Most professional installers design standalone solar electric systems with about three to five days’ backup. That is to say, they size the battery bank so that it can supply your needs for three to five cloudy days. Generally, the sunnier climate, the fewer batteries you will need. That said, I think a three-to-five cloudy-day storage capacity is best for most climates. While creating a more substantial storage capacity may give homeowners greater peace of mind, few beginners realize that PV modules produce electricity in cloudy weather, although at a reduced rate. The system doesn’t go dead the minute a few clouds block the sun.
To determine how many batteries are needed to provide sufficient backup, you’ll need to run through some fairly complicated calculations that require computing your household’s average daily consumption of electricity in amp-hours.
Suppliers should be able to help you determine your needs, especially if they have worked with you to size your solar electric system. They’ll also help you determine how many amp-hours of electricity you’ll need to store in your battery system to ensure that you can achieve the desired number of days of autonomy.
Once you determine the number of amp-hours, all you have to do is to select a battery you like. Next, divide its storage capacity in amp-hours (minus its discharge limit—how deeply it can be discharged) into the total amp-hours of electricity you need to get by for three days. This tells you the number of batteries you’ll need in your system. For example, let’s say you need 3,500 amp-hours of electricity to provide three days of back-up power. The Surrette S460 batteries you are thinking about buying store 350 amp-hours of electricity each (at 6-volt DC). If they can be discharged by 80 percent, they effectively supply about 280 amp-hours each. To determine how many Surrette S460s you will need, simply divide 3,500 amp-hours by 280 amp-hours, which gives you 12.5 batteries.
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