BECOMING YOUR OWN POWER COMPANY

In the last column I talked about electrical pressure and asked
the question, "What is being pressurized in an electrical
system?". The answer, as I am sure you all remember, is
*current*.

Current is the equivalent of water moving through the system.
Current is measured in *amps* the way water is measured
in gallons. When we speak of the number of amps of electricity
an electrical device uses we refer to the amount of current that
needs to flow into the device to make it operate.

So we now have two electrical values, volts - the measurement
of electrical pressure, and amps - the measurement of electrical
current flowing through the system. These two values taken together
show the total amount of energy available in a system (or produced
by a PV panel, or consumed by an appliance). This measurement
of energy is sometimes referred to as *volt-amps* because
its value is determined by multiplying volts x amps. The more
common term for volt-amps is *watts*.

The PV panels on the roof of our house are rated in watts. Each panel will produce 100 watts of energy in full sun. These panels have a convenient feature that allows me to wire them for a 12 volt system or a 24 volt system. In the 12 volt configuration a panel produces 6.6 amps at a full charge of 15 volts (when the batteries are full). In the 24 volt configuration a panel produces 3.3 amps at a full charge of 30 volts. Both configurations produce 100 watts of energy.

This calculation (amps x volts = watts) is a useful one to know. If you know the wattage consumption of an appliance you can calculate how much current it needs at a certain voltage because watts/volts = amps.

Let's say you have a 120 watt stereo. At 120 volts AC you can calculate the amount of current it will consume. 120 watts/120 volts = 1 amp. However, since you are providing the energy to run that stereo from your battery bank, which is at 24 volts DC, you also need to know how much current it will draw from the batteries. 120 watts/24 volts = 5 amps.

The amount of energy consumed (in watts) is the same, but the
amount of current needed depends on the voltage. An *inverter*
is used to change the 24 volt DC battery bank power to 120 volt
AC to run your household appliances. This inverter takes 120
watts of energy from the battery bank and supplies it to the stereo
at 120 volts. The inverter pushes 1 amp of current at 120 volts
to provide the stereo the 120 watts it needs to operate. But
it has to take 5 amps of current at 24 volts from the batteries
to provide the same 120 watts.

This conversion of the lower voltage energy stored in a battery bank to the higher voltage energy typically used in a home is an important one to understand. It will allow you to calculate how much energy you need to store in your batteries to provide the electricity you will need to run your house.

Because a battery's energy capacity is rated in amphours (the number of amps used x the number of hours they are used) you will need to consider how much time you use the various electrical devices in your house.

Let's say you use the 120 watt stereo for 3 hours a day (unless,
of course, you have teenagers). Using the calculation above it
works out to 3 amphours of current at 120 volts or 360 *watt-hours*
(.36 kilowatt-hours). (120 watts/120 volts = 1 amp, 1 amp x 3 hours
= 3 amphours, 3 amphours x 120 volts = 360 watt-hours).

To calculate how much current (in amphours) the batteries must provide you simply divide the 360 watt-hours by 24 (the voltage of the batteries) and you have a value of 15 amphours that will be used from the battery bank to run the stereo.

In the next article I will talk about how to determine your energy
usage and how to properly size your battery bank.

Wagonmaker Press

Thomas W. Elliot NEXTReturn to Article List