must be matched. A 3-k W
inverter can’t handle the
power produced by a 5-k W
DC array. If you install a
3-k W array, you may consider putting in a 5-kW
inverter to allow expansion of the array later. Also
be aware of economies of
scale. Labor costs may be
lower, per unit, for a large
system. If a 5-k W system
can be installed in a day,
you won’t save labor costs
by buying a 3-k W system,
which may also take a day.
Net metering >> Your utility company may not
offer net metering. If it does, it will do so under
terms usually determined by state law or by your
public utility commission. The rate the utility will
pay for power you sell will therefore vary widely
state to state and city to city. Some utilities may pay
you more for power during high-load periods, as
in summertime late afternoons, when heavy use of
air conditioners creates high demand. Check with
your utility company for details or look up your local
information at dsireusa.org.
Maintenance >> Modules are tough and usually
carry a 20- to 25-year warranty. They need to be
cleaned occasionally, which is usually a matter of
hosing off dust and leaves. If appropriate, consider
how you’ll clear snow off the modules. The inverter
processes hundreds of volts for several hours a day.
It needs to be mounted in a cool, shaded place.
Inverter life expectancy is about 10 to 15 years.
PHoto b Y lAVonne e WinG / Pix YJACk PreSS
Installation location is
critical to PV performance.
The array should face the
sun. This usually means due
south, though if you have
a heavy air-conditioning
load in the late afternoon,
you may want to point the
array southwest. The array
should not be shaded by
trees, towers or chimneys
during any part of its productive day. The array
should be tilted upward at
the correct angle to optimize seasonal exposure —
typically at the angle of your
latitude, so it gets sunlight
at a right angle at spring
and fall equinoxes. Some
arrays can be adjusted for
the seasonal sun angle. The
ideal location is on a south-facing pitched roof. If the
array needs to be elevated
above the roof surface, it
places additional torsional
loads on the roof structure
during wind storms.
Since 1999, Rex and LaVonne Ewing’s off-grid mountain home has been powered entirely
by solar and wind energy. The ground-mounted 1,940-watt photovoltaic array is supple-
mented by a 1,000-watt wind turbine. Domestic water is heated by a roof-mounted solar
collector array of 30 evacuated tubes, with an on-demand water heater as backup.
Thin-film modules are made by depositing or printing
photosensitive materials on a glass, metal or plastic substrate — even on roofing tile. They’re considerably less
efficient, so you may need to give them a lot of space to generate the same amount of power as a smaller single-crystal
or multicrystalline system. Thin-film modules are relatively
inexpensive and, depending on the substrate, can be very
robust and flexible. One practical use is to glue a flexible-plastic thin-film module directly to a metal roof.
off the Grid
If you plan to live far from the main road, it may be too
expensive to run a power line in from a utility pole. In that
case, you’ll need to generate your own electricity and store
it in a bank of batteries for use at night. Off-grid homes use
massive heavy-duty, deep-cycle batteries — similar to the
batteries used in forklift trucks. Batteries should be housed
in a stable, weather-proof space, insulated against very hot
and very cold weather but isolated from living spaces.
Batteries are charged with DC from a charge controller.
The charge controller can take power directly from a PV
array, from a wind turbine, from a diesel generator or from
a hydroelectric source — anything that can feed steady DC
can be a charging source. Current taken off the battery goes
to the inverter for household use.
If you live on the grid, a battery storage system is usually unnecessary. It can, however, substitute for a back-up
generator, assuring that you’ll have power and heat during
utility company outages. That means your small business
can have reliable refrigeration in hot-weather brown-outs,
and you can operate a fan-driven furnace when an ice storm
takes out power lines. GS
This article is adapted from the Solar Energy Resource Guide
2008, published by the NorCal Solar Energy Assciation (norcal
solar.org), a chapter of the American SolarEnergy Society.