The classic electric boiler is usually the largest consumer of electrical energy in the household. It draws its energy for hot water preparation from the power grid. The amount of energy can vary depending on the user's hot water requirements. As a rule, around 3 kWh of electricity is required per day. This corresponds to around 50 litres of hot water per day or 1,095 kWh of electricity per year . With average electricity prices of €0.30/kWh, the annual cost of producing hot water is €328.50 .

The solar boiler draws its energy for hot water production primarily from the power of the sun. The photovoltaic modules connected directly to the boiler generate electricity, which is converted into hot water in the boiler via a heating element.

For example, if two solar modules with a maximum output of 400 Wp each (0.8 kWp total output) are connected to a boiler, they generate around 0.8 kWh of energy per hour on a sunny day. If the sun shines for 4 hours on that day, the photovoltaic modules would produce 3.2 kWh of electricity (4x 0.8 kWh). This covers the boiler's entire electricity requirement for that day. You do not need to get electricity from the grid and have reduced your electricity costs by 100% .

Since the sun does not shine every day of the year, the water usually has to be reheated on very cloudy days. This can be done via the 230V power grid.

Experience and simulations show that, depending on user behavior, the energy costs for hot water preparation with the photovoltaic boiler are reduced by around 75%. In other words, over the course of the year, around 75% of the energy required is provided free of charge by the photovoltaic modules. According to the previously calculated example, the financial savings of a photovoltaic boiler compared to the classic electric boiler are 246€ per year .

cost accounting:

A profitability calculation is then carried out to determine the payback period in years and the annual return.

Cost of solar hybrid boiler: €699
Costs of 2x photovoltaic modules each 400 Wp: €220
Cable costs: 50 €
Substructure costs: 100 €
Total cost: 1,069 €
Savings per year: €246
Payback period: 4.35 years

Payback period = investment / savings = €1,069 / €246 = 4.35 years

Return = savings * 100 / investment = €246 * 100 / €1,069 = 23%

The payback period in this example is 4.35 years. After this time, you will have saved your initial investment and you can now enjoy free hot water. The return on your invested capital is 23%!

On particularly sunny days , for example when the boiler is already fully heated up in the morning, you can feed excess electricity directly into your home power grid using the fothermo switching device in combination with a micro PV inverter.

In the example shown above, the boiler was fully heated after 4 hours of sunshine. With the surplus electricity feed-in, you can feed the photovoltaic electricity generated during the rest of the day into your home network. In this example, you would save another €50 in electricity costs per year.

If you want to reduce your electricity costs even further, you can use four photovoltaic modules instead of two. This gives you the advantage that the hot water is reliably heated even on very cloudy days and that you can feed more surplus electricity into your home network. The cost-effectiveness analysis then looks like this:

Cost of photovoltaic hybrid boiler: 699 €
Costs of 4x photovoltaic modules each 400 Wp: €440
Cable costs: 70 €
Substructure costs: 160 €
Cost of switching device 169€
Cost of Micro PV Inverter 149 €
Total cost: €1,687

Savings per year - Boiler: 246 €
Savings per year – surplus feed-in: 143 €
Total savings: €406
Payback period: 4.15 years

Payback period = investment / savings = €1,687 / €406 = 4.15 years

Return = savings * 100 / investment = 406 € * 100 / 1,687 € = 24%

In this example, you save €406 in electricity costs every year. In many cases, this is about 50% of the annual electricity costs. After about 4 years, your original investment costs are paid back and you can use your self-generated free hot water and electricity. The return on your invested capital is 24%! Incidentally, you also save about 590 kg of CO2 compared to electricity from the German electricity mix (2022).