Calculate the usable and nominal battery capacity for a PV system from PV yield, demand and a target autarky.
This calculator gives a first battery-size estimate for covering a target share of annual demand with PV and storage.
Complete the inputs and calculate.
Derive usable and nominal battery capacity from PV, demand and target autarky.
PV yield, annual demand, target autarky, usable fraction and round-trip efficiency.
Target autarky is an annual share, not supply on every winter day. Usable fraction protects the battery from charge limits; efficiency and reserve make nominal capacity larger than shiftable energy.
Average daily balance estimates shiftable surplus. Usable fraction and efficiency convert it to nominal capacity.
A 10 kWh battery with 90% usable fraction provides about 9 kWh usable; seasonal generation can still miss the target autarky.
Sources and limits: Transparent annual-average method; use hourly simulation with power limits and reserve for investment decisions.
Derive usable and nominal battery capacity from PV, demand and target autarky.
Energy calculators balance quantities over time and separate useful energy, losses and grid import. This matters because power is instantaneous while energy is its time integral.
Average daily balance estimates shiftable surplus. Usable fraction and efficiency convert it to nominal capacity.
PV yield, annual demand, target autarky, usable fraction and round-trip efficiency. Target autarky is an annual share, not supply on every winter day. Usable fraction protects the battery from charge limits; efficiency and reserve make nominal capacity larger than shiftable energy.
Enter annual PV yield (or system data to estimate it), annual consumption, the target autarky, the battery's usable fraction and round-trip efficiency. The calculator returns the required usable and nominal storage capacity.
A 10 kWh battery with 90% usable fraction provides about 9 kWh usable; seasonal generation can still miss the target autarky.
Target autarky is an annual share, not supply on every winter day. Usable fraction protects the battery from charge limits; efficiency and reserve make nominal capacity larger than shiftable energy. An annual value describes a scenario, not a guarantee for every day. Efficiency, coincidence, temperature, driving profile and tariff assumptions can change practical results more than rounding.
Energy in kWh per year, capacity in kWh, shares and efficiency in percent.
A first rough sizing of battery storage capacity for a planned or existing PV system, or checking how much a higher target autarky increases required storage capacity.
Transparent annual-average method; use hourly simulation with power limits and reserve for investment decisions.
Common mistake: Don't choose target autarky independent of the ratio between PV yield and annual consumption – a battery can only shift PV surplus in time, not generate energy. If annual PV yield can't energetically cover the desired autarky at all, the calculator explicitly flags this instead of returning a misleadingly small capacity. Don't confuse usable fraction with nominal capacity; a 10 kWh nominal battery with 90% usable fraction delivers only about 9 kWh of usable energy.
If the desired target autarky would need more energy than annual PV yield minus direct consumption can ever supply as surplus, the target is unreachable with any battery size – this is flagged explicitly instead of showing a meaningless small capacity.
Usable fraction (typically 80 to 95%) and round-trip efficiency reduce the energy actually shiftable below the battery's nominal capacity.
The calculation uses an average annual balance; in winter, with lower PV yield and higher consumption, a battery can temporarily lack enough surplus to charge despite sufficient annual energy overall.
PV yield and demand are reduced to an average daily surplus. Usable fraction and round-trip efficiency convert that surplus into nominal capacity.
The average-day balance does not replace a seasonal simulation with battery power, reserve, temperature and charge limits.
Inputs, units and assumptions remain traceable in the result. Planning and approval require measured data, manufacturer documentation and a time-series simulation.