edited by Liz Merry
and Seth Masia
Electric power is mea- sured in watts. A kilowatt (k W) is 1,000 watts, and a megawatt (MW) is 1 mil- lion watts. You buy elec- tricity in kilowatt-hours (k Wh). For instance, if you run a 100-watt light bulb for an hour, you’ve used 100 watt-hours. If you run it for 10 hours, you’ve used 1 k Wh, for which the average household would be billed 11 cents. A 1-k W photovoltaic array can pro- duce 1 k W in direct sun. If sunlight falls on it five hours a day, it may produce
5 k Wh that day.
An appropriate roof-mounted array faces south (or close to it) with no trees, chimneys, poles
or towers to throw shadows on the arrays.
Photovoltaic (PV) technology produces electricity from sunlight using a solid-state lectronic module with no moving parts. It’s a mature technology, invented at Bell Labs in the 1950s.
For residential application, PV falls into two main categories: grid-tied, where the home generates its own electricity but can also draw power from the utility company at
night; and off-grid, where the home is located too far from
a utility cable and must generate all its own power, storing
it in batteries for use at night.
tied to the Grid
A basic home PV system consists of PV cells connected
together in glass-protected weatherproof modules, fastened
side-by-side on a rack to form an array. In a grid-tied system, the modules produce direct current (DC), which
flows to an inverter. The inverter changes high-voltage
DC to 110-volt alternating current (AC) that powers the
household electric system: wall and ceiling lamps, outlets
and large appliances.
Excess power from the inverter may flow out of the
house, through the utility company’s electric meter and
into the city-wide grid. When this happens, the meter may
run backward, and the utility will credit the outflowing electricity against electricity purchased from the grid at night.
This process is called net metering.
In an off-grid system, DC power flows from the modules through a charge controller (also called 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
Today’s commercially available PV panels come in
Single-crystal (or monocrystalline) modules are the
most efficient — that is, one square meter produces the
most electric power. They must be mounted in a rigid
Multicrystalline (or polycrystalline) modules are
made of cells cut from multiple crystals, grown together
in an ingot. They are more structurally robust and slightly
Microinverters >> Some newer grid-tie systems
replace the large central inverter with several microinverters, attaching one microinverter to the back of
each PV module. Power coming off the roof is 230
volts AC and can tie directly to the household service panel. A major advantage is that, if one module
is shaded or broken, voltage of the entire system is
unaffected. You can track the performance of each
individual panel on a home computer.
Know your load. If your
family uses 600 k Wh of
electricity each month,
that works out to about
20 k Wh per day. If you get
an average of five hours
of direct sunlight daily,
you’d balance your electric use with about 4 k W
of net-metered photovoltaic power. A 2-k W system
would offset about 50 percent of your bill.