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Electric instantaneous water heater with PV power?

Electric instantaneous water heaters are generally considered energy efficient. The reason for this is that the heat is only generated at the moment the water is consumed and no lossy heat storage is required. But is this really the case?

Unfortunately, if we consider the high power required at the moment of use, the technology of the electric instantaneous water heater does not match at all with photovoltaic systems. These produce energy during the day, when little hot water is usually needed. Even if the fluctuating production and the hot water demand coincide in time, there is usually not enough power available to produce the heat without drawing from the power grid. In other words, a photovoltaic system does not necessarily produce all the power needed to operate the electric instantaneous water heater when it is needed.

Does an electric instantaneous water heater make energy sense?

A small calculation example: If you take an average shower five times a week, you will use around 375 liters of water - if we assume a consumption of 75 liters per showering session. 18,000 liters of water per year that has to be heated.

Unlike heating elements with hot water storage tanks, which heat a large amount of water and keep it warm, an instantaneous water heater heats the water needed directly when you open the hot water tap. Quick hot water at the "push of a button"? That means high electricity consumption.

Older instantaneous water heaters, where the water temperature is controlled hydraulically in several stages, are of course different from modern devices, which can be adjusted quite precisely and work far more efficiently. Nevertheless, at the time of their use, instantaneous water heaters require a lot of electricity. 


Advantages and disadvantages of the electric instantaneous water heater

However, electric instantaneous water heaters are by no means simply bad. They have advantages and disadvantages, just like other devices.

Advantages of an electric instantaneous water heater:

  • No heat loss: Since instantaneous water heaters do not have a hot water storage tank, there can be no energy loss when it is not in use. Moreover, since these heaters are usually installed very close to the point of use, hardly any heat is lost through the pipe.

  • Unlimited hot water: Instantaneous water heaters provide unlimited hot water because it is heated as it passes through.

  • Compact design: Since instantaneous water heaters do not require a storage tank, they are small and compact and can be integrated almost anywhere.

  • No Legionella problem: Legionella (bacteria that can cause various diseases) likes warm, stagnant water. This is a problem that can occur with hot water tanks, but not with electric instantaneous water heaters.

  • Low initial cost: An instantaneous water heater is usually quick, cheap and easy to install.

Disadvantages of an electric instantaneous water heater:

  • High electricity costs: Heating water quickly and effectively using electricity incurs high costs.

  • High primary energy consumption: Power plants have varying degrees of efficiency when producing electricity, which means that some of the energy cannot be put to use. Further energy is lost on its way through the power grid. This is certainly true not only for instantaneous water heaters, but for all electrical appliances that require a power supply.

  • No cheaper night-time electricity: Due to a lack of storage, instantaneous water heaters cannot use cheaper night-time electricity.

  • Poor controllability: Hydraulically controlled instantaneous water heaters produce hot water only when there is sufficient water pressure. So it can happen that it suddenly gets cold while showering, because hot water is drawn from another place, e.g. the tap in the kitchen.

If you save water, you save electricity - but do you have to?

One thing is clear: the less hot water we need, the less has to be heated by the electric instantaneous water heater and the less energy is consumed. So the common rules for using less hot water also help save electricity. 

  • Do not let water run unused

  • Take a shower instead of a bath

  • Use water-saving shower heads

  • Avoid pre-rinsing dishes by hand if you use a dishwasher

Saving water is certainly always a good idea, because it is a precious commodity. But electricity? Do we really need to save electricity?


You can save electricity, but you are not forced to

Again and again, we are confronted with the general attitude that we - as a society - must use electricity or energy sparingly. We should act in a sustainable and resource-saving way.

However, we at my-PV are happy to point out that this claim is only correct if we are not talking about photovoltaic power. After all, solar energy is almost always available to us and will not "run out", at least during our lifetime. Professor Timo Leukefeld, who gave the keynote speech at the opening of our new company building, also advocates an intelligently wasteful use of solar energy. >>

Click here for the presentation by Timo Leukefeld (in German). 

Those who base their energy supply on solar power and maximize their own consumption no longer have to worry about saving electricity. Nevertheless, electric instantaneous water heaters are a suboptimal solution in the solar electric powered household.  


Electric instantaneous water heater or storage heater?

If the house's own electricity is produced by means of a photovoltaic system, a storage tank for heat is required, which - similar to a battery - stores the energy of the day's production. A very simple hot water boiler is sufficient for this purpose. If this is heated according to the photovoltaic production by a controllable electric heating element, photovoltaically heated hot water is also available in the morning and evening, i.e. exactly when it is usually needed. Consumption peaks are cushioned by the energy content of the hot water storage tank.

Of course, this concept means that heat losses occur across the surface of the storage tank. However, these are in no relation to the non-PV-covered electricity demand that electric instantaneous water heaters bring with them at the time of their use.


Cost comparison electric instantaneous water heater vs. hot water tank

For a typical single-family home with hot water needs for 4 people, a 400 liter boiler, an annual electricity consumption of 4,000 kWh and a 5 kWp photovoltaic system, the following figures speak for themselves.

Although the annual heat loss of the hot water tank is about 600 kWh, the electricity consumption from the public grid is 1,500 kWh less with this solution than with an electric instantaneous water heater. From a technical point of view, an instantaneous water heater is therefore well suited for secondary taps where only small amounts of hot water are consumed for short periods, for example in the kitchen or at the hand basin in the toilet, but not for providing the main demand in the bathroom.

If we add to these results an economic consideration taking into account the price of electricity and the feed-in tariff, then for Austria* The controlled heating element also results in an annual saving of 220 euros in operating costs compared to the instantaneous water heater. In Germany**The price advantage is around 240 euros per year.

All in all, this is a clear result in favor of controlled heating elements, both in terms of sustainability and in economic terms!

* 20 ct/kWh purchase, 4 ct/kWh feed-in rate

** 30 ct/kWh purchase, 10 ct/kWh feed-in rate

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