More on Understanding Electricity

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.

All Contents © 1997
Wagonmaker Press
Thomas W. Elliot


Return to Article List