Retrofit solar thermal: when is it worth it??

If you want to protect the environment and your wallet at the same time, it is advisable to retrofit conventional heating systems with solar thermal energy. The solar collectors turn solar radiation into usable heat that you can use in your household for hot water preparation and heating support. But when is it worth upgrading, and how big is the potential savings, really?

Solar thermal system for water heating and heating support

First, you need to consider how you want to use your solar thermal system. There are 2 options here:

In the first model, the stored heat is used only to produce hot water. The investments are correspondingly smaller and less expensive. If you retrofit solar thermal, you have to expect an initial cost of 4.000 to 5.000 euro calculate. In the summer, the solar thermal system can provide 100 percent of your hot water needs. Over the year, the system brings it to about 50 percent, which is also your expected cost savings.

If you want to use your solar thermal system for backup heating as well, there will be an initial cost of 8.000 to 10.000 euros. Such combined systems not only produce hot water, but also feed the residual heat into the heating circuit. On an annual average, 20 percent of the total heat demand can be met by solar thermal energy.

Tip: by the way, if you want to retrofit solar thermal, the best time to do it is in the summer. Get started on planning early in the year and purchase your system in the spring. If start-up difficulties occur, you can bridge temporary failures in the summer months relatively easily.

Retrofit solar thermal: requirements

Conveniently, most rooftops can easily be equipped with a solar thermal system. Nonetheless, you should first address the structural requirements: not every roof promises high energy efficiency. Roofs that meet the following criteria are particularly well suited:

• Roof orientation to the south or southwest
• Available unshaded roof space of at least 3 to 7 sq. Ft. (depending on the use of the system and the size of the household)
• Roof pitch of 30 to 50 degrees for water heating and 45 to 60 degrees for central heating backup

If your property does not meet these requirements, you should not immediately discard the idea of using solar energy. It is best to consult a specialist, because you may be able to retrofit your roof with solar thermal energy after all. For example, if your roof is oriented to the east or west, you will have to reckon with energy losses of around 10 to 30 percent. However, if you have more surface area available for this purpose, you can compensate by adding solar collectors. Sloping substructures allow even flat roofs to be fitted with a solar system.

If you want to use your solar thermal system for heating support, it must be compatible with your current heating system. Gas and oil heating systems are particularly frequently retrofitted with solar energy, but other systems can also be sensibly retrofitted.

Heating systems that can be retrofitted with solar thermal energy:

Good to know: if your oil or gas heating system is already more than 30 years old, retrofitting is no longer worthwhile according to the building energy act (GEG). In this case, it is better to have the heating system replaced completely and switch to an environmentally friendly gas-solar hybrid heating system. The project is subsidized by the state with up to 40 percent of the costs.

Inexpensive solar thermal retrofitting thanks to government subsidies

The purchase price for a solar thermal system seems rather high compared to the cost savings of 20 percent for heating support and 50 percent for hot water preparation. However, solar retrofitting becomes much more attractive thanks to state subsidies.

Through the federal office of economics and export control (BAFA), the state subsidizes the purchase of solar thermal systems with 30 percent of the costs. The only prerequisite for this is that the solar collectors of your choice bear the "solar keymark" label and are included in the BAFA list of eligible solar systems.

If you cannot or do not want to pay the remaining sum out of your own pocket, you can also turn to the kreditanstalt fur wiederaufbau (kfw). Via kfw program 167, this supports retrofitting with solar thermal systems with low-interest loans.

Important: if you would like to take advantage of government subsidies for your solar thermal system, you must apply for them before signing the purchase contract. You are not entitled to any retroactive subsidies.

Retrofitting solar thermal: how sensible is it??

Due to attractive subsidies, retrofitting with solar thermal energy is a good idea in most cases. You save money in the long run and significantly reduce the CO2 emissions of your household. However, it is not possible to put an exact figure on how high the savings potential will be in individual cases. This depends largely on how much heat you need in your household and how efficiently your solar system works.

The savings potential also depends on a number of external factors over which you yourself have no influence. If you retrofit your heating system with solar thermal energy, you make yourself independent of fossil fuels and their price development. The CO2 price, which is planned for 2021 as part of the climate package, is likely to cause prices to rise further. The earlier you break away from your supplier's pricing policy, the greater your savings potential.

Conclusion: retrofitting solar thermal almost always makes sense

Although solar thermal systems with a purchase price of 4.000 to 10.000 euros is a proud investment, but the permanent cost savings will quickly show up in your account. Solar retrofits are made even more attractive by generous government grants and low-interest loans. Since solar thermal systems can be installed on almost any roof, there's no longer any reason for property owners not to look into the environmentally friendly alternative.