Tankless water heaters: electronic models and their advantages and disadvantages

Electronic instantaneous water heaters for water heating are inexpensive and easy to install. However, they require a lot of electricity, which can drive up operating costs. The good news is that you can replace outdated electronic instantaneous water heaters with economical, new ones. This means that the investment pays for itself in a short time.

What is an electronic instantaneous water heater?

Electronic instantaneous water heaters provide hot water in the kitchen and bathroom in millions of German households. According to figures from the Federal Association of Energy and Water Industries, around 20% of the population in Germany use this type of hot water heating. Around 10% use gas-powered instantaneous water heaters.

Experts understand an electronic instantaneous water heater to be a device into which cold water flows. There is a heating element inside. It heats up due to the current and releases this heat into the water. The hot water then flows to the tap or tapping point in the kitchen or bathroom.

The instantaneous water heater is the second most popular method of water heating in German households. However, central water heating is much more common and also more efficient.

The different models of instantaneous water heaters

Depending on the design, a distinction is made between three different types of electronic instantaneous water heaters.

Semi-electronic instantaneous water heaters: Cost-effective, but not very efficient

Until the 80s, there were only hydraulically controlled instantaneous water heaters. These devices adjust the flow rate based on the water pressure. In this way, they achieve a fixed water temperature. In most cases, you could choose between two temperature settings. This is why experts also call this form of instantaneous water heater semi-electronic.

Semi-electronic instantaneous water heaters have significant disadvantages. If the water pressure fluctuates, it has a direct effect on the temperature. This happens, for example, when several family members turn on taps at the same time. However, studies have shown that even a deviation of a few degrees Celsius in temperature when showering significantly reduces comfort. Therefore, semi-electronic instantaneous water heaters are clearly at a disadvantage in terms of comfort.

In addition, they do not work efficiently. The fixed temperature is often too high. Most of the devices deliver a temperature of 55°C. This is also the maximum temperature that instantaneous water heaters are allowed to provide for showering. To avoid getting out of the shower crab red, you have to mix in plenty of cold water to get to the pleasant 40 to 42°C.

However, this wastes precious water. Even more serious for the wallet: The unnecessarily high temperature of the hot water consumes an extremely large amount of electricity. This quickly outweighs the advantage that semi-electronic instantaneous water heaters cost the least to buy.

Electronic instantaneous water heaters: Select your own water temperature

Electronic instantaneous water heaters are better. They are able to control the temperature of the hot water more precisely to a selected target temperature. With these hot water appliances, the user can set the desired temperature. If the instantaneous water heater is located near the tap, the line losses are low. That's why 40°C is often enough for a pleasant shower feeling. On hot days in summer, maybe even 36 to 38°C.

Electronic instantaneous water heaters have a temperature sensor that measures the temperature of the incoming cold water. On the basis of this information, the electronic control system adjusts the output of the heating element so that the desired target temperature is reached at the end. The lower the temperature when exiting the instantaneous water heater, the lower the power consumption.

The addition of cold water in the shower is now also superfluous. That's why electronic instantaneous water heaters save a lot of electricity and water compared to semi-electronic devices.

Fully electronic instantaneous water heaters control best

The best way to control the desired target temperature is to use so-called fully electronic instantaneous water heaters. They also include temperature sensors. These measure both the cold water temperature in the inlet and the hot water temperature at the outlet. The control system of these devices immediately notices if there are minor deviations and adjusts.

Fully electronic instantaneous water heaters also measure the flow rate. You can adjust these precisely with the help of a motor valve. This enables a target temperature to the exact degree – especially with pressure fluctuations.

Fully electronic instantaneous water heaters usually have a comfortable display. They show the current hot water temperature, but also the water consumption and electricity demand. In addition, the user can control such devices remotely with a smartphone or tablet.

Fully electronic instantaneous water heaters are particularly economical. Compared to semi-electronic instantaneous water heaters, you can save up to 30% on water and energy. With information on energy consumption on the display, these devices also help you save energy. Because even the most efficient device is not much help if it is not used carefully. Showering twice as long always means twice as much energy and water consumption.

The operating costs of an electronic instantaneous water heater are high

It is worthwhile to make sure that the hot water of electronic instantaneous water heaters is used sparingly. This is because instantaneous water heaters are among the biggest power guzzlers in the household. In order to heat the water quickly, they need a high electrical output of typically 21 kilowatts. This means that they consume the most electricity in the household after the electric stove.

Power consumption of an electronic instantaneous water heater: An example calculation

You take a bath: filling the bathtub takes 15 minutes. For this, you need 5.25 kilowatt hours of electricity with a 21 kilowatt output of a semi-electronic instantaneous water heater. At a cost of 32.63 cents per kilowatt hour, a bath costs 1.70 euros.
A shower of ten minutes devours 3.5 kilowatt hours. This corresponds to about 1.14 euros. If you take a bath twice a month and shower once a day, you will have 24 baths and 320 showers a year. This calculation takes into account 21 days of absence due to vacation or other reasons.
The electricity consumption from washing hands is comparatively low. For the sake of simplicity, it can therefore be neglected in the calculation.
In total, this gives you 1246 kilowatt hours per person per year for the electricity consumption of the instantaneous water heater. This corresponds to costs of around 407 euros per person.

Replacing a semi-electronic instantaneous water heater is worthwhile

If we assume the favourable case that a fully electronic instantaneous water heater saves 30% of electricity consumption, this means a saving of 132 euros per person per year. In addition, there is the saved water consumption of seven cubic meters.

The replacement of an old semi-electronic instantaneous water heater with a modern fully electronic device costs about 800 euros when installed by a specialist company. This means that this investment pays off in a one-person household after less than five and a half years. For a four-person household, the replacement pays off after less than a year and a half.

The advantages of electronic instantaneous water heaters

The advantages of hot water heating with electronic instantaneous water heaters are obvious. The heating of water in the flow does not require a heat storage tank. Therefore, there is no storage heat loss.

In addition, dangerous legionella have no chance without storing warm water. You can find out more about this in the article "Preventing Legionella: Danger Detected, Danger Averted".

Low investment costs and little space requirement also speak in favor of electronic instantaneous water heaters. They are also low-maintenance. Only descaling of the instantaneous water heater may be necessary from time to time.

The disadvantages of electronic instantaneous water heaters

However, this is also offset by clear disadvantages. Electronic instantaneous water heaters are only climate-friendly if you use green electricity from wind or solar energy. Otherwise, their high electricity consumption will drive up the carbon dioxide pollution of the atmosphere even further.

The cost of electricity can also be a heavy burden on users of electronic instantaneous water heaters. In addition, the devices require three-phase current. If the cross-sections of the power cables are too small, there may also be a risk of fire.

The advantages and disadvantages of electronic instantaneous water heaters at a glance

Alternatives are difficult to implement in existing buildings

For all these reasons, it makes sense to think about alternatives. However, there are not too many possibilities. The conversion of a decentralised hot water supply to a central one that is linked to the heating system is costly. But it can be worth it. Especially if modernization measures are planned anyway.

A decentralised alternative is hot water heat pumps. These devices use electricity much more efficiently than electronic instantaneous water heaters. However, a hot water heat pump always contains a heat storage tank. It therefore needs a little more space.

In principle, you can also couple an environmentally friendly solar thermal system with an electronic instantaneous water heater. Then the sun provides the warm water. The instantaneous water heater only has to heat when the sun is not shining sufficiently - especially in winter.

You need space on the roof for the solar panels of the solar system and space in the basement for a hot water tank. In addition, you will need to add a hot water pipe that leads from the basement to the bathroom.

Conclusion: Weigh up the pros and cons

Are you planning a comprehensive bathroom renovation, would you like to install new tiles and new bathroom equipment? Then you should discuss the alternatives to an electronic instantaneous water heater with your trusted heating company and weigh them up conscientiously.

In new buildings and in the case of comprehensive renovations of existing buildings, the central hot water supply should be given preference over electronic instantaneous water heaters. This is because modern heating systems – such as heat pumps, solar thermal energy or gas condensing boilers – prepare hot water in a much more energy-efficient way than electronic instantaneous water heaters with their high electricity consumption.