Solar hot water: costs, advantages and disadvantages, requirements

When you generate hot water with solar energy, you not only reduce your energy costs, but also your CO₂ emissions. In this article, you will find out what costs are involved and what the advantages and disadvantages of solar hot water are. 

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How much hot water demand can I cover with solar?

In total, you can cover up to 60% of your hot water needs with solar energy. In summer, up to 100% is possible. To achieve this, the solar system must be optimally dimensioned and designed for hot water production.  

How does solar hot water production work?

With solar thermal energy, solar radiation is captured by collectors, converted into heat energy and stored in a hot water tank. Depending on the size of the system, you can then use this heat for various purposes, for example to heat water or to support the heating system.

Solar hot water production takes place in three steps: 

1. heat absorption: The solar panels capture sunlight and convert it into heat. Flat plate collectors are most commonly used, but there are also evacuated tube collectors. These are more efficient and are suitable when space is limited or conditions are not ideal;
2. heat distribution: In a solar thermal system, pipes connect the solar panels for hot water to the storage tank. The solar fluid, a mixture of antifreeze and water, transports the thermal energy. The network includes important components such as the expansion tank, the safety valve and the solar pump for safe and reliable operation;
3. heat storage: Normally, the hot solar fluid heats the drinking water in the hot water tank. This prepares the hot water and stores the heat energy. However, it is also possible to use the heat energy to support the heating. A buffer tank is then used for this. Here, heating water is heated instead of drinking water.

What are the advantages and disadvantages of solar hot water?

Solar water heating has its advantages and disadvantages. First and foremost, solar thermal systems are highly efficient and have a lifespan of over 25 years. However, the investment costs are high and structural measures must be carried out to lay the pipe network.

How much does solar water heating cost? 

Solar water heating costs between €4,000 and €6,000. The price for the solar panels is between €1,500 and €2,500, and up to €1,000 for the hot water tank. Other components such as pipes, fittings, solar pumps and controllers cost up to €1,500. You pay between €500 and €1,000 for installation.

A typical solar thermal system for hot water preparation in a single-family home usually consists of a collector area of four to five square meters and a hot water tank of around 300 liters. The costs are broken down as follows:

Is solar energy worth it for hot water?

Solar thermal energy is considered beneficial from an environmental point of view as it helps reduce emissions. The economic viability of solar thermal energy depends on various factors such as energy prices, consumption habits, building characteristics and current heating systems.

Example: A solar thermal system generates around 3,000 kWh of heat energy per year. At a gas price of €0.12 per kilowatt hour, this means an annual saving of around €400. Over a period of ten years, the savings amount to €4,000, and after 20 years, they are €8,000. This means that the investment costs are usually paid off within 10 to 15 years. In addition, you save up to €601% in energy costs annually.

Solar energy for hot water is even more worthwhile with government funding programs that further shorten the payback period for solar thermal systems.

What subsidies are available for solar water heating systems?

The Federal Office of Economics and Export Control (BAFA) supports solar systems for hot water preparation with up to 30% of the eligible costs. The eligible costs consist of 50% for specialist planning and construction supervision, 30% as an income bonus and a maximum of 20% for the climate speed bonus.

Can I retrofit solar for hot water?

You can also retrofit solar for hot water into existing heating systems. To do this, you need a sufficiently large roof area, the installation of a pipe network and sufficient space to set up the hot water tank. Depending on the size of the system, you can also use the solar thermal system to support the heating. 

Tips and requirements for planning

The size of a solar hot water system depends on the hot water requirement, usage behavior and local conditions. An optimally designed system completely covers the demand between April and October. 

What is my hot water consumption (determine consumption)?

First, you need to determine your hot water consumption. You should also take future needs into account. As a guideline, you should assume a daily hot water requirement of 60 to 80 liters per person. 

What size should the collectors be and how many do I need?

For the above-mentioned guideline, you need about 0.8 to 1.5 square meters of roof area. Usually, 4 to 6 square meters of collector area are sufficient for single-family homes. 

The solar collectors should be large enough to heat the water effectively. The type of collector affects the energy yield:

• flat-plate collectors have a lower efficiency and therefore require approximately 1.0 to 1.5 square meters per person;
• vacuum tube collectors achieve a higher efficiency and require only 0.8 to 1.2 square meters per person.

Factors such as roof orientation, inclination and shading play a role in the dimensioning of the collectors. Roofs facing south and with a roof inclination of between 20 and 50 degrees are suitable. It is important that the roof surface is free of shading, as this reduces the yield.

You should also global radiation when designing the collectors. In Germany, this is an average of 1,100 kilowatt hours per square meter. However, solar radiation is higher in the south and reaches up to 1,300 kWh/m². In the north, it is between 900 and 1,000 kWh/m². 

How big should the drinking water tank be?

Expect 50 to 80 liters per square meter of collector area. For a four-person household with about six square meters of collector area, you will need a storage tank with a capacity of 300 to 480 liters.

Does solar also work as heating support?

Solar also works as heating support. The solar collectors are connected to a buffer tank instead of a hot water tank. Instead of drinking water, the heating water is heated by the solar fluid. However, the efficiency of solar for hot water is higher. During the winter there is little solar energy available, when the heat demand is at its highest. 

The alternative: hot water with photovoltaics

Due to high energy prices, hot water preparation with photovoltaics is becoming increasingly attractive. With photovoltaics, you can generate both electricity and heat, so you only need one system. Although more roof space is required to install a PV system, the pipe network is no longer required.