Battery Calculator
Find out which battery configuration suits your situation best and calculate your payback time.
Calculating…
Why is the recommended battery not bigger if I have a lot of surplus?
The size of your battery is not set by how much sun you have left over, but by how much you use in the evening and at night. A battery only pays off if it is empty again every morning — whatever is still in it earned nothing that night.
That is why every extra kWh of battery returns less than the one before. The first module raises your self-consumption by roughly 20 percentage points, the second by 10, the third by barely 2. The price per module stays the same. Beyond a certain point every extra module only lengthens your payback time. We stop where the last module still pays for itself.
And why does my neighbour with lower consumption sometimes get a bigger battery than I do?
Because it is not your yearly consumption that counts, but when you consume. A household of 2,500 kWh with an electric car charging in the evening can draw 8 kWh from the battery every night. A household of 4,500 kWh that is mostly home during the day uses the sun directly and needs far less at night. The second household uses more, but needs a smaller battery — and pays it back faster.
What is this calculation based on?
Production. We use the VREG synthetic production profile for PV installations for 2026 (decision BESL-2025-142, annex 2), hour by hour across a full year. Yield: 950 kWh per kWp per year — a cautious value for a Belgian roof with mixed orientation.
Consumption. Your consumption is spread across the year following a household profile with a morning and evening peak, higher in winter. A heat pump is spread according to degree days (mostly December–February), an electric car according to a home-charging profile centred between 6 and 10 pm.
Prices. € 0.27 per kWh for electricity you take from the grid. That is the price you really save: energy, proportional grid use, levies and excise duties, including 6 % VAT. For electricity you feed in we count € 0.05 per kWh — the average injection tariff in Belgium in September 2026. The capacity tariff is € 54.23 per kW per year excl. VAT (Fluvius, 2026 tariff) and is calculated on the average of your twelve monthly peaks, with a minimum of 2.5 kW. We treat the price you enter as a variable price: energy, proportional grid use, levies and VAT, without fixed costs such as metering and without the capacity tariff. We count that last one separately, so no euro is counted twice.
Battery. Round-trip efficiency 88 %, 96 % of the capacity usable, charge and discharge power 2.4 kW per SolarFlow inverter. The battery optimises self-consumption: charge on surplus, discharge on shortfall.
What we leave out. Electricity price increases (every increase shortens the payback time), battery capacity loss over the years, interest, maintenance and any installation costs. An indicative estimate, not a quote.
Indicative estimate based on average profiles and 2026 tariffs. Your actual yield depends on your roof orientation, your consumption pattern, your energy contract and your grid operator. This is not a quote and not financial advice.