Page last updated Oct 20, 2021 @ 10:42pm

Alternative Energy Sizing


System Sizing Information

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


Conservation plays an important role in keeping the cost of a photovoltaic system down. The use of energy efficient appliances and lighting as well as non-electric alternatives wherever possible can make solar electricity a cost competitive alternative to gasoline generators and in some cases, utility power.

Cooking, Heating & Cooling

Conventional electric cooking, space heating and water heating equipment use a prohibitive amount of electricity. Electric ranges Use 1500 watts or more per burner, so bottled propane or natural gas is a popular alternative to electricity for cooking. A microwave oven has about the same power draw, but since food cooks more quickly, the amount of kilowatt hours used may not be large. Propane and wood are better alternatives for space heating. Good passive solar design and proper insulation can reduce the need for heat. Evaporative cooling is a more reasonable load, and in locations with low humidity, the results are almost as good. One plus for cooling - the largest amount of solar energy is usually available when the temperature is the 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, an RV, or a boat, low voltage DC lighting is usually the best. DC wiring runs can be kept short allowing the use of fairly small gauge wire. Since an inverter is not required, the system cost is lower. When an inverter is part of the system, the house will not be dark if the inverter fails and the lights are powered directly by the battery.

In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of DC fluorescent lights, which have 3 to 4 times the light output per watt of power used compared with incandescent types. Halogen bulbs are 30% more efficient and actually seem almost twice as bright as similar wattage incandescents 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 large stand alone system with AC lighting, the user should have a back up inverter or a few low voltage DC lights in case the primary inverter fails. It is a good idea to have a DC powered light in the room where the inverter and batteries are in case there is a problem. 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 - 10 gallons of LP gas/month. If an electric refrigerator will be used in a stand-alone system, it should be a high efficiency type. SunFrost refrigerators use 300 - 400 watt hours of electricity/day while conventional AC refrigerators use 3000 - 4000 watt hours of electricity/day at a 70 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.

Major Appliances

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. These 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 500 watt-hours per load. If the appliance is used 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 press, etc.), their standard electric motor 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 on 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.

Small Appliances

Many small appliances such as irons, toasters 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.


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