For example, if your daily power consumption is 10kWh and you want a backup power option for three days, the total battery capacity required would be 30kWh. It is important to note that battery systems are not 100% efficient, so it is recommended to add a safety margin of around 20% to the calculated total battery capacity.
Divide your total power figure (number of Watts) by 10 to get Amps when run on 12V supply. So, if your total power figure is 300 Watts, to get Amps; 300W ÷ 10 = 30A from the battery How many hours will my Battery give me? The capacity of your battery will be given as a number of Amp hours (Ahr) with the most common sizes being 85Ahr and 115Ahr.
If you want enough power for 3 days, you'd need 30 x 3 = 90 kWh. As discussed in the post above, the power in batteries are rated at a standard temperature, the colder it is the less power they have. So, with batteries expected to be at 40 to supply 10 kWh, with this data you'd multiply by 1.3 to see you would need 13 kWh of batteries.
For example, let’s say your power consumption is moderate, at 71Ah per day. You effectively just need a battery that will provide your electrical devices for a few hours. An 80 Ah battery, for example, only has around 72Ah usable capacity for LiFePO4 batteries, and 40Ah for lead-acid batteries.
Here's how you can make sure you pick the right battery capacity: Calculate your daily energy use in watt-hours. Add up the power consumption of all devices you plan to use. Convert this figure into amp hours (Ah). This shows how much current a battery can deliver over time.
To calculate the total battery capacity required, multiply the daily power consumption by the desired autonomy days. For example, if your daily power consumption is 10kWh and you want a backup power option for three days, the total battery capacity required would be 30kWh.