Common terminology
You’ll need to understand some basic indus-
try terminology in order to work with solar PV
systems.
✓ Load: Any device that uses electricity. An
inverter is a load, as is a washing machine or
a light bulb. When determining system size,
you add up all the various loads, and how
much they are being used in your home.
✓ Volt: The unit of force, or electrical pres-
sure, that causes electrons to flow through
a conductor (wires and electronic equip-
ment). Volts are abbreviated as V, or some-
times E (which stands for electromotive
force). Most household electrical systems
operate on 240 VAC, or 120 VAC.
✓ AC: Alternating current. The flow of elec-
tricity goes in both directions, back and
forth, and in household electrical sys-
tems it flows back and forth 60 times per
second (referred to as Hertz, or Hz). In some
regions, the line voltage and frequency (the
number of Hz) from the grid may vary quite
a bit, and this can cause problems with
some of your household loads. It may also
require you to specify an inverter that can
handle the fluctuations.
✓ DC: Direct current, where the flow of elec-
tricity is in only one direction. The output of
a solar panel is DC. Since your home uses
AC, this DC current must be converted
into AC to be useable, and this is what an
inverter accomplishes.
✓ Ampere, or Amp: The amount of current
that is flowing, due to the presence of a
voltage. Amps are denoted by I (for inten-
sity of current).
✓ Resistance (denoted by R): The amount of
impedance to electrical flow. The higher
the resistance, the lower the current for a
given voltage.
✓ Watt: A unit or power, and is equal to
Voltage times Current, or I X V. It’s an
instantaneous measurement; power can
vary from second to second, as you switch
the various loads in your home off and on.
The output of a solar PV system is listed
in watts, and this varies with the sunlight
striking the panels.
✓ Watt-hour: A unit of energy, and this is what
your solar system produces over a period
of time. At any given time, your system is
outputting a certain power level (watts), but
over time it produces energy, and ultimately
energy production is what you are after.
Typically, we’re more concerned with kWh,
which simply means one thousand watt-
hours. Your utility bill charges you for how
many kWhs you use per month (Chapter 17
explains the various rate structures, or the
way your power company charges you for
the energy you use). A 100-watt light bulb
burning for one hour uses 100 watt-hours
of energy, or 0.1 kWhs.Off-grid solar electrical systems all use the same basic components, with other
elements added according to need. Figure 18-1 shows a typical off-grid system.
Here are the functions of each part:
✓ Charge controller: The charge controller feeds current into the battery
bank at the required voltage. Good charge controllers draw the best
performance out of the batteries and are very important for economics
because they influence efficiency.
✓ Battery bank: The battery bank is typically made up of six or more indi-
vidual batteries connected with stout cables in either series or parallel
arrangements.
✓ Inverter: The inverter changes DC to AC voltages suitable for use with
household equipment. An inverter is optional if you use DC loads
exclusively.
✓ DC loads controller: You may be using both DC (boat, RV, and auto appli-
ances) and AC loads (standard household appliances). The DC loads con-
troller maintains the proper currents and voltages into the DC loads.
✓ AC generator: As a backup power supply, the AC generator isn’t strictly
necessary but is usually part of any off-grid system in order to prevent
blackouts when the sun is weak for extended periods of time.
✓ Transfer switch: The transfer switch alternates the power source
between either the inverter output (when battery power is available) or
the AC generator.
✓ AC loads controller: This device includes appropriate fuses and switching
means and maintains the voltages and currents used by the AC appli-
ances connected to the system.