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Electricity and Basic Electronics
an inverter is needed to convert dc power to
ac. The inverter also performs the following
tasks:
• Synchronizes the grid to the PV system.
• Turns on and connects to the grid after
sunrise.
• Matches the frequency of the PV system
and the grid.
• Monitors the voltage and current
generated.
• Turns off and disconnects from the grid
around sunset.
• Collects and saves energy data.
• Provides a safety disconnect during
outage from storms.
When a PV system is used to supply
energy to the grid, it is a two-way system.
When there is a surplus of energy in the PV
system, the electricity is fed into the grid.
When there is a shortage of energy in the
PV system, the grid feeds it electricity. If a
storm causes an outage on the grid, people
repairing the electrical lines could be shocked
from the PV system unless it is automatically
disconnected. A PV system that performs all
of these functions is called a line-tied or grid-
connected installation.
If a PV system is installed on a home, the
owner can sell electricity to the local power
company. Any excess photovoltaic energy that
is not needed to run the home would be fed
into the power grid. This spins the home’s
electric meter backward. The excess energy
provides a credit against the electricity from
the power company that is used in the home.
This is known as net metering.
However, the homeowner may choose not
to sell the electricity to the power company.
Stand-alone PV units provide the owner with
an off-the-grid power supply that is indepen-
dent from the power company.
Uses and examples
PV systems can be used in many different
applications. They are ideal for areas where it
is diffi cult or impossible to run power lines,
Figure 24-16. For example, farmers may
use photovoltaics to power electric fences
in pasture areas. Some mountain homes or
other isolated vacation areas use PV systems
Figure 24-16. You may have wondered how
the LED eyes in Figure 17-30 got the power to
operate. Here is the installation of the photovol-
taic panel that charges the batteries needed to
light the eyes. (Courtesy of Joseph Matt)
Figure 24-15. This roof is being installed with
photovoltaic roofi ng shingles. (Southface)