The Simple Math of Electrical Solar Power Systems

If you're evaluating solar power systems, here is a quick guide to help you calculate your energy useage and the number of solar modules and storage batteries needed to meet it.

| October/November 1998

Solar power systems come in a variety of form factors and a range of electricity generating capacities.  We've compiled an Energy Needs Table of Facts to help you determine the size of system you need for the amount of power you typically use. Also, bear in mind the hours of sunlight per day available to your system will vary depending on location. In North America, the further south you live the more hours of sunlight you'll have and the more electricity you can potentially generate.

Energy Used and Lost

In addition to the electricity used by appliances, the system itself also consumes some power. For example: you do not get back all the energy you put into a battery. Each day a battery is left unused (and without any additional charge), it can lose 1% to 2% of its charge, and even a properly charged battery is still not 100% efficient. An inverter also uses some energy to convert electricity from DC to AC, and current is lost to electrical resistance as electricity travels through wires. To be on the safe side, system design should assume 30% DC waste and 40% AC waste.

Solar modules should be installed at the correct 'tilt-angle' to achieve the best year-round performance. Generally, this is an angle equal to the site's latitude plus 20%, with modules facing south in the northern latitudes and north in the southern latitudes.

Battery Storage

Batteries are a major component in solar systems. A number of different types and capacities are available. Many small to medium sized systems can use photovoltaic or marine grade batteries. These are designed to be deep-cycled (discharged and recharged) many times and are generally maintenance free. They are available in capacities of about 120 Amp-hrs.

Batteries must be able to store enough energy for daily operations. A reserve should be considered so you will have additional capacity to operate the loads during anticipated periods of cloudy, sunless weather. This reserve capacity is referred to as system 'autonomy' and is rated in clays. The amount of autonomy needed varies. For critical loads such as telecommunications, you may want 10 or more days of autonomy, for a residence perhaps five days, and only a day or two for a vacation cabin.

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