The classic electric boiler is usually the largest consumer of electrical energy in the household. It gets its energy for hot water preparation from the power grid. Depending on the hot water requirement, the amount of energy can vary depending on the user. Typically around 3.5 kWh of electricity is required per day. This corresponds to around 55 liters of hot water per day or 1,277 kWh of electricity per year. With average electricity prices of €0.31/kWh, the annual hot water production costs are €396.02.

The photovoltaic boiler draws its energy for producing hot water 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 three standard photovoltaic modules each with a maximum output of 430 Wp (1.29 kWp total photovoltaic output) are connected to a boiler, they will generate around 0.7 kWh of energy per hour on an average day with changing weather. After 5 hours, the photovoltaic modules have produced 3.5 kWh of electricity (5x 0.7 kWh). The boiler's entire electricity requirements are covered for that day. You don't have to draw electricity from the grid and you have reduced your electricity costs for heating water by 100% that day.
Since the sun doesn't shine every day of the year, the water has to be reheated on, for example, very cloudy days. This can be done via the power grid or, for example, via an existing gas, oil or pellet heating system.

Experience and simulations show that, depending on user behavior, the energy costs for hot water preparation are reduced by around 75% with the photovoltaic boiler. In other words: over 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 around €301 per year.

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

Cost of photovoltaic hybrid boiler: €699
Costs of 3x photovoltaic modules each 430 Wp: €330
Cost cable: €50
Cost of substructure: €120
Total cost: €1,199
Savings per year: €301
Payback period: 3.99 years

Payback period = investment / savings = €1,199 / €301 = 3.99 years

Return = savings * 100 / investment = €301 * 100 / €1,199 = 25.1%

The payback period in this example is 3.99 years. After this time, you will have saved your initial investment and can now use free hot water. The return on your invested capital is 25.1%!

The graphic opposite shows how often the photovoltaic hybrid boiler has to automatically reheat via the power grid in order to additionally heat the water when there is a high demand for hot water and at the same time relatively bad weather.

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

In the example shown above, the boiler was completely heated up after a few hours of sunshine. It could also be seen in the graphic that a relatively large amount of photovoltaic electricity could not be used in the summer months. With the excess electrical feed-in via the fothermo switching device, you can feed the photovoltaic electricity generated for the rest of the day into your home network at up to 800 W. In this example, you would reduce your electricity costs by around €104 per year.

If you want to reduce electricity costs even more, you can use four photovoltaic modules instead of three. This gives you the advantage that the hot water is heated reliably even on very cloudy days and that you can feed more excess electricity into your home network. The economic analysis then looks as follows:

Cost of photovoltaic hybrid boilers: €699
Costs for 4x photovoltaic modules each 430 Wp: €440
Cost cable: €70
Cost of substructure: €160
Cost of switching device 169€
Mirkro PV inverter costs €149
Total cost: €1,687

Savings per year - Boiler: €323
Savings per year - surplus feed-in: €104
Total savings: €427
Payback period: 3.95 years

Payback period = investment / savings = €1,687 / €426 = 3.95 years

In this example with excess feed-in into a micro PV inverter, you save €427 in electricity costs every year. That's €126 more per year than in the first example when using the photovoltaic boiler exclusively. After about 4 years, your original investment will be paid back and you can use the free hot water and electricity you generate yourself. The return on your invested capital is 25.3%. By the way, you also save around 590 kg of CO2, compared to electricity from the German electricity mix (2022).

Return = savings * 100 / investment = €426 * 100 / €1,687 = 25.3%

The graphic opposite shows the energy fed into and consumed in the company's own power grid (green) over the course of the year. It should be noted that the photovoltaic boiler is always supplied with energy first and only the surplus is fed into the power grid via an 800 W micro PV inverter.

Especially in the summer months, there is photovoltaic surplus, which can be used directly in your own home network. Since the photovoltaic yield is significantly lower in the winter months, there is hardly any excess feed-in during this time.

In the graphic opposite you can clearly see that due to the over-dimensioning of the photovoltaic modules, hardly any energy has to be used to reheat the water (blue) in the summer months. The photovoltaic modules almost completely cover the hot water requirement. In this specific example, 81% of the annual hot water requirement is covered by photovoltaics. That's about 16,200 liters of 65 degree water.