How to Size Your Residential Solar Electric System

Determining how large a residential solar electric power system is required for your home is dependent upon your usage and your goals for the system. Being completely free from utility power may be impractical due to your location, the space required for solar panels and cost. Typically, homeowners target offsetting 40-50% of their average monthly electrical usage as a reasonable goal for their solar electric power system.

Home power usage varies based on the number and efficiency of your appliances, whether you heat or air condition with electricity, hot water source and your cooking and clothes drying power sources.

Your best source for information on your consumption is your electric bill. Your electric bill will have your usage in kilowatt hours. To determine your average monthly usage, gather the kilowatt usage from your last twelve bills and divide by twelve to get your average monthly usage. You may also be able to call your utility company for the information. Using a year’s worth of information enables you to factor in seasonal fluctuations. You will also want to know your peak usage requirements.

Cold northern locations usually find peak electrical usage during the winter months, when days are shorter and lights are used longer. Warm locations find peak usage is during the summer when air conditioning is used.

Once you know how much electricity you consume, talking with a solar installer can help narrow the options. A professional will know the specifics of available residential solar systems and can match your requirements with systems in the marketplace.

The output of your solar electric system depends on how far north you are and the weather patterns in your area. Maps such as these from the US Renewable Energy Laboratory can help you determine how much solar radiation you can expect for your location.

Weather and the number of daylight hours play the greatest role in how much power your solar electric system will generate. Solar panels produce electricity when struck by direct sunlight, regardless of air temperature. A clear, bright cold day is almost as good, as a summer day when the sun is overhead. In summer, the longer daylight hours enable your system to generate power longer. On cloudy days, your system will produce power – but not as much.

The number of solar panels required to reach your target system output will vary by manufacture and whether the panels are fixed or moving solar panels. Panels that move with the sun can be continuously angled to optimize direct sun exposure.

Typical solar panels are two to four feet wide and four to six feet long. Each panel will generate between 10 and 300 watts depending on the type and efficiency of the panel. One hundred watts is a reasonable number to use for rough estimates on the number of panels required for your application.