Mr Willcox said: ‘The price of gas and electricity has gone up considerably since the house was finished, so the very high efficiency of our insulation and heating systems combined mean we just keep on saving more money all the time.

‘My view was that I was happy to invest in something if I could get a payback. As it happens, the payback has been greater every year.’

The Government recently cut tariffs paid to homeowners to export solar power into the grid, making the panels less cost-efficient.

Installing the same solar system would now cost £5,000, but the tariffs paid for the electricity would be worth only around £350 a year under a contract that runs for a maximum of 20 years.

Using these figures, it would take more than 14 years for the cost of the solar panels to be recouped

Rainwater taken from gutters is used to flush toilets. Mr Willcox thinks the water harvesting system has helped cut bills by 60 per cent