How big should your solar battery be: calculator

How the calculator works

The calculator simulates a whole year, hour by hour: how much energy your system produces, how much the house uses in the same hour, how much goes into the battery and how much comes back to the house when there is no sun. It does this for every usable capacity from 0 to 20 kWh (more if consumption is high) and shows where each extra kWh stops adding much self-sufficiency.

How to use it

1. Enter your electricity consumption for one year, from your bill.
2. Enter the system’s power in kWp and the annual yield per kWp: for an existing system it is one year’s production divided by the kWp; for a planned system, the PVGIS estimate [4] for your address.
3. Choose the orientation: south, or east-west.
4. If you are considering a specific battery, enter its usable capacity. With the electricity price and the feed-in payment, the calculator also estimates how much your spending changes in a year.

How to read the result

• Self-consumption: the share of the energy produced that is not fed into the grid. It rises with the battery, because the midday surplus is used in the evening.
• Self-sufficiency: the share of the house’s consumption that does not come from the grid. This is the number that usually matters to anyone asking “how big?”.
• Where the curve flattens: up to the first capacity shown, each extra kWh adds at least 2 points of self-sufficiency; beyond the second, less than 1. These are thresholds we chose to read the curve, not advice: the full table shows every capacity and the gain from each one.
• Cycles in a year: the energy returned by the battery divided by its capacity. A bigger battery does fewer cycles, because in winter it does not fill up and in summer it only partly empties.

An example from the forums

In a 2023 discussion on energiesparhaus.at, the answers to the question “how big should the battery be?” ranged from 5 to 12 kWh, and some advised against buying one, without a calculation based on the asker’s data [12]. With 4,500 kWh of consumption and 6 kWp facing south open the example in the calculator: without a battery, self-sufficiency is about 37%, the curve flattens between 6 and 8 kWh, and with 8 kWh self-sufficiency reaches about 70%.

What it does not tell you

The calculation uses a standard load profile [1], not yours: a heat pump, an electric car or working from home can shift the result a lot. The battery returns 90% of the energy it takes in, a value within the range measured by HTW Berlin on systems on the market [6]; charging power and standby consumption are neither limited nor counted. We do not say whether the battery pays off: purchase price, lifetime, incentives and financing are left out. All sources and choices are in method and sources.