PV BASIcS
SETH MASIA
a regulator), an electronic device that produces
a smooth flow of current at the desired voltage.
From the charge controller, the power can go to
a set of storage batteries and then on to an
inverter, or it can go directly to the inverter.
The inverter changes high-voltage DC to 110-volt
AC that powers the household electric system: the
wall and ceiling lamps, outlets and large appli-
ances. If the system is grid-tied, excess power from
the inverter may flow out of the house through the
utility company’s electric meter, into the city-wide
grid. When this happens, the meter may run back-
ward, and the utility will credit the outflowing elec-
tricity against electricity purchased from the grid at
night. This process is called net-metering.
System components 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-k W 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-kw system can be installed
in a day, you won’t save labor costs by
buying a 3-kw system, which may also
take a day.
PV cells come in three versions:
• Single-crystal (or monocrystalline) modules
are the most efficient — that is, 1 square meter
produces the most electric power. They must be
mounted in a rigid frame.
• Multicrystalline (or polycrystalline) modules
are made of cells cut from multiple crystals, grown
together in an ingot. They are more structurally ro-
bust and slightly less efficient.
• Amorphous silicon (thin-film) modules are
made by depositing or printing photosensitive ma-
terials 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 gener-
Installation location. 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
made adjustable 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
Inverter boxes convert direct current from the
solar cells to alternating current for use on the
household circuit and to feed back out to the
utility grid.
ate the same power, but they’re fairly cheap and
depending on the substrate can be very robust and
flexible.
• 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. In-
verter life expectancy is 10 to 15 years.
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 bat-
teries for use at night. Off-grid homes use mas-
sive, heavy-duty deep-cycle batteries — similar
to the batteries used in fork-lift trucks. Batteries
should be housed in a stable, weather-proof
space and insulated against very hot and very
cold weather but isolated from living spaces.
Batteries are charged with DC from a charge con-
troller. The charge controller can take power direct-
ly from a PV array, or from a wind turbine or diesel
generator or hydroelectric source — anything that
can feed steady DC can be a charging source. Cur-
rent taken off the battery goes to the inverter for
household use.
If you live on the grid, a battery storage system is
usually unecessary. It can, however, substitute for a
backup generator, assuring you’ll have power and
heat during utility company outages. That means
your small business can have reliable refrigeration
in hot-weather brownouts, and you can operate
a fan-driven furnace when an ice storm takes out
power lines. r
This article is adapted from the 2000 Solar Energy
Resource Guide, published by the NorCal Solar Energy
Association ( norcalsolar.org), a chapter of the American Solar Energy Society
