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Economic consideration

photovoltaic boiler vs. classic electric boiler

approx. €250 savings per year
approx. 23% return per year
Short payback period of usually 3 to 5 years.
Reduce hot water costs by up to 100%

The photovoltaic boiler generates up to 100% of your hot water needs using photovoltaic electricity. Since the sun doesn't send you a bill, you can almost completely reduce your running costs for hot water heating. The following text describes a simplified economic analysis of photovoltaic boilers or heating elements.

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 kWh of electricity is required per day. This corresponds to around 50 liters of hot water per day or 1,095 kWh of electricity per year. With average electricity prices of €0.30/kWh, the annual hot water production costs are €328.50.

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 two standard photovoltaic modules, each with a maximum output of 400 Wp (0.8 kWp total photovoltaic output), are connected to a boiler, they will generate around 0.8 kWh of energy per hour on a sunny day. If the sun shines for 4 hours that day, the photovoltaic modules would produce 3.2 kWh of electricity (4x 0.8 kWh). The boiler's entire electricity requirements are covered for that day. You don't have to draw electricity from the grid and have reduced your electricity costs by 100%.

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 €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 photovoltaic hybrid boilers: €699
Costs of 2x photovoltaic modules each 400 Wp: €220
Cost cable: €50
Cost of substructure: €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 can now use free hot water. The return on your invested capital is 23%!

Additional savings: excess feed into the power grid

around €400 savings per year
approx. 24% return per year
Short payback period of usually 3 to 5 years.
approx. 580 kg CO2 savings per year

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 micro PV inverter.

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

If you want to reduce electricity costs even more, you can use four photovoltaic modules instead of two. 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 of 4x photovoltaic modules each 400 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: €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 on electricity costs every year. In many cases this is around 50% of the annual electricity costs. After about 4 years, your original investment costs will have been paid back and you can use the free hot water and electricity you generate yourself. The return on your invested capital is 24%! By the way, you also save around 590 kg of CO2 compared to electricity from the German electricity mix (2022).