The viability of the project depends on assumptions about the future price of oil and gas, though in some cases projects are fully competing already. "There are millions of buildings where a solar thermal system is already paying for itself in its lifetime – between six and seven years in good sunny conditions," says Piria. Most people in the industry are increasingly confident, given the annual rise in fossil fuel prices since the 1990s.

And even where the technology might take a long time to pay for itself – such as at the Bewleys Hotel at Dublin Airport in rainy Ireland, there may be other commercial advantages for the customer. "The payback period for the solar installation alone is not too good," says George Sonneborn, the project manager who introduced it, "even with the energy price situation that we currently have. But we never looked at this just from the payback point of view as we are using the installation for advertising purposes."

In the medium term, the future looks positive for solar thermal, which sold €3bn worth of kits, compared to €9bn in 2006 for finished solar PV panels. Matthias Fawer, an analyst at Swiss investment firm Sarrasin Chiswell, compares solar thermal to heating oil costs. He estimates that it will break even on average across Europe when heating oil reaches the price of €78 per hundred litres – €28 higher than current prices.

"At the moment oil and gas is still cheaper but in the next 20 years we do forecast that fossil fuel prices will go up and that will make a difference to the viability of the technology too," confirms Christoph Lapczyna, business development manager at solar PV and solar thermal producer Conergy, one of the few independent solar thermal providers in Europe.

In the majority of European cases, gas or oil is indeed still the dominant heating source, and it is still less complicated to go with the status quo, because of infrastructure issues that inhibit renewables development, particularly in the case of heating. Sales staff must also overcome the awkward fact that a conventional boiler is needed as a backup system in any case when home owners buy solar thermal. That could mean an average total cost of around €5,500 for a house in a cloudy country like Germany for example, requiring a backup 30% of the time.

To overcome this problem and the fact that the system, which contains moving parts such as pumps, incurs higher maintenance costs than PV, industry activists have argued for years for an incentive scheme comparable with the tariffs or obligation schemes that support most clean electricity development in Europe.