Home Battery Size and Payback

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Home battery: how big and does it pay off?
Example values, check them for your case. Many guides online are written by sellers, get several quotes.
Inputs
yearlyUse = 3500 kWh Part of the use that happens in the evening and at night (solar cannot cover it directly)
nightShare = 45 % Verbraucherzentrale (via 42watt.de): about 300 to 700 € per kWh including installation, falling trend
batteryPrice = 450 € / kWh
usableDepth = 90 %
roundTripEfficiency = 92 %
cyclesPerYear = 250 BDEW Strompreisanalyse 08/2026: household average 37.0 ct/kWh
electricityPrice = 0.37 € / kWh Assumption: check the current feed-in tariff (EEG) for your system
feedInTariff = 0.08 € / kWh LFP batteries last 15 to 20 years or 6000 to 10000 full cycles (reduco.ai, 42watt.de)
lifetime = 15 years Daily need that the battery should cover
dailyNight = yearlyUse * nightShare / (365 days) to kWh / day
batterySize = dailyNight * (1 day) / usableDepth to kWh
batteryCost = batterySize * batteryPrice to € What each stored kWh is worth: you avoid buying power instead of selling it cheaply
valuePerKwh = electricityPrice * roundTripEfficiency - feedInTariff
yearlyGain = batterySize * usableDepth * cyclesPerYear * valuePerKwh / (1 year)
paybackTime = batteryCost / yearlyGain to years
lifetimeGain = yearlyGain * lifetime - batteryCost Cross-check: Verbraucherzentrale rule of thumb, at most about 1.5 kWh per 1000 kWh of yearly use (via 42watt.de)
ruleOfThumbSize = yearlyUse * 1.5 / 1000