The size of a solar electric system depends upon the amount of power that is required (in watts), the amount of time it is used (hours), and the amount of energy available from the sun in a particular geographic area (sun hours per day). The user has control over the first two variables, while the third depends upon the location.

Propane and wood are better alternatives for space heating. Good passive solar design can reduce the need for heating. Evaporative cooling is a more reasonable load, and in locations with low to moderate humidity, the results are almost as good. One plus for cooling is that the largest amount of solar energy is usually available when the temperature is highest.

Lighting requires the most study since so many options exist in type, size, voltage, and placement. The type of lighting that is best for one system may not be right for another.

The first decision is whether your lights will be run on low voltage direct current (DC) or conventional 110 volt alternating current (AC). In a small home, RV, or a boat, low voltage DC is usually best. DC wiring runs can be short, allowing use of fairly small gauge wire. Since an inverter is not required, the system cost is lower. If an inverter is part of the system, the house will not be dark if an inverter fails since the lights are powered directly off the battery. In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of low voltage fluorescent lights, whicdh have 3 to 4 times the light output per watt of power use compared with incandescent. Halogen bulbs are 30% more efficient and actually seem almost twice as bright as similar wattage incandescent because of the spectrum of light they produce. Twelve and 24 volt replacement ballasts are available to convert AC fluorescent lights to DC.

In a very large installation or one with many lights, the use of an inverter to supply AC power for conventional lighting is cost effective. In a larger stand alone syustem with AC lighting, the user should have a backup inverter of a few low voltage DC lights in caser the primary power fails. AC light dimmers will not function on AC power from inverters unless they have pure sine wave output. Small fluorescent lights may not turn on with some "load-demand start" type inverters.

Gas powered absorption refrigerators are a good choice in small systems if bottled gas is available. Modern absorption refrigerators consume 5 to 10 gallons of LP gas per month. If an electric refrigerator will be used in a stand alone system, it should be a high efficiency type. SunFrost refrigerators use 300 to 400 watt-hours of electricity per day while conventional AC refrigerators use 3000 to 4000 watt-hours of electricity per day at a 70 degree average air temperature. The higher cost of good quality DC refrigerators is made up many times over by savings in the number of solar modules and batteries required.

Standard AC electric motors in washing machines, larger shop machinery and tools, "swamp coolers," pumps, etc. (usually 1/4 to 3/4 horsepower) require a large inverter. Often a 2000 watt or larger inverter will be required. The inverter will get warm or hot when running these loads, which may shorten its life. The electric motors are sometimes hard to start on inverter power, they consume relatively large amounts of electricity, and they are very wasteful compared to high efficiency motors, which use 50% to 75% less electricity. A standard washing machine uses between 300 and 400 watt-hours per load. If you use the more than a few hours per week, it is often cheaper to pay more for a high efficiency appliance (if one exists) rather than make your electrical system larger to support a low-efficiency load. For many belt-driven loads (washers, drill-presses, etc.) their standard electric motors can often be easily replaced with a high-efficiency type. These motors are available in either AC or DC, and come as separate units or as motor-replacement kits.

Vacuum cleaners usually consume 600 to 1000 watts, depending upon how powerful they are, about twice what a washer uses, but most vacuum cleaners will operate on inverters larger than 1000 watts because they have low surge motors.

Many small appliances such as irons, toaster, and hair dryers consume a very large amount of power when they are used, but by their nature require very short or infrequent use periods, so if the system inverter and batteries are large enough, they may be usable.

Electronic equipment, like stereos, televisions, VCR's, and computers have a fairly small power draw. Many of these are available in low-voltage DC as well as conventional AC versions, and, in general, DC models use less power than their AC counterparts. A portable stereo "boom-box" that runs on 8 or 10 D-cell batteries will usually work on 12 volts DC. Some have a DC input, or you can connect wires from the battery contacts to the 12 volt system. This should be done by someone experienced in electronics repair.

Static inverters convert DC power stored in batteries to AC power that can be used as needed. Selecting the static power inverter that is needed for a stand alone power system requires the determination of:

• Maximum load
• Maximum surge
• Output voltage required
• Input battery voltage
• Optional features needed.

The size of an inverter is measured by it maximum continuous watts output. This rating must be larger than the total wattage of all of the AC loads you plan to run at one time. The size of the inverter can be determnined from a tag or label on the appliance, usually located near where the power cord enters, or from the owners manual. If the inverter is expected to run induction motors, like the ones found in automatic washers, dryers, dishwashers, and large power tools, it must be designed to surge, or deliver power many times its rating for short periods of time while these motors start.

This has one function, which is to change DC power to AC power that will be fed into the utility grid. A power system with this type of inverter uses the utility company as a storage battery. When the sun is shining, your electricity comes from the PV array, via the inverter. If the PV array is making more power than you are using, the excess is sold to the utility power company through a second electric meter. If you use more power than the PV array can supply, the utility company makes up the difference. This type of system makes the most sense if you have utility power, because there are no batteries to maintain or replace, but it has a very long payback period and may not be cost-effective at today's electric rates.

Another factor in the choice of an inverter is the output waveform. Inverters deliver power in one of three basic waveforms: square wave, modified square wave (sometimes called modified sine wave), and pure sine wave. Utility power is delivered as pure sine wave.

Lead-acid batteries are the most common in PV systems because their initial cost is lower and because they are readily available nearly everywhere in the world. There are many different sizes and designs of lead-acid batteries, but the most important designation is whether they are deep-cycle batteries or shallow cycle batteries.

Shallow-cycle batteries, like the type used as starting batteries in autos, are designed to supply a large amount of current for a short time, and stand a mild overcharge without losing electrolyte. Unfortunately, they cannot tolerate being deeply discharged. If they are repeatedly discharged more than 20%, their life will be very short. These batteries are not a good choice for a PV system.

Deep-cycle batteries are designed to be repeatedly discharged by as much as 80% of their capacity so they are a good choice for power systems. Even though they are designed to withstand deep cycling, these batteries will have a longer life if the cycles are shallower. Try to routinely stay above 50% capacity. All lead-acid batteries will fail prematurely if they are not recharged completely after each cycle. Letting a lead-acid battery stay in a discharged condition for many days at a time will cause sulfation of the positive plate and a permanent loss of capacity. An electronic desulfater can be added to a battery to minimize sulfation and greatly extend battery life.

Nickel-cadmium (ni-cad) batteries are nearly ideal for all types of power systems, but their initial cost is very high. New ni-cad batteries cost 10 times as much as similar capacity lead-acid batteries, and reconditioned ni-cads cost about 3 times as much as lead-acid batteries. Ni-cad batteries can last 10 times as long as lead-acid batteries and a smaller capacity battery can be used.

Since ni-cad batteries can be discharged 100%, and they can stay in a discharged condition for long periods of time without damage, a smaller battery can be used. The fact that their capacity does not get lower at low temperatures, and that their voltage is very stable from full charge to full discharge, even under large loads, makes a smaller battery a workable choice. Depending upon loads and weather conditions, a 30% to 50% smaller battery many be used.

Ni-cads can operate down to -10 degrees F with only a small reduction in capacity and they will not be damaged if they freeze. This allows them to be used in areas with severe temperature swings without having to oversize capacity and without providing a heated space.

Charging efficiency of ni-cad batteries is about the same as that of new lead-acid batteries, and it stays the same for the life of the battery. Their overall eficiency is higher, because they have an extremely slow self-discharge rate. if the power system is increased at a later time, additional ni-cad batteries of the same type can be added to the old bank without any problem, something that should never be done with lead-acid batteries.

Ni-cad batteries require slightly higher charging voltage than lead-acid batteries to bring them up to full charge. This may cause some difficulty with equipment designed for lead-acid batteries. Many AC power driven battery chargers will not go up to the 16.5 volts required to completely charge them. Solar modules on the other hand may be usabel without regulation, since their peak charging voltage is usually between 16 and 17 volts. This higher full charge voltage will cause most inverters to turn themselves off, so they may not work near the end of a charging cycle. Even though these batteries reach this voltage near full charge, the voltage drops immediately upon discharge, so the condition is only temporary.

Nickel-iron batteries are very similar to nickel-cadmium. They use an alkaline electrolyte similar to ni-cad batteries that allows them to withstand low temperatures. Their charge and discharge voltage is about the same, as is their long life. Many old nickel-iron batteries that were made 50 years ago in the U.S. are still working fine.

One major difference between nickel-iron and ni-cad batteries is discharge rate. Nickel-iron batteries cannot deliver the extremely high currents that ni-cads can, so if you plan tohave heavy loads, you will need to have a larger capacity battery bank.

One of the most interesting aspects of nickel-iron batteries is that they are made withour toxic lead or cadmium, which solves a future disposal problem.

The size of the battery bank required will depend upon:

• the storage capacity required
• the maximum discharge rate
• the minimum temperature that the batteries will be stored at (lead-acid only).

Temperature has a significant effect on lead-acid batteries. At 40 degrees F they will have 75% of rated capacity, and at 0 degrees F, that drops to 50% of rated capacity.