The size of the battery bank you need depends on several factors and is specific to your needs and to your home. A qualified energy installer should look at the following to determine what you need:
1) What kinds of loads you would like to run while the power is out
Your installer should look at which appliances, lighting, and other loads you want to power in an outage in order to determine how much energy they will use over the time period you want to keep them running. To size your battery system, the installer will add up the required number of watt-hours per electrical load over the desired backup period and the maximum number of watts you’ll need at any given time during a backup period. Bigger loads like electric stoves, electric water heaters and whole-house air conditioners may not be able to be backed up.
2) Suitable space in or outside your home to place batteries
Depending on the type, batteries may need to be located inside or outside. If located outside, and depending on batter chemistry, they may need to be placed in a shaded, temperate area. Your installer might need to adjust the size of your battery system to accommodate your available space.
3) How long the batteries are able to run without being re-charged
A battery system that operates your appliances and lights for one day would be smaller than a system that can operate the same equipment for two days without being re-charged. Your installer will guide you through how long you want to be able to run your appliances, but for most battery backup systems the standard length of run time is one day, especially if you have solar on site to re-charge your batteries.
4) Your budget
Because batteries can be expensive, most people size their systems to only power critical electrical loads while utility service is out. Your installer will help you decide which loads you want to power with your battery given your budgetary constraints.