Solar Electricity 101:
An Introduction & Overview of how Solar Electric Power Works

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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 wither 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 a 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.

Many water towers are physically shaped like a mushroom. Electrically speaking, batteries are mushroom shaped as well.  A tower designed to produce 50 p.s.i. for household pressure might be built like this (image to the right).

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 if 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 nee 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.  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.

A 1 watt load that is powered for one hour will consume 1 watt-hour.

(1W x 1hr = 1Wh)

A 100 watt load consumes one hundred watts of power per hour. A 100 watt load powered for two hours will consume 200 watt-hours.

(100W x 2hr = 200Wh)

A 100 watt load could consist of a 12 volt appliance drawing 8.3 amperes. 

(12v x 8.3amps = 99.6 watt)

A 100 watt load could also consist of a 120 volt appliance drawing .83 amperes. 

(120v x .83amps = 99.6 watt)

Another unit of measure that you will come across is the kilowatt.  A kilowatt is 1000 watts.  A kilowatt-hour could result from a 100 watt load being powered for 10 hours or a 1000 watt load being powered for 1 hour.

(100W x 10hr = 1000Wh or 1kWh)
(1000W x 1hr = 1000Wh or 1kWh)

NOTE:  the terms 110 volt, 117 volt and 120 volt, all refer to the same common household AC current.

The Basic Idea is Simple  Photovoltaic modules (solar panels) convert sunlight into electricity.  Wire conducts the electricity to batteries where it is stored until needed.  ON the way to the batteries, the electrical current passes through a controller (regulator) which will shut off the flow when the batteries become full.

For some appliances, electricity can be used directly from the batteries.  This is "direct current" and it powers "DC" appliances such as car headlights, flashlights, portable radios, etc.  To run most appliances found in the home, however, we need to use "alternating current" or "AC", the type which is found in wall sockets.  This we can produce utilizing an inverter which transforms DC electricity from the batteries into AC. The inverter's AC output powers the circuit breaker box and the common outlets in your home.

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