Is an immersion heater worth it? The calculation – laid out honestly

As a simple guide: total investment divided by the annual economic benefit. The annual benefit is calculated from the usable surplus quantity multiplied by the difference between avoided heating costs and lost feed-in tariff. Ongoing additional costs or subsequent maintenance should be taken into account if they are relevant. It is also important to only include the surplus solar power that can actually be used for domestic hot water heating at that time. The calculation is therefore always an object-specific approximation and not a fixed product feature.

A low feed-in tariff lowers the opportunity cost of self-consumed solar power and can make self-consumption more economically attractive. Nevertheless, there must be a sufficient usable surplus, a suitable storage system, and an overall positive balance sheet. A low tariff alone does not make an immersion heater economical; the decisive factors are which heating costs are actually replaced and the total amount of the investment.

This can make sense. After the funding period ends, systems over 20 years old can continue to feed into the grid under current legislation; the feed-in remuneration is based on the annual solar market value minus a marketing fee. For 2025, the annual solar market value was 4.51 cents per kWh; the marketing fee for 2026 is 0.23 cents per kWh. The annual market value for 2026 will not be determined until after the end of the year. Whether additional self-consumption is economical therefore depends on your actual feed-in remuneration, the usable surplus, and the investment costs.

Probably not if there is little usable surplus, high installation or conversion costs, an unsuitable storage system, or if existing water heating is already very inexpensive and efficient. A long calculated payback period can also argue against the investment. In the case of district heating, heat pumps, or other low-cost heat sources, one should therefore calculate very carefully using their own tariffs and system values.

Use the actual feed-in tariff for your own PV system. It depends on factors such as the commissioning date, the system capacity, and the type of feed-in. For new building PV systems up to 10 kWp with partial feed-in, the fixed feed-in tariff for systems commissioned from August 1, 2026, is currently 7.70 cents per kWh; older systems and systems over 20 years old may have significantly different values.