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If you're not familiar with how electricity works, you'll find it beneficial to read this section to gain an understanding of what the terms you'll encounter really indicate and how they apply to your alternative energy system, regardless of the source(s) of power. I have determined that learning comes much easier and is retained for a longer time if we understand the principles behind the calculations. In short, once you figure out how it works everything becomes much simpler. So, let's start with the two most basic electrical terms, which are voltage and current.
Voltage (E) is perhaps best understood as the equivalent of pressure. Picture the tire on an automobile. Typically, it is inflated to about 30psi (pounds per square inch). A bicycle tire is smaller, yet is often pumped up to 50psi or more. The wide, rear tire on an ordinary lawn tractor may only have an air pressure of 10psi. So, you can see that tire size does not necessarily correlate with pressure, just as the size of a storage battery is not an indicator of its internal voltage. There are 1.5-volt button-sized cells and D-sized flashlight batteries that are far larger, yet also supply 1.5 Volts. But, back to the pressure analogy and a hypothetical experiment. If you were to make an identical pinhole through the rubber on each of the tire sizes mentioned above and put them into a large tub filled with water, which of the three do you think would produce the greatest stream of air bubbles? It would be the bicycle tire, for it has the greatest amount of pressure to force the air out through the tiny hole. Now, which of the tires do you think would produce a stream of bubbles for the longest time? It would be the automobile tire, for it is the largest and thus contains the greatest volume of air of the three sizes. We're a bit ahead of ourselves here talking about capacity, but you'll soon see that this has a counterpart in an alternative energy system. For our purposes, Voltage will be expressed in Volts (V).
Current (I) may be viewed as the rate of flow in electrical circuits as well as in our tire analogy, above. Obviously, the pinhole in the bicycle tire has more airflow than the same-sized hole in the other tires that have less pressure. But, what if we were to take our low-pressure lawn tractor tire and instead make a nail-sized puncture in it. Which of the two tires would produce the greater amount of bubbles now? Well, assuming the diameter of the hole we made is more than 5 times the size of the pinhole in the bicycle tire, the tractor tire would now take the lead in escaping air, even though it contains less pressure. Were we to puncture the car tire with a nail, it would produce more bubbles for it has a greater air pressure than the tractor tire. Which tire will produce a stream of bubbles from a nail-sized hole for the longest time? That is a good question! Assuming the automobile tire at 30psi has more than three times the internal area as the lawn tractor tire at 10psi, we can assume it would win.
By this analogy, I hope you see that electrical current is a measure of the rate of flow of electrons. You can have a high voltage but only a small amount of current flow as represented by the bicycle tire with a pinhole, or a low voltage with a large amount of current flow such as in the case of a lawn tractor tire with a nail-sized hole in it. It is also possible to have both a high voltage and a high flow of current. This is what happens in an electrical short circuit in household wiring and it can be a dangerous situation. Just as a sudden blowout in a tire releases the pressure almost instantaneously in a great blast of air, the sudden rush of current at a significant level of voltage through a wire can generate intense heat that may start a fire. That is why it's so important to treat electricity with care. Current may be expressed in Amps, or milliamps (mA or 1/1,000th of an Amp) in alternative energy.
Power (P) is the product of voltage (in Volts) and current (in Amps) and represents the amount of "work" we can expect from a given circuit. Expressed in Watts, it is the most significant figure you will encounter in your research. For our purposes it may also be expressed in kilowatts (kW=1,000 Watts) or milliwatts (mW=1/1,000th of a Watt). There is a simple little drawing you should try to memorize that will allow you to calculate values. Picture it drawn on a piece of paper in the following manner. An uppercase P with a horizontal line directly beneath it. Right below that line and centered with the P are an uppercase I beside an E. The P stands for power (in Watts), the I represents current (in Amps) and the E stands for voltage (in Volts). If you know two of the values, you can determine the third simply by "covering over" the letter representing the unknown quantity. For example, P = I E (I times E). I = P over (divided by) E. E = P over (divided by) I. Why not jot this handy "cheat" on a piece of paper and play with it until it's familiar to you, because the "pie drawing" is a most handy thing to know. I learned it in high school electricity class and have used it ever since. Just remember to convert any value that may be a fraction of its full unit to the decimal equivalent beforehand. For example, 200mA would be 0.2 in the calculation.
Let me complicate this power thing a bit and advise you that when it comes to electrical consumption, there is something referred to as volt-amps (VA) which is more precisely the product of voltage and current. Watts as a form of power measurement works fine so long as the device consuming the energy is purely resistive in nature, such as the filament of an incandescent lamp. However, add some inductance or capacitance to the circuit such as found in a typical power supply or CFL lamp and suddenly there's a less-than-ideal power factor (pf) that amounts to phantom electricity which isn't reflected in the wattage rating. For this reason, use the (always higher) VA rating in appropriate situations when determining the load being presented to a DC to AC inverter, assuming your system uses one.
Resistance (R) in an electrical circuit is an important concept to understand because it explains a great deal of the "mystery" surrounding electricity. Every part of an electrical circuit presents some resistance to the flow of electrons. This can sometimes be useful, such as in heating the filament of a light bulb or the elements in a toaster. However, it's not such a good thing when it comes to the transmission of electricity, especially low voltage DC. Resistance in a circuit causes voltage loss and when you're talking about low voltages, this can become a significant concern. Using the leaking tire as an example, the size of the hole would be the equivalent of the amount of resistance to current flow in an electrical curcuit - the larger the hole, the less the resistance to the flow of electrons. If an electrical device we're using generates or consumes a lot of power, using thin wire for the connection would be the equivalent of a pinhole in the 10psi lawn tractor tire. In a typical 12-Volt system, several Volts might be lost in just a thirty foot length of cable. That is one reason why it's important to size the wire according to the load requirements. The other reason is that the lost electricity creates heat, and it's very possible to melt insulation or start a fire in extreme cases. Copper is one of the best conductors of electricity and that's why it is used in most electrical wiring. You may sometimes see aluminum used since it is less expensive, but because it does not conduct as well, a thicker wire is required for the same current flow.
Review and sample calculations
To review, when we speak of voltage we're essentially talking about the force that is pushing the electrons through a conductive material such as a piece of wire. When talking about current, we're referring to the amount or quantity of electron flow. Power is the product of voltage and current, literally Volts times Amps. Power is greatest when the voltage and current flow are both high. Reducing either voltage or current reduces the wattage and the reverse is also true. For a given situation (all other things remaining equal), increasing the voltage will cause the current flow to increase. Adding series resistance to a circuit decreases current flow. Resistance to the flow of electrons converts electrical energy to heat.
Here are some sample calculations for you to check out.
The electrical principles outlined above will apply to all alternative energy systems, be they constructed around solar, wind, water, or human-powered generation. In the interest of safety, please don't embark on the journey to "energy independence" until you understand basic electrical theory.
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