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The two calculators on this site give estimates, not measurements. We have not measured batteries or heat pumps ourselves. This page says where each figure comes from, which choices we made and what is left out.

The load profile

We use the standard load profile for households, H25, from BDEW, the German energy industry association [1]. It gives the consumption for every quarter-hour over twelve months and three types of day (weekday, Saturday, Sunday and public holiday), for a consumption of 1,000,000 kWh in a year. It was derived from the data of 62 grid operators for 2018 and 2019 and already contains a small effect of photovoltaic self-consumption [2]. We transform it as follows:

• we apply the BDEW dynamisation function, which makes consumption vary over the year [2];
• we use the calendar of the chosen PVGIS weather year: national public holidays as Sundays, 24 and 31 December as Saturdays [3];
• we add up the four quarter-hours of each hour and rescale the year to the consumption you enter.

A standard profile is an average of many houses: yours may use more in the evening, less at midday, or have a heat pump and an electric car that the profile does not contain.

The PVGIS weather year

Production comes from PVGIS, the tool of the European Commission’s Joint Research Centre [4], with the data used under its conditions [5]. We downloaded the hourly series from 2005 to 2023 for Kassel, in the centre of Germany, for 1 kWp with system losses of 14%: modules facing south tilted at 35° and, for east-west, half the modules facing east and half facing west tilted at 15°. We use a real year, not an average day: a real year also contains the cloudy weeks in which the battery does not fill up, which an average day hides. The year is 2005, the one with production closest to the median of the 19 years: 981 kWh/kWp facing south and 811 kWh/kWp east-west. The calculator keeps the shape of that year and rescales it to the yield per kWp you enter.

The battery

Every hour, solar energy goes to the house first; the surplus charges the battery up to its usable capacity and the rest is fed into the grid; when the sun is not enough, the battery covers consumption as long as it has energy. Assumptions:

• the battery starts empty on the first of January;
• it returns 90% of the energy it takes in. In HTW Berlin’s laboratory measurements the average efficiency of the batteries alone ranged from 87.9% to 97.1%, charging from photovoltaics reached 98.2% and discharging to the house ranged from 91.2% to 97.8% [6]: 90% for the whole round trip is a value within these ranges, not a measurement;
• no power limit and no standby consumption: with a battery system that is small compared with the PV array, or with high standby consumption, the real result is lower;
• one-hour steps: within the hour, production and consumption offset each other, and this makes self-consumption look a little higher than a meter reading every minute would measure.

Where the curve flattens. For every capacity from 0 kWh upwards, in steps of 1 kWh, we calculate how many points of self-sufficiency the last kWh adds. We show the last capacity at which the gain is still at least 2 points per kWh and the last at which it is at least 1. The thresholds are our own choice for reading the curve, and we state them next to the result.

The heat pump

The house’s useful heat is the gas consumed times the boiler efficiency; the heat pump has to deliver the same heat, with a single annual performance factor (JAZ) for the whole year. The comparison uses the prices you enter and, if you enter them, the standing charges. As a guide to the JAZ we cite Fraunhofer ISE’s measurements in existing houses [7]. The values in the example are the BDEW averages for 2026: 37.0 ct/kWh for household electricity [8] and 11.93 ct/kWh for gas in single-family houses [9]. In the battery calculator, the example uses a feed-in payment of 7.70 ct/kWh, the rate for systems up to 10 kW commissioned between August and December 2026 [10].

What we do not calculate, on purpose

• Incentives, grants and subsidised loans, including those from KfW: they change often and some are loans.
• Financing, instalments, leasing and rental of PV systems and batteries.
• Tax: VAT, income tax, rules for those who sell energy.
• Return on investment and payback time: the battery calculator estimates how much your spending changes in a year at the rates you enter, not whether the purchase pays off.
• Installation and electrical work: sizing the heat pump, connections, grid connection and paperwork with the grid operator. These are done with a qualified installer.
• Battery safety and where to place it in the house.
• Any “worth it or not” judgement without your numbers.

Advertising

On some German-language pages, below the calculator, there are links to partner services: comparisons of electricity tariffs for heat pumps and quote requests to installers. They are labelled as advertising, kept separate from the result and do not change it: the calculation is the same with or without those links, and there is no partner form on our pages.