Next - AC verses DC
Electricity can be thought of as a flow of electrons through a conductor, generally wire. This flow is often compared to the flow of water through a pipe.
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 water through a pipeline at high speed requires high pressure. Pressure in water is measured in p.s.i., pounds per square inch. You can envision 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 do the work of knocking someone off their feet if they get in the way. Similarly, the "pressure" of electron flow is called voltage and is measured in volts. Generally speaking, the higher the voltage of an electrical current, the more force behind it.
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 spigot twice as big as another with the water in both at the same pressure, twice the amount of water will flow from the larger. The amount of flow in electricity is called amperage or "current" and is measured in amperes, or "amps" for short.
Taking our analogy further, a battery stores electricity much as a water tower stores water. The taller this tower, the higher the pressure present 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 load, electricity begins to flow. The higher the voltage of a battery bank, the greater the "pressure" of the electron flow. And just as with a tower of water, as electricity is drained from the battery, the pressure (voltage) slowly drops.
Most of the water available in such a tower is available from 45 to 60 p.s.i.. Once drained below 40 p.s.i., usage will rapidly deplete the supply at an ever decreasing pressure. 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. When drained below 12 volts, little amperage remains.
Just as a pump designed to fill such a tower would need to be able to produce at least 60 p.s.i. (that is, be able to lift 138 feet,) so does a solar PV module need to be able to produce at least 15 or 16 volts in order to charge a 12 volt battery.
Electrical power (the ability to do work) is a function of pressure (voltage) and amount (amperage). Double either one and you double the power the current is carrying through the circuit. The rule "VOLTS MULTIPLIED BY AMPERES EQUALS WATTS" defines this relationship. This is known as Ohm's Law. The watt is the measure of the power of electricity and will be our basic unit of measure for determining the size of our electrical loads.
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