This article is part of our special report Solar Power.
Solar heating systems can provide over half of households' hot water needs, according to the largest ever field trial of the green energy devices, conducted in the UK.
But the year-long study, undertaken at 88 homes by the Energy Saving Trust, also reveals that the solar water heating systems will save most owners just £55 a year despite costing between £3,000-5,000, prompting calls from green campaigners for clarity on government subsidies for them.
Residential payment levels under the [UK] government's £860m renewable heat incentive (RHI) scheme, announced in March, are not due to be announced until 2012. But if on a par with those for large-scale solar heating systems, householders could expect payments of around £96 annually. The true figure is likely to be higher because smaller installations will probably attract bigger subsidies.
The trust also revealed that, between 1 August and 9 October this year, 707 people took out new £300 grants for solar thermal systems. The government has put aside £15m for the renewable heat premium payment (RHPP) scheme, which runs until March 2012 offering grants for a range of green heating technologies including groundsource heat pumps and biomass boilers. The separate RHI payments, similar to those made to owners of solar panels and wind turbines under the feed-in tariffs for green electricity generation, will launch after the RHPP.
Solar water heating systems work by using the sun's energy to heat water or anti-freeze in collectors on the roof of a building. The heated water or anti-freeze is then usually pumped to a hot-water cylinder to be stored until the hot water is needed.
Overall, the trust said it was impressed by how well solar heating systems were working in the UK. The field study found the systems provided a median of 39% of households' hot water needs, rising to 60% for the best and plummeting to 9% for the worst-installed one. The trust had previously believed around 35-40% would be a typical figure, based on laboratory tests.
Jaryn Bradford, senior technical manager for the Energy Saving Trust, and author of the report, said: "This is a technology that works, and works well in the UK." He said the main impact on a system's performance was how well insulated the home's hot water tank and pipes were.
The study also showed the pumps powering the systems used between 1 and 23% of the energy generated, though would cost just £8 in electricity annually for a typical system. Some systems had pumps powered by free electricity using solar photovoltaic panels.
The market for solar water heating in the UK has continued to grow despite the economic climate – up 18.1% in 2010, compared with 13.1% for Europe overall, though the trust said that was largely because the UK was starting from such a low baseline. There are an estimated 140,000 homes with solar water heating in the UK.
Founder and CEO of Good Energy, Juliet Davenport, said: "This pioneering research demonstrates that households with solar hot water generators really can benefit, provided their generator has been correctly installed and they understand how to make the most of it. Solar thermal panels are a fantastic, cost effective way for the UK to reduce our demand on imported energy whilst cutting our bills and also our carbon emissions."
Friends of the Earth's energy campaigner, Tony Bosworth, said: "Unless we end the nation's dependency on increasingly expensive fossil fuels, homes and businesses will continue to pay the price through soaring power bills. The government's renewable heat incentive can't be delayed any longer – ministers must provide households with real cash incentives to help families switch to clean energy to heat their homes."