Common question #2: If I only have panels able to generate 1 kilowatt, how can I run my 1.2 kilowatt hair dryer?

Answer: The power you can draw does not depend on the power being generated by the panels at one instant. Rather it depends on how much power you have in your batteries and the size of your inverter. If you have a 4 kilowatt inverter and 16 kilowatt hours of battery storage, you can run 4000 watts of appliances (such as your washing machine, microwave oven and power saw) all at once. If you have a net metering PV system, then you can use solar electricity and utility power at the same time!

Common question #3: Can I run my electric dryer, hot water heater or heating system? Answer: Yes and No. If you wanted to buy a more than one inverter you can run these items with no problem. For houses, there is a more cost effective solution: use another energy source for these items. Electric resistance heating is the most expensive way to dry clothes, heat water or heat your house. It is much cheaper to either use gas or propane powered dryers and heaters or use a clothes line and solar hot water heater, if you would rather not burn fossil fuel. Only in suburban USA is electric resistance heating common. Everywhere else, gas, wood or solar heat is the norm.

Common question #4: Can I run my refrigerator, freezer and washing machine? Answer: Yes, absolutely. These appliances can take many watt-hours (a unit of energy) but only a few hundred watts (a unit of power). You can even buy high efficiency refrigerators/freezers and washing machine motors to minimize their energy use.

Photovoltaics

Photo is Greek for Light and Voltaic is after Allesandro Volta pioneer in the study of electricity. Thus photovoltaic means *light to electricity*. The photovoltaics (PV) convert sunlight directly to direct current (DC) electricity by use of a semiconductor. The most common type is made from the chemical element silicon. Silicon is one of the most common substances on Earth. For example, sand and glass are made of silicon dioxide. Our entire electronic age is built upon silicon. Almost all integrated circuits (chips) are made of silicon. What is a semiconductor? The essential miracle of a semiconductor p-n junction is that an electric field is maintained purely by virtue of the material. In other words, no external voltage need be applied. For a solar cell, this means that when light hits the cell, electrons are bumped from atoms and drawn to one side of the junction by the internal electric field. Current flows. You get electricity.

Silicon PV comes in several varieties. Shell PV panels are made with single-crystal silicon. The cells are cut from a single crystal of silicon. Single crystals give the highest efficiency of conversion of light to electricity, 15 to 17%. Solarex panels are made of polycrystalline silicon. These are close in efficiency but polycrystalline cells are easier to make because small crystals take less time to grow than big ones. Astropower PV panels are made of microcrystalline silicon, which again is easier to make. A very different type of cell is made of amorphous silicon, that is completely non-crystalline. Efficiencies of these cells is about 6%, much lower than crystalline. However, amorphous silicon is a thin film material which can be made in ultra thin ( micrometers) on thin backing. It can be made into flexible products such as the Unisolar PV shingles and PV Laminates.
There is one other type of PV which is common in the aerospace industry, gallium arsenide. These cells can be 30% efficient (some even higher) and are resistant to radiation damage (in the environment of low orbit). They are still very expensive and used only on spacecraft.

Because the solar cell is a solid state device, it can last for decades. The only part of a panel which can degrade is the material which encapsulates the relatively fragile cells. Some of these encapsulants will brown if subjected to multiplied sunlight (such as by a focusing mirror), or after many decades of regular exposure. The panels are not fragile. They are subjected to hail tests (1 inch hail at 50 miles per hour) at the factory. All indications are that solar panels installed today, will still be working perfectly in 30 years. Most panels have a 25 year warranty from the manufacturer.

Manufacturers of PV include: GE, Sharp, Shell, Kyocera, BP, Atlantis and Unisolar.

Charge controller

The charge controller is a solid state electronic device which measures the charge state of the batteries and turns the current from the PV on if the batteries need charging or off if they don’t. This protects the batteries from over-charging which can shorten their life. Charge controllers are made by several manufacturers, including Trace and Morningstar.

Batteries

Stand-alone solar electric systems use lead acid batteries to store power for cloudy weather or for nighttime. Lead acid batteries can be made to withstand deep-cycling, that is discharging them to near-empty. They do not have any *memory* like NiCds do. They can even be re-invigorated if they are abused, for example, sulfation can be reversed, more water can be added, etc. If charged properly, good lead acid batteries will last for years. The larger deep cycle batteries last 10 to 15 years.

Inverters

An inverter is another electronic device which takes the direct current from the batteries and changes it to alternating current (AC). Most households run on AC because AC wiring is cheaper and somewhat safer than DC wiring. Inverters are made to convert from 12,24, 48, 225, or 400 volts DC to 120 or 240 AC. The AC output of inverters is either a modified sine wave or true sine wave. Get a true-sine-wave inverter for whole-house systems and net metering systems. Inverters for net metering systems also have to meet the IEEE-929-2000 standard and be UL listed.

Wiring

Since solar electric systems are basically DC, they have wiring requirements different form AC systems.

To highlight some of these:

All disconnects and fuses have to be rated for DC

Large wires between the panels and controller are required to avoid voltage loss if the panels are far from the controller. For example, a 24 volt system of panels 150 feet from the controller will require wire 0.6 inches in diameter, costing $1.1/foot.

A large switch with fuses is required between the inverter and batteries to protect the inverter. Batteries are capable of putting out thousands of amps.

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