Conservation
Conservation plays an important role in keeping down the cost of a photovoltaic system. 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 winter heating. Evaporative cooling is a more reasonable load than air conditioning 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
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, a home 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 backup 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.
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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.
Refrigeration
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 70º 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 A C 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.
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