Dunkelflaute, German for “dark doldrums”, describes the extended winter spells across Northern Europe when wind drops and cloud cover persists. For a home solar system, the effect is severe even without the calm. A 4 kWp array that produces 20 kWh or more on a June day may manage only 1 to 3 kWh on an overcast December day, while household demand is at its highest. Solar self-sufficiency in December and January is close to zero, and a battery sized to store summer surplus spends the winter largely idle. Our analysis of how NESO manages Dunkelflaute covers the effect on the national grid.
This changes the case for the battery. In winter, its value comes from tariff arbitrage, not from storing solar. On an off-peak tariff, the battery charges from the grid at around 7p/kWh overnight and supplies the home through the day and evening, replacing electricity that would otherwise cost 25p/kWh or more. After a round-trip efficiency of about 90%, each stored kWh still saves roughly 17p. See charging a home battery from the grid in winter for configuration details.
To size for winter, start with the home’s peak-rate consumption on a typical winter day, which is total daily use minus whatever already runs in the off-peak window. A gas-heated home may need 8 to 12 kWh. A home with a heat pump can need considerably more. Allow for usable rather than nameplate capacity, and check that the inverter’s charge rate can fill the battery within the off-peak window. A 13.5 kWh battery charging in four hours needs a charge rate of well over 3 kW.
Sized this way, the battery earns its keep through all twelve months. For homes without panels, the same reasoning applies, as our battery arbitrage guide explains.