Sizing a Battery Bank

Properly sizing a battery bank is key to designing a reliable off-grid system. The principal goal when sizing a battery bank is to install a sufficient number of batteries to carry your household or business through periods when the wind or wind and sun (in hybrid systems) are not available.

Battery banks are typically sized to meet the need for electricity for three days. Longer reserve periods — five days or more — may be required for some areas. As noted in Chapter 3, backup fossil- fuel generators are often included in off-grid systems. Backup generators can reduce the size of the battery bank and are used to equalize the batteries. For more details on wiring and sizing battery banks for off-grid systems, you may want to check out Dan’s book, Power from the Wind. Because batteries are expensive, it’s a good idea to make your home as efficient as possible. This will reduce the size of your wind system and battery bank.

Wiring a Battery Bank

Batteries are wired by installers to produce a specific voltage and amp-hour storage capacity. Small renewable energy systems — for example, those used to power RVs, boats and cabins — are typically wired to produce 12-volt electricity. The electronics in these applications run entirely off 12-volt DC electricity. Systems in off-grid homes and businesses are typically wired to produce 24- or 48-volt DC electricity. The low-voltage DC electricity, however, is converted to AC electricity by the inverter. It also boosts the voltage to 120- and 240-volts, commonly used in homes and businesses.