The Basics of Solar Electricity
 

Before purchasing a photovoltaic system, it is a good idea to have a basic understanding of electricity. Simple familiarity with basic electrical terms and concepts will enable you to better understand your renewable energy system and use it with confidence.

The building blocks of an electrical vocabulary are voltage, amperage, resistance, watts and watt-hours. Electricity can simply be thought of as the flow of electrons (amperage) through a copper wire under electrical pressure (voltage) and is analogous to the flow of water through a pipe. If we think of copper wire in an electrical circuit as the pipe, then voltage is equivalent to pressure (psi) and amperage is equivalent to flow rate (gpm).

To continue with our electricity to water analogy, a battery stores energy much as a water tower stores water. Since a column of water 2.31 feet tall produces 1 psi at the base, the taller the water tower the higher the pressure you get at the base. As you can see from the picture to the right, the mushroom shape design of a water tower allows it to provide a large volume of water to end users at between 40-60 psi. Once drained below 40 psi which occurs near the neck of the tower, continued water usage will rapidly deplete the water supply at an ever decreasing pressure. Although a 12 volt battery is not physically shaped like a water tower, it has most of its stored electricity available between 12 volts to 12.7 volts. When drained below 12 volts, little amperage remains and the battery voltage will decrease rapidly.

In a simple system, a power source like a solar module provides the voltage which pushes the amperage through a conductor (wire) and on through a load that offers resistance to the current flow which in turn consumes power (watts). Power is measured in watts and is the product of voltage multiplied by amperage. Energy is power (watts) used over a given time frame (hours) and is measured in watt-hours or kilowatt-hours (1 kilowatt-hour equals 1000 watt-hours). For example, a 100 watt light left on for 10 hours each night will consume 1000 watt-hours or 1 kilowatt-hour of energy. A kilowatt-hour is the unit of energy measurement that the utility company bills you for each month. Electrical appliances are rated in terms of how many watts (or amps) they draw when turned on. To determine how much energy a particular appliance uses each day, you need to multiply the wattage by the number of hours used each day. See the load evaluation sheet on page 15 for more information on electrical load calculations.

When wiring solar modules or batteries together in an renewable energy system, remember that connecting two of them in series (+ to -) doubles their voltage output, but keeps their amperage (or amp-hour capacity) the same. Connecting two of them in parallel (+ to +, - to -) doubles their amperage output (or amp-hour capacity), but keeps their voltage output the same. For example, most solar modules have a 12V nominal output so you would need to wire four of them in series (+ to -) to charge a 48V battery bank. The amperage output from these four solar modules in series is the same as that of a single solar module. Similarly, you would need to wire four 6V 350 amp-hour (AH) L-16 size batteries in series (+ to -) to configure them for 24V operation and then connect two strings of four batteries in parallel (+ to +, - to -) to obtain a 700 amp-hour capacity battery. See Appendix F for more information on battery wiring.

The discussion above of voltage and amperage leads to the subject of wire size. The amount of current that you can send through any electrical circuit depends on three things; the size or gauge (AWG) of the wire being used, the voltage of the system and the one way wire run distance. All wire (Cu and Al) has a listed resistance per 1000 feet with a larger gauge wire having a lower resistance value than a smaller one. The longer the distance and lower the voltage, the larger gauge wire you will need to use to minimize the voltage drop.

As a "rule of thumb", if your solar array consists of 4 or more, 60 watt or larger solar modules and is 50 feet or more away from the battery bank you should consider setting your system up at 24 or 48V instead of 12V. See the voltage drop tables in Appendix B at the back of the catalog for more information on wire sizing for 12, 24 or 48 VDC.