Photovoltaic heat vs. solar thermal

Price differences between solar thermal systems and photovoltaic heat

Since 2015 we have been creating price comparisons for heat from photovoltaics and solar thermal energy. Accordingly, we always compare a current photovoltaic module with a corresponding solar thermal flat-plate collector that has been available on the market for years.

The graphic shows how prices have developed in recent years: The price for flat-plate collectors has remained almost unchanged between 2015 and 2020. The reason for this are raw materials such as copper and aluminium, which are still used in large quantities in production.

By 2020, on the other hand, there was a sharp fall in in prices for photovoltaics, with a price reduction of around 65 percent per watt peak of the modules. Unfortunately, this trend changed in 2022: Due to the difficult political and economic situation, the prices for both solar collectors and photovoltaic modules have risen again – both price groups to almost the same extent. Unfortunately, we cannot estimate whether, how and when this price increase will flatten again.

In addition, unfortunately, the costs for craftsmen have also increased, if you can find any with free capacities at all. The installation costs (apart from the craftsman) and the maintenance effort when using photovoltaics remain negligible compared to solar thermal energy. Pipes, pumps, valves, expansion tanks and antifreeze are no longer required.

Conservation of resources with solar thermal systems vs. photovoltaics

Compared to solar thermal energy, photovoltaics for water heating also offer enormous potential for saving resources. Photovoltaic heat offers savings of more than 90 percent in the consumption of copper in the pipes alone, which are necessary for the energy transfer from the roof to the hot water storage tank.

A calculation example for this: If you take a standard 2.25 mm thin electric cable (diameter without insulation) and compare it with a copper pipe with an inner diameter of 16 mm, then the cross-sectional area of ​​the cable is 4 mm² and the pipe is 53 mm². In a direct comparison, the consumption of copper for the lines is 93% lower with photovoltaic heat than with solar thermal energy.

Differences in working methods

With both photovoltaics and solar thermal energy, energy is obtained from solar radiation. However, there is a clear difference: photovoltaic systems produce electricity, solar thermal systems produce heat. In the case of photovoltaics, solar cells combined in modules are used to generate electricity. Solar thermal, on the other hand, uses collectors to generate heat.

The question that arises here is: do we need electricity or heat – or both? The direct current generated by photovoltaics is converted into alternating current by an inverter. This can be used in a variety of ways in the house – including for heat generation! Photovoltaic electricity is either consumed immediately, routed to a battery storage system, fed into the power grid or even used for the generation of heat.

This diverse use of heat from solar thermal energy is not possible. Energy from solar panels can only be used for heating purposes.

Differences in efficiency - and the associated error in reasoning

Solar collectors theoretically convert up to 80 percent of the sun's energy into heat, but some of this energy is lost in the subsequent circulation of the heated water. Therefore, an effective system efficiency of around 50 percent is assumed for solar thermal energy. Crystalline photovoltaic modules convert about 20 percent of the sun's energy into electricity, but the losses are minimal. The 20% is therefore often used as the overall efficiency of photovoltaic systems.

However, this purely numerical comparison of the efficiencies is not really relevant. On the one hand, the energy source is free anyway, on the other hand, we almost always have enough space available on our roofs to easily compensate for the difference!

In addition, electricity can be used for various applications (sectors). The efficiencies of the two systems must therefore be placed in a larger context:

• What can the generated energy (heat or electricity) be used for in the household?

• What are the comparative costs if you had to buy this energy?

• What happens to the surplus that is not used immediately? Can this be saved or used in another way?

• What are the acquisition, installation and maintenance costs?

• What is the lifespan of the technology?For comparison: the service life of photovoltaic modules is 25 to 35 years, for solar collectors between 10 and 20 years.

Only when all of these factors are related to one another does a comparable picture emerge, in which photovoltaics is clearly in the lead.

What is the market saying? Which technology is ahead?

The figures from the German Solar Industry Association e. V. from 2021 clearly show: The increase in reported photovoltaic capacity in Germany continues to increase exponentially, while the development of solar thermal energy is slowly stagnating.

For us at my-PV, one thing is certain: Photovoltaics are more versatile, cheaper and more resource-saving.

More about your options with the power of the sun.