November 2, 2022

Heating element combined with PV system Q&A part 2

my-PV GmbH

Electric immersion heaters are used in various areas – more on this in the first part of the "Heating Element Combined with PV System" series. Now it's all about the questions regarding practical use.

How long does it take for a heating element to heat up water?

This is a question that we hear quite often. It should be no surprise that the heating-up time depends on several factors. Nevertheless, we have written a short and conclusive answer:

The length of time it takes to heat the water within a storage tank using a heating element that is powered by photovoltaic is primarily dependant on the current solar radiation.

The length of time needed to bring the water within the storage tank to the desired temperature is also dependent on the hot water demand of the individual/family/commercial application and the amount of heated water that is being drawn from the storage tank while the heating element is working to bring the entire tank up to the desired temperature.

A third variable that should be taken into consideration when answering this question is the power of the heating element itself. When accounting for these three variables and assuming constant conditions, we are able to make the following theoretical calculation.

In theory, it works like this: 1.16 Wh of energy is required to heat 1 liter of water by 1 degree. For example, a tank with 300 liters of water needs around 17.4 kWh of energy to be heated from 10 °C to 60 °C. A 2 kW immersion heater like our ELWA needs about 8.5 hours for this. The stronger the immersion heater, the greater the rate of energy transfer, the faster the water is heated.

Good to know: For a more customized and year-round view of the system, we recommend a comparison with our my-PV Power Coach.


How many kW should an immersion heater have for my water storage tank?

How many kW your immersion heater actually needs depends on the size of your photovoltaic system and your water storage tank. A general guideline is to make the immersion heater as small as possible but as large as necessary. For systems below 6 kWp and a heat storage tank of around 300 liters, immersion heaters with an output of 3 kW are usually sufficient. For example our AC ELWA-E. For larger photovoltaic systems, we then recommend a 9 kW immersion heater, which can be linearly controlled by AC•THOR 9s. This must be connected in three phases and requires a different type of protection than a normal household circuit.


Can I retrofit my photovoltaic immersion heater?

If you have a photovoltaic system on the roof but are not yet using a PV immersion heater for your hot water preparation, then you should reconsider this fact. Retrofitting a hot water storage tank with a photovoltaic immersion heater is relatively straightforward: simply screw it in and connect the device to the signal source. The water in the tank can immediately be heated using excess solar power.

Retrofitting an existing standard immersion heater is just as easy. Our AC•THOR or AC•THOR 9s is perfect for this and turns an uncontrolled heating element into a photovoltaic-compatible heating element. The entire retrofit is then purely electrical, there is not even any intervention in the system hydraulics. The trades are also clearly separated.


Is my immersion heater calcifying? Do I have to descale?

Hard water can, of course, lead to calcification of every heating element, including the PV heating element. This happens in particular when the target temperatures are set above 60 °C. These lime scale deposits are nothing out of the ordinary, but something to look out for if you want to keep the performance and lifespan of your photovoltaic heater high.

Lime scale is a poor conductor of heat and thus also reduces the heat dissipation. The consequences are

  • less hot water
  • increased energy consumption
  • and higher costs.

It is therefore advisable to decalcify the water reservoir and the immersion heater regularly. When and how often depends on the hardness of the water. If the water is very hard, cleaning should take place once a year. If the water is rather soft, cleaning every three years may be sufficient. In any case, we recommend an annual check of the system to ensure that no damage is caused by lime scale.


How much heat does a water storage tank lose in 24 hours?

When there is an excess of energy, the storage system heats up, when there is less energy (e.g. overnight) it releases the heat again, as heat always dissipates to spread out evenly and keep equilibrium. This is physically understandable, but for many customers it is an unpleasant side effect. Therefore, the question of how much heat a water storage tank loses within 24 hours is quite justified.

Depending on the insulation and surface, a storage tank cools down by one to five degrees within a day. It is therefore important to protect the storage tank against heat loss. Regular hot water storage tanks already have the appropriate insulation integrated. You can recognize this from a certificate or from the fact that at room temperature, the outer surface of the tank is room temperature or only slightly warmer than room temperature.

A particularly large amount of heat is lost at the connections of the heat accumulator. A single connection that is often poorly insulated loses around 0.1 kilowatt hours of energy per day. These heat losses, albeit small, can to a large extent be avoided. The best thing to do is to ask your trusted installer how the connections, but also the entire water tank, can be insulated exceptionally well.


PV immersion heater or heat pump? What makes more sense for hot water?

Another question that is often asked is whether a heat pump for the preparation of hot water makes more sense than a heating element with photovoltaics. Here are a few considerations:

Heat pumps produce around 3 kWh of heat from one kilowatt hour (kWh) of electricity. This adds 2 kWh of environmental energy. This ratio is more or less constant and unchangeable.

With photovoltaic heat, the proportion of environmental energy results from the size of the PV system. With appropriate dimensioning, a PV heating element can easily achieve a better ratio than a heat pump. In addition, in times when there is too little solar yield, it is by no means absolutely necessary to bring in the residual heat by using expensive grid electricity. my-PV offers this option, but practically any other heat generator can also take over the post-heating, which means that, depending on the type of heating, the heat can be generated almost completely without mains power. Apart from that, a heating element is cheap to buy and also very low-maintenance. The technology is reliable and works for years without losses.

Which solution is better, depends on the circumstances. If the primary concern is to use excess electricity to heat water, the economic efficiency of a heating element is very high. And finally, an infinitely variable heating element can use the excess photovoltaic with absolute precision, regardless of whether it is 100 or 1000 watts. A heat pump cannot do that. It is in our opinion that the clear winner is the immersion heater with photovoltaics.

If you want to calculate what savings you can achieve using photovoltaic electricity and the my-PV products, then try out the my-PV Power-Coach. If you have any further questions, we look forward to answering them in one of our next posts or in our FAQ. Let us know on social media or send us an e-mail at This email address is being protected from spambots. You need JavaScript enabled to view it..


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We at my-PV get more out of photovoltaics than just electricity, we move your building technology into the future. Use your solar power yourself instead of feeding it into the public grid: Electricity, hot water, heating and charging options for your e-car. With our products, you can use your self-generated photovoltaic electricity for your own needs without any detours. This way, you save costs in the long term and act environmentally friendly!



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