The best method that we've found is to convert both your power consumption and your power production into WATT HOURS that way we can compare apples to apples. Step one is to determine the individual wattage rating for each load that you intend to run off solar. Look on the back of each appliance and try to locate a label which indicates the wattage used by the appliance, if it doesn’t give you the wattage then it may tell you the amount of volts and amps that the appliance uses.

Remember volts times amps equal watts. Once you have written down the wattage rating for each appliance, you then need to determine the amount of time each appliance will run during the day. For example let's say that you have a television that runs for three and a half hours a day, then write down 3.5 hours, or let's say you have a computer that runs for two hours and 15 minutes, then write down 2.25 hours, or a microwave that runs for 45 minutes, then write down .75 hours.

Next take the wattage rating of each appliance and multiply that by the amount of time it will run, that will give us the WATT HOUR rating. For example a television that draws 200 watts and runs for three hours (200 x 3 = 600) will use 600 watt hours, or a toaster that draws 1100 watts and runs for 15 minutes (1100 x .25 = 275) will use a 275 watt hours. Add the up all of the watt-hour ratings for each appliance and that will equal your total power consumption for each day.

Let's say for example that you only had five hours of full sunlight then five hours times two hundred watts would only be 1000 watt hours so you would be at a deficit, so you would need to either add another 40 watts of solar panels or reduce your power consumption by 200 watt hours.

There are two cell technologies that are prevalent in today's market, they are referred to as polycrystalline and monocrystalline silicon. Some manufacturers will use one or the other technologies in the manufacture of their product some will use both.

Solar cells that are created from monocrystalline or (single crystal) technology are cut from a silicon boule that is grown from a single crystal, in other words a crystal that has grown in only one plane or (one direction). Single crystalline are more expensive to manufacture and typically have a slightly higher efficiency than do conventional polycrystalline cells resulting in smaller individual cells and thus typically a slightly smaller module.

Solar cells that are created from polycrystalline or (multicrystalline) technology are cut from a silicon boule that is grown from multifaceted crystalline material, or a crystal that grows in multiple directions. Conventional multicrystalline solar cells typically have a slightly lower efficiency resulting in larger individual cells and thus typically a slightly larger module. All of this has changed with the advent of the new silicon nitride multicrystalline cells which are rated as high or even higher efficiency than similarly sized monocrystalline cells.

It's important to keep in mind that a 100 watt module is a 100 watt module whether it was made from polycrystalline cells or monocrystalline cells.

Amorphous or thin film technology has for years been touted as the technology of the future that would offer a reduction in the cost of manufacturing solar modules. Many companies have entered and promptly exited the market with little success. Thin film or Amorphous technology has a lower efficiency rating so thus panels that are manufactured from this process tend to be substantially larger in size requiring a greater roof area for a typical installation. Questions concerning life expectancy remain unanswered.

One of the most misunderstood parts of a solar power system is the battery or battery bank, and that is where our class begins. Some solar battery banks use wet cells, like golf cart batteries, while others use sealed or gel cell batteries, and each have different temperature, mounting, and ventilation requirements.

Every battery is designed for a specific type of charge and discharge cycle. Car batteries have thin plates to keep their weight down and are designed for a heavy discharge lasting a few seconds, followed by a long period of slow re-charge. A 6-volt golf cart battery (size T-105) is the minimum battery I recommend for a residential solar application. You will need to buy these in "pairs" to make 12 volts. Golf cart batteries have very thick plates and are designed for hours of heavy discharge each day, followed by a fast recharge in only a few hours each night. This is similar to the duty cycle of a residential solar application, only in reverse. A solar battery must be able to provide long periods of deep discharge each evening and night, followed by a full recharge in only a few hours of sunlight each afternoon. Very few batteries can take a deep discharge-recharge cycle every day, and the 6-volt golf cart battery is the least expensive and easiest to find locally that can.

For some reason, everyone wants to use a sealed marine battery for their homegrown solar system. I strongly recommend that you do not. Included is a photo showing a sealed marine battery that "exploded" after being connected to a small solar charger for several months.

Even though this was a small 12-volt DC 5-amp solar charge controller powered from a single 50-watt solar photo-voltaic module, this was enough energy to gradually overcharge the battery and evaporate all of the electrolyte even though this battery was "sealed." A low electrolyte level can expose the plates which will gradually warp or "grow" in thickness as they oxidize. This can cause an internal short circuit and ignition of the hydrogen gas.

Plate damage can also occur when there is a large buildup of sediment after the upper plate areas become exposed from reduced water levels and begin to "flake" off. Most liquid acid batteries do not vent gasses while discharging. However, near the end of a typical charging cycle, when the battery is almost "full," the sulfuric acid and water electrolyte will begin to break down into hydrogen and oxygen—a very explosive combination.

When ignited by a nearby spark or flame, an "explosion" can result, but this flash lasts only a fraction of a second, which is usually too fast to ignite nearby walls. However, this is still a very explosive reaction, with plastic battery parts becoming acid-covered shrapnel. While using a hand grinder one day in a shop, I accidentally directed the sparks towards several car batteries being charged about 30 feet away. There was a very loud explosive sound with acid and plastic hitting every wall of the large shop, yet I did not see a flame and there was no fire. Regardless, it was not a pleasant experience.

Always wear eye protection and acid proof gloves when working around batteries, and have lots of water and baking soda nearby. This will neutralize any acid spills from battery refilling and prevent further corrosive damage.

A typical 6-volt golf cart battery will store about 1 kilowatt-hour of useful energy (6 volt X 220 amp-hr X 80% discharge = 1056 watt-hours). Since this would only power two 50-watt incandescent lamps for 10 hours (2 X 50 X 10 = 1000 watt-hours), your alternative energy system will most likely require wiring several batteries together to create a battery bank. Since each golf cart battery weighs almost 65 pounds, there are weight considerations as well as battery gas venting issues to think about.

An area of a garage or storage building having a concrete floor is the most common location for a battery bank, although some large systems have their own specially designed battery room. I am going to assume you are installing a much smaller system and will only require four to eight batteries.

If you need more than the 220 amp-hr capacity contained in each golf cart battery, I suggest switching to the larger "L-16" size traction battery, having a 350 amp-hour rating, which may allow using fewer batteries. This battery is the same length and width as a golf cart battery, but is much taller and twice as heavy. This is an excellent battery for solar applications and can take very heavy charge-discharge cycling. This industrial rated battery may be more difficult to find, as it is only available from battery wholesale distributors.

Batteries can lose over half of their charge when exposed to extreme temperature swings, so be sure your proposed battery location stays in a 50° to 80° F range, or you will need to insulate the battery box. Since liquid batteries require refilling and battery terminal cleaning to remove corrosion several times each year, the floor area selected should be able to take an occasional acid spill and water wash down.

Battery venting is very important as discussed earlier, and if you build an enclosure around your batteries, it should be designed to direct all vented gasses to the outside. A 2-inch PVC pipe makes a good vent, but be sure it is located at the highest point in your battery enclosure where the lighter hydrogen gas will accumulate. Be sure it also includes a screened vent cap to keep out rain and insects. Do not locate your battery bank near a gas water heater or other open flame appliance that could ignite any accidental hydrogen accumulation.

A battery box can be built using standard 2 x 4 framing construction, with pressure treated plywood lining the interior surfaces. A hinged top door is needed for periodic battery maintenance, and should include a gasket to prevent gases from entering the room. Note how the top of the site-built battery box shown in these photos slopes up to a high rear area where two PVC vent pipes are located. The interior plywood surfaces of this wood frame construction were painted with several coats of fire and acid resistant paint. Since batteries lose capacity with lower temperatures, your batteries should not rest directly on a cold un-insulated concrete floor.

Pressure treated 2 x 4s on edge, spaced every 6 inches and covered by a fiberglass laminated concrete board, makes an excellent base for your battery box. This heavy sheet material is sold in most building supply outlets as a backing behind ceramic tile work in wet shower stalls, and is usually available in smaller 2-foot by 4-foot sizes. By careful planning, you may be able to use the entire sheet without cutting or splicing.

If you can afford to invest in the more expensive gel or absorbed glass matte (AGM) batteries, you will have more flexibility in locating your battery bank, since these batteries do not need to be refilled and do not normally generate explosive gasses. The photo shows a large battery bank with the batteries mounted close together in a vertical steel rack. You do not need a vapor proof enclosure or vent pipe when using these batteries, however they cost almost 30 percent more without providing any additional life or storage capacity.