Off-Grid Battery Bank Calculator

A battery bank has to store enough energy to carry your loads through the hours the sun is not charging it — overnight, and through cloudy stretches. The sizing starts from your daily load in watt-hours and multiplies it by the days of autonomy you want, which is how many full days the bank should run with no charging at all. One day is a light buffer; two to three days is the common off-grid target; more is prudent in cloudy climates.

That product is the usable energy the bank must deliver. Real batteries should not be drained flat, so the calculator divides by the usable depth of discharge for your chemistry to get the gross capacity you actually have to buy. Flooded and AGM lead-acid banks are planned around `50%` depth of discharge to protect cycle life; lithium (LiFePO4) is commonly planned at `80%`. Finally, dividing gross watt-hours by your system voltage — `12`, `24`, or `48 V` — converts the answer into amp-hours, the number printed on a battery.

The output is a planning figure. Cold temperatures reduce usable capacity, inverter standby draws a little around the clock, and every extra day of autonomy is more battery to pay for, so round to real battery blocks and lean conservative.