain the information
by phoning your utility company. The company may even make the information available
to you via its website.
When studying your utility bills, look for
the kilowatt-hours consumed each month.
Calculate the total for each year, and then
calculate a yearly average. Look for trends in
energy use. Are you using more electricity now
than two or three years ago? Or less? Is con-
sumption steady? Perhaps you’ve installed a
big-screen TV or an air conditioner. Perhaps
you’ve replaced an inefficient furnace or refrig-
erator with a new, more frugal model. Plan for
energy use going forward, with the equipment
you have now or expect to obtain in the future.
That way, you’ll avoid oversizing or undersiz-
ing your PV system.
Determining electrical demand in a new
home is more difficult. There’s no track record,
just some design assumptions. If the new home
is of roughly the same size as your old home,
you plan to use similar appliances and you don’t
expect dramatic changes in lifestyle, electrical
consumption may be similar to that in the old
home. But that’s not often the case. You may be
building in a new climate or with a different ori-
