Solar panels are one of the least cost-effective ways of combating climate change and will take 100 years to pay back their installation costs, the Royal Institution of Chartered Surveyors has warned.
In a new guide on energy efficiency, Rics said that roof panels for heating water and generating power are unlikely to save enough from bills to make them financially viable in a householder's lifetime. In the case of solar panels to heat water for baths and showers, the institution estimates the payback time from money saved from electricity and gas bills will take more than 100 years – and up to 166 years in the worst case.
Photovoltaic (PV) panels for power – and domestic, mast-mounted wind turbines – will take between 50 and 100 years to pay back.
Given that the devices have a maximum lifetime of 30 years, they are never likely to recoup the £3,000 to £20,000 cost of their installation, according to Rics' building cost information service. Instead, it suggested people wanting to cut fuel bills should insulate lofts and cavity walls, install efficient light bulbs and seal windows.
Joe Martin, author of Rics' Greener Homes Prices Guide, said there was an argument for installing solar panels but it was not an economic one. "We wanted to bring some reality to this because there are a lot of missionaries out there. The whole push for household renewable power is that you can do these things and make back money but that's not true on existing property," he said.
The solar power industry accused Rics of failing to take account of the rising cost of energy and other financial benefits of renewable power in its figures. Jeremy Leggett, of Solar Century, said: "They are grossly irresponsible."
Rics assessed the cost, annual savings, disruption and payback time of various energy-saving methods and gave each an overall rating of one to five stars.
Solar panels for heating and power and wind turbines generating between 3kW and 5kW merited two stars. Smaller 1.5kW turbines of the type installed on roofs paid back in 25 years, received a three-star rating.
By contrast, cavity wall insulation had a five-star rating: spending £440 would save £145 a year in fuel bills, paying back in three years, while an investment of £325 in extra loft insulation would save £60 annually, paying back in five years.
The figures were compiled before energy companies put up bills by up to 30 per cent last month and ignore state subsidies.
Last year, the Department for Trade and Industry slashed grants for the installation of household renewable power by 83 per cent, infuriating the fledgling micro-generation industry which complained the move rendered solar panels unaffordable to all but the wealthy.
Jeremy Leggett, executive chairman of Solar Century, complained that Rics' figures failed to assume any rise in energy prices, when a conservative estimate of 10 per cent a year would transform the calculations.
In addition, Rics had failed to take account of a number of other benefits – renewable obligations certificates worth £160 a year to householders from next year; reductions in energy consumption of up to 40 per cent for schemes with a meter; the rising payments from energy companies for spare electricity put back into the national grid; and the increased value of an energy-efficient home.
He estimated the current payback of power-generating PV panels was 13 years.
Rics countered by saying it had not taken account of maintenance costs and that it deliberately chose not to include "ifs" in its figures. "I doubt however you do the sums, they [solar panels] make sense," a spokesman said.