Electricity Basics

In this analogy, if you wish to have increased flow through the pipeline, you will need either a bigger pipe or you will have to push the water (or electricity) through at a more rapid rate. To push the water through a pipeline at high speeds requires high pressure. Pressure in water is measured in psi (pounds per square inch). You can imagine water under high pressure squirting out very rapidly from a nozzle, such as a fire hose, with enough speed and force (power) to carry it to great heights or to the work of knocking someone off their feet if they get in the way. Similarly, the "pressure" of electrons flowing is called voltage and is measured in volts (V). Generally speaking, the higher the voltage of an electrical current, the more force behind it.

Amperage is Like Volume of Water Flowing Through a Pipe

The amount of flow at a given pressure is determined by the size of the cross-section of the pipe. If you were to open a water hose twice as big as another with the water in both at the same pressure, you will get twice as much water flowing out of the larger one. The amount of flow of electricity is called amperage or "current" and is measured in amperes, or "amps"(A) for short.

Taking the water analogy further, a battery stores electricity much as a water tower stores water. The taller this tower, the higher the pressure of the water is at its base. If you open a valve at the base, water will flow out at a high pressure. In the same way, if you flip a switch connecting batteries to a light bulb or some other load, electricity begins to flow. The higher the voltage of a battery bank, the greater the "pressure" of the electrons flowing the wire. And just as with the water tower, as electricity is drained from the battery, the pressure (voltage) slowly drops.

Most of the water available in a typical water tower is available at a pressure of 45 to 60 psi. Once drained to below 40 psi any additional drain will cause the pressure to decrease even more rapidly. This is because the majority of the water is stored up in the huge tank at the top of the water tower. The lower pressure occurs when the water has been all the way down to the base of that large tank. In the same way, a nominal 12-volt battery has most of its stored electricity available from just below 12 volts to 12.6 volts. Once the battery is drained below 12 volts, there's little amperage that remains (similar to how there's little water left in the water tower once the tank has been drained to just the supply tubes at the base of the tank).

Just as a pump designed to fill a tower that provides 45 to 60 psi of pressures would need to be able to produce a little more than 60 psi (requires that the pump lifts the water 138 feet), so does a solar electric panel (PV module) need to be able to produce at least 15 to 16 volts in order to charge a 12 volt battery.