Home / fothermo News / Reduce 75% of the cost of the electric water heater.
75 % der Kosten des elektrischen Durchlauferhitzers senken.

Reduce 75% of the cost of the electric water heater.

The very high costs of producing hot water using an electric instantaneous water heater can be reduced by around 75% per year with the photovoltaic off-grid boiler. The photovoltaic boiler is used to preheat water for the instantaneous water heater. The water heater only starts heating when the hot water from the photovoltaic boiler has been used up. This will save you around 600 kWh to 900 kWh per year or €200 to €300.

The structure

The photovoltaic off-grid boiler is installed directly in front of the instantaneous water heater. The boiler preheats the cold water to 65 °C using electricity from the photovoltaic modules. As soon as hot water is needed, the already hot water from the photovoltaic boiler flows into the instantaneous water heater.

Since the water is already hot, the instantaneous water heater is not used, does not use any electricity and therefore does not incur any costs. Only when the boiler's hot water has been used up does the instantaneous water heater switch on automatically and generate hot water using electricity from the socket.

In this way, around 70% to 80% of a household's hot water costs can be saved.

Below is a graphic showing the structure of the system.

What is the optimal boiler size? 10 L, 30 L or 80 L photovoltaic off-grid boiler:

The recommended boiler size differs depending on the application. Around 10 liters to 30 liters of 65-degree hot water are required per shower:

- 10 liter boiler: Use in camping. Or for individuals who shower very sparingly.
- 30 liter boiler: For 1-person households.
- 80 liter boiler: For 2-person households and more.

How much hot water can I generate per year using the photovoltaic modules?

An 80 liter photovoltaic boiler in combination with two photovoltaic modules in an economical two-person household uses around 600 kWh of electricity from the photovoltaic modules per year.

This corresponds to 10,000 liters of 65 degree hot water. Or about 25,000 liters of 34 degree shower water mixed with cold water.

At an electricity price of 30 cents/kWh, this corresponds to an annual saving of around €200. In a multi-person household, correspondingly more.

What happens when the photovoltaic boiler is fully heated in summer?

The answer: Either the boiler simply switches off, or you can simply feed excess energy into your power grid via a micro PV inverter.

This will save you additional electricity costs. So you can use all of your electricity yourself. No electricity is lost. In this article you will find more about excess feed-in in a micro PV inverter with the fothermo switching device.


Save around 600 kWh of electricity per year

A large part of the energy comes from the photovoltaic modules. The remaining energy comes from the instantaneous water heater.

Reduce electricity costs by around €200 to €300 per year

You also become less dependent on rising electricity prices. You produce your own hot water.

3 - 5 years payback period

After a few years, your investment will have been paid off and you can produce your own free hot water.

Why is the boiler called an off-grid boiler?

Offgrid can be translated as “far from the grid”. The area of ​​application for the off-grid boiler is wherever I either have no power grid (e.g. mountain/garden hut or camper) or in applications where the water does not have to be reheated directly via the photovoltaic boiler.

This is the case when the photovoltaic boiler is combined with an instantaneous water heater or a gas boiler. In case of doubt, these take over the additional heating.

80 liter photovoltaic off-grid boiler

  • 550W heating power

    550 W photovoltaic heating power

  • easy installation

    No electrician is necessary for installation as the entire system works in safety extra-low voltage.

  • Registration free - island complex

    Since no electricity is fed into the grid, the photovoltaic system does not need to be registered.

  • 200 liters of 34 degree shower water

    At a maximum boiler temperature of 65 °C, you can mix around 200 liters of 34 degree shower water.