How big a battery do I need: sizing home storage sensibly

The size of a home battery is a balance between cost and benefit. Too small and it cannot cover the evening load; too large and it sits almost empty and slows the payback. Here is how to judge it sensibly instead of guessing.

Establish what you actually consume

The number you are after is not the total monthly consumption, but the energy that passes through the house between sunset and sunrise — when the sun is no longer producing. For a typical household that is usually a few kilowatt-hours per evening.

Separate the peaks

• The evening base load — fridge, router, lighting, standby devices.
• Evening appliances — cooking, water heater, washing machine, dishwasher.
• Peaks that can be moved — car charging, water pre-heating, irrigation.

A simple formula for a first estimate

Start from what you genuinely draw while there is no sun. If in the evening you pull an average of 6 kWh from the grid and you want to cover about 70% of it, the target usable capacity is roughly 4 kWh. Divide by the depth of discharge and by the efficiency factor to arrive at the nameplate rating on the label.

The reserve (MIN_SOC) people often forget

A battery is never drained to zero. HoumeOS protects a configured minimum (typically 20–30%) so you keep lights, internet and security when it matters. The capacity you can actually use is the band between a full charge and that reserve — so the number on the label is not what you have at your disposal.

Solar surplus changes the maths

If you produce during the day but do not consume, the battery has something to store and works at full pace. If almost all of the surplus is already absorbed by the water heater and the flexible appliances, the benefit of larger storage is small — the money would buy capacity that gathers dust.

What it costs and when it pays back

At roughly €0.23/kWh for bought electricity and €0.06/kWh for exported, the battery earns the difference on every kilowatt-hour it keeps at home instead of buying back in the evening. Annual savings depend on prices and habits, but the honest warning is that the exact payback is individual — we calculate it from real site data, not from a spreadsheet.

An honest conclusion

The better strategy is to first shrink the evening load with automation, and only then size the battery for what genuinely remains. That way you pay for capacity that is actually used.