Price per stored kWh: the number that compares batteries fairly
Price per watt hour ignores cycles, aging and losses. How we calculate the cost of each kWh a battery delivers over its life, with a worked example.
Why price per Wh misleads
Two batteries with the same capacity and price can deliver very different amounts of energy over their life. One may be rated for twice as many cycles, another may lose more energy in conversion, and the usable capacity is always lower than the figure on the label. In its 2023 test of power stations, Stiftung Warentest measured at most 430 Wh from a model sold as 512 Wh, about 84 percent. That test is now old, but the gap it showed is typical.
Our formula
We divide the price by the energy the battery can realistically deliver: usable capacity in kWh, times the number of cycles you will actually use, times the remaining capacity at the end of life. The result is a conservative cost per stored kWh. It does not include the electricity you put in, only the cost of the battery itself.
A worked example with round numbers: a battery costs 1,000 euros, stores 1.92 kWh and is rated for 6,000 cycles down to 70 percent. On paper that is 1,000 / (1.92 × 6,000 × 0.7), about 12 cents per kWh. But a balcony battery may run about 250 cycles a year, and aging limits it to perhaps 12 years, so about 3,000 cycles. Then the result is about 25 cents per kWh, twice the paper figure.
Losses and standby
Every kWh that goes in comes out a little smaller, and the electronics draw power even when nothing happens. The HTW Berlin storage inspection of 2026 found standby consumption from 4 W in the best home battery to 64 W in the weakest. Over a year, 60 W of standby is more than 500 kWh. These figures come from home batteries, not balcony units, and no independent ranking of balcony batteries exists yet. Ask for the standby figure before you buy.