13 maggio 2026|Generale

Using an Immersion Heater with a Balcony Power Plant

Balcony power plants are easy to install and therefore becoming increasingly popular. But what should be done with surplus electricity – could an immersion heater be an alternative for domestic hot water heating?

Balcony power plants are usually small photovoltaic systems whose microinverters are allowed to deliver a maximum output of 800 watts (in Germany and Austria). Installation and operation are simple, the cost savings are easy to calculate, and the investment – compared to a larger PV system – is very manageable. The often complicated registration or approval requirements are eliminated, making balcony power plants an easy way to reduce electricity costs – even in apartments.

Using Electricity from the Balcony Power Plant Yourself

Unlike a grid-connected photovoltaic system with several kWp, balcony power plants are generally not eligible for feed-in tariffs from the grid operator. This means that any surplus electricity not consumed directly on-site is fed into the public grid without compensation – a nice bonus for electricity providers, but far less attractive for the owner.

In fact, the amount of energy effectively given away is considerable. In a typical two-person household with a 2 kWp balcony power plant – the maximum permitted module capacity – and an 800 W inverter, the system generates around 1,719 kWh per year. Without additional measures, as much as 1,031 kWh remains unused and flows into the public grid without compensation. Extrapolated to the more than one million registered balcony power plants in Germany alone, this represents an enormous amount of energy that is essentially being “gifted” to grid operators.

What Happens When All Appliances Are Supplied and Surplus Power Still Remains?

This is exactly where the untapped potential lies. As is common with larger PV systems, an immersion heater can heat domestic hot water during the day in order to make the best possible use of solar energy. A continuously variable power control ensures that every excess watt from the balcony power plant is directed specifically into the hot water tank instead of being wasted in the grid.

Calculations for a two-person household with a 120-liter hot water tank and an integrated 3 kW immersion heater clearly demonstrate the potential: with continuously variable surplus power control, around 972 kWh per year from the balcony power plant can be used for domestic hot water heating. This allows 47% of the annual hot water demand to be covered by solar energy, while grid feed-in drops from 1,031 kWh to only around 165 kWh – a reduction of approximately 84%.

Interestingly, a comparison with a 2 kWh battery storage system shows that a hot water tank can actually store more energy than a battery of this size. Combining battery storage with continuously variable hot water heating reduces grid feed-in to just 11 kWh per year – meaning that virtually the entire energy yield is consumed on-site. All details regarding the calculations can be found here.

Hot water with photovoltaics – even off-grid possible

Provided there is enough space available, there is an especially elegant alternative or addition to a traditional balcony power plant: off-grid domestic hot water heating using a continuously regulated solar-electric heating element. Instead of a balcony PV module with an integrated microinverter, several standard PV modules with MC4 connectors are connected directly to a DC power manager – without an inverter, without a grid connection, and without any approval requirement!

With the my-PV product SOL•THOR, this can easily be implemented. The DC power manager converts solar energy directly and with virtually no losses into heat by supplying a heating element directly with PV-generated DC power. The result: solar energy can be used entirely without permits, because the PV modules are not connected to the public electricity grid and are used solely for domestic hot water heating.

Just how efficient this solution is can be seen in calculations based on three PV modules – enough to cover 50% of the annual domestic hot water demand of a two-person household.

Two reference projects demonstrate just how well this works in practice

Townhouse in Nuremberg, Germany

How efficient an off-grid PV heating solution can be in real-life operation is demonstrated by a townhouse near Nuremberg. There, a 2.2 kWp PV system with just five modules has been supplying a 150-liter domestic hot water tank autonomously with solar energy since the beginning of 2025. The my-PV screw-in heating element is controlled via the SOL•THOR – without an inverter and without grid approval.

The estimated solar yield of 11 to 15 kWh per day during summer is more than sufficient to provide hot water for the two-person household. As a result, the oil-fired heating system is relieved on sunny days and heating oil is saved – because the heating system can remain switched off for several months.

Incidentally, the project was realized as a final thesis project by prospective mechanical engineering technicians at the Rudolf Diesel Technical School. The full reference project from Nuremberg can be found here.

To the Reference Description

Single-family home in Auckland, New Zealand

A pilot project in Auckland demonstrates the potential of a slightly larger off-grid system combined with the SOL•THOR. In this installation, the SOL•THOR converts the electricity generated by a 2.7 kWp PV system directly into hot water using an existing 3 kW heating element in a 180-liter electric boiler. The PV array consists of six Suntech modules rated at 440 W each.

The results after more than one year of operation are impressive: of the annual domestic hot water demand of 4,072 kWh, 2,627 kWh are supplied directly by solar energy – corresponding to a solar coverage rate of around 65%.

The payback period for the SOL•THOR is approximately three years. The full reference project from New Zealand can be found here.

Conclusion: Heat unlocks the full potential of a balcony PV system

Whether as a grid-connected solution with continuously variable surplus control via the AC•THOR or as an off-grid variant using the SOL•THOR, domestic hot water heating with photovoltaic electricity is the key to unlocking the full potential of a balcony PV system.

Instead of feeding valuable solar energy into the public grid without compensation, the energy is used exactly where it matters most: within your own home.

Balcony PV System

for Hot Water

Use the surplus solar power from your balcony PV system not only for household appliances, but also for domestic hot water heating. Discover how my-PV can help you increase self-consumption, use energy more efficiently, and reduce the amount of electricity fed into the grid without compensation.

Learn more now!

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