2. Calculate the power demand of the circuits you want to back up over the duration you want to back up

Once you've determined the circuits you want to back up and for how long, it's time to calculate the power demand this will entail. For example, if you want to back up your EV charger, your refrigerator, your air conditioner, and the lights and outlets in your home, for four hours each night in order to avoid peak demand charges, you'll need to determine the amount of power each appliance draws per hour.

In this case, let's assume the EV charger draws 1,000 watts per hour of use (watt hours, or, in this case, 1 kilowatt-hour), and is used for two hours each night to "top off" the battery charge. The A/C draws 3,000 watts per hour, the refrigerator 800 watts per hour, and the lights and outlets draw another approximately 1,000 watts per hour, all running continuously through the 4-hour period. This means the power demand is 5,800 watt-hours (5.8 kWh) for two hours and 4.8 kWh for two hours, for a total of 21.2 kWh of power demand each night. This demand can typically be met with two battery modules, such as the Tesla Powerwall. The exact time of usable backup will depend on user behavior, too - plugging in a few large devices and appliances and running them continuously will cause the batteries to drain faster, and being judicious with energy use will allow the batteries to back up for longer.

The same approach can be taken to determine the power demand to back up other circuits over other lengths of time, as well. Simply add up the approximate power demand of each circuit and appliance, and factor in the length of time they'll need to run (whether intermittently or continuously) and you'll be able to determine the number of battery modules necessary to achieve your goals.