Home Battery Size and Payback

Calculations

{"type":"doc","attrs":{"background":{"dim":35,"color":"#0f0e11","url":"https://images.unsplash.com/photo-1766507680004-1c71007aefe5?q=80&w=2340&auto=format&fit=crop&ixlib=rb-4.1.0&ixid=NHwxMjA3fDA%3D"}},"content":[{"type":"heading","attrs":{"level":1},"content":[{"type":"text","text":"Home battery: how big and does it pay off?"}]},{"type":"paragraph","content":[{"type":"text","text":"Example values, check them for your case. Many guides online are written by sellers, get several quotes."}]},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Inputs"}]},{"type":"numbox","attrs":{"code":"yearlyUse = 3500 kWh"}},{"type":"paragraph","content":[{"type":"text","text":"Part of the use that happens in the evening and at night (solar cannot cover it directly)"}]},{"type":"numbox","attrs":{"code":"nightShare = 45 %"}},{"type":"paragraph","content":[{"type":"text","text":"Verbraucherzentrale (via 42watt.de): about 300 to 700 € per kWh including installation, falling trend"}]},{"type":"numbox","attrs":{"code":"batteryPrice = 450 € / kWh\nusableDepth = 90 %\nroundTripEfficiency = 92 %\ncyclesPerYear = 250"}},{"type":"paragraph","content":[{"type":"text","text":"BDEW Strompreisanalyse 08/2026: household average 37.0 ct/kWh"}]},{"type":"numbox","attrs":{"code":"electricityPrice = 0.37 € / kWh"}},{"type":"paragraph","content":[{"type":"text","text":"Assumption: check the current feed-in tariff (EEG) for your system"}]},{"type":"numbox","attrs":{"code":"feedInTariff = 0.08 € / kWh"}},{"type":"paragraph","content":[{"type":"text","text":"LFP batteries last 15 to 20 years or 6000 to 10000 full cycles (reduco.ai, 42watt.de)"}]},{"type":"numbox","attrs":{"code":"lifetime = 15 years"}},{"type":"heading","attrs":{"level":2},"content":[{"type":"text","text":"Daily need that the battery should cover"}]},{"type":"numbox","attrs":{"code":"dailyNight = yearlyUse * nightShare / (365 days) to kWh / day\nbatterySize = dailyNight * (1 day) / usableDepth to kWh\nbatteryCost = batterySize * batteryPrice to €"}},{"type":"paragraph","content":[{"type":"text","text":"What each stored kWh is worth: you avoid buying power instead of selling it cheaply"}]},{"type":"numbox","attrs":{"code":"valuePerKwh = electricityPrice * roundTripEfficiency - feedInTariff\nyearlyGain = batterySize * usableDepth * cyclesPerYear * valuePerKwh / (1 year)\npaybackTime = batteryCost / yearlyGain to years\nlifetimeGain = yearlyGain * lifetime - batteryCost"}},{"type":"paragraph","content":[{"type":"text","text":"Cross-check: Verbraucherzentrale rule of thumb, at most about 1.5 kWh per 1000 kWh of yearly use (via 42watt.de)"}]},{"type":"numbox","attrs":{"code":"ruleOfThumbSize = yearlyUse * 1.5 / 1000"}},{"type":"paragraph"}]}