Solar Battery Storage Sizing Calculator

Start from the load you want covered: daily kWh, plus any EV charging and heat pump use, multiplied by the days of autonomy you want. Subtract the solar energy you use directly during the day, then divide by the depth of discharge to get nameplate capacity. A home using 10 kWh a day with decent solar typically lands between 6 and 10 kWh of battery; add an EV and the figure roughly doubles.

Depth of discharge (DoD) is the share of a battery's nameplate capacity you actually cycle. Lithium systems are commonly run at 80 to 90 percent; discharging deeper accelerates degradation and shortens warranty life. The practical effect is that a 10 kWh battery at 80 percent DoD gives you 8 usable kWh, so the calculator divides your storage need by the DoD to find the size to buy.

Only the share you want the battery to supply. A full EV charge is 40 to 80 kWh, far beyond domestic storage, so most owners charge the car directly from the grid on an off-peak window and size the battery for the house. A heat pump can add 10 to 15 kWh a day in winter. Enter realistic daily figures rather than worst-case ones, or the recommendation balloons.

It is the number of days of consumption the battery should cover without solar input. One day means storing enough to ride through the evening and overnight, which suits most grid-connected homes and businesses. Two or more days only makes sense for backup-critical or off-grid sites, and capacity scales linearly, so cost rises fast. The calculator multiplies your total daily load by this figure.

Only if the system is designed for it. Standard grid-tied inverters must shut down in a power cut to protect engineers working on the network, taking the battery offline with them. Backup requires an inverter with islanding capability and a changeover arrangement, usually covering a dedicated essential-loads circuit rather than the whole board. If outage cover is the goal, specify it explicitly: it changes the hardware, not just the capacity.