The basic building block of a lead-acid battery is a 2-volt cell. A battery bank is a collection of connected 2-, 6-, or 12-volt batteries that supply power to the household in case of outages or low production from renewable energy sources. The batteries are wired together in series to produce 12-, 24-, or 48-volt strings. These strings are then connected together in parallel to make up the entire battery bank. The battery bank supplies DC power to an inverter, which produces AC power that can be used to run appliances. The decision to select a 12-, 24-, or 48-volt battery bank will be determined by the inverter’s input, the type of battery you select, and the amount of energy storage you require.
To determine the number of batteries you need, you must first determine how much energy storage you need in kilowatt-hours (kWh). If you are connected to the utility grid, you can use your monthly utility bill to calculate past energy usage for your household. (Keep in mind that implementing energy-efficiency measures in your home is a preliminary step to installing a solar electric system. Reducing energy consumption and installing energy-efficient appliances are far cheaper than purchasing larger solar electric systems.) A second way to determine your required kWh of energy storage is to multiply the wattage of your appliances by the number of hours you use them in a day. Because watts = amps x volts, if you require 1,000 watt-hours (or 1 kWh) per day, and if you have a 24-volt battery bank, then you need 42 amp-hours of useful storage. Because you cannot fully discharge lead-acid batteries, you would need to install a larger battery to get the needed 42 amp-hours of capacity.
Over the lifetime of the solar electric system, batteries will be the most expensive component of the renewable energy system in an off-grid home due to maintenance and replacement costs. Initial costs for residential batteries range from $80 to $200 per kWh.