In 2001, Andrew Briggs bought Tanners Farm, a five-bedroom, 18th-century property near Henley in Oxfordshire, for £1.9m. But having installed an emergency generator, fully lit outdoor swimming pool, security lighting and air conditioning, he soon realised that he was going to be paying rather a lot for energy.

“We were looking at the bills, and, suddenly, it dawned on us there were things we could do to reduce them,” says Briggs, 42, who sold the Pocket Phone Shop – the company he had set up with Simon Jordan, chairman of Crystal Palace Football Club – to One 2 One, for £73m in 2000.

Briggs embarked on a £40,000 energy-saving programme. This included replacing all the outside lighting with low-energy LED lights, installing a solar-panel system to supply hot water and heat the pool, and putting in a stretch of photovoltaic panels to generate electricity.

Thanks to all this eco-kit, Briggs’s energy bill is now £800 a month – half what he was paying before. He also makes £100 a quarter by selling his excess electricity back to the National Grid.

So far, so green. Then Briggs decided to put Tanners Farm on the market and commissioned a home information pack (Hip), compulsory for properties with four or more bedrooms since August. The energy performance certificate (EPC), the most significant element in the pack, gave him a disappointing E rating (where A is best and G, worst), with 46 points out of 100 – below the national average of 49.

“I was horrified,” says Briggs. “The government is trying to persuade people to include energy-saving measures, and the same government is forcing people down the route of Hip packages. But if they’re not taking things into account, there’s not really an incentive to do it.”

The problem lies in how energy certificates are calculated, which varies from company to company. Sava, the firm that carried out the assessment on Tanners Farm, spends 30 minutes on each property, using a technique that relies on a number of assumptions that underestimated quite how energy efficient Briggs’s property had become. Sava’s procedure assumes photovoltaic systems will offer only a certain limited power output – Briggs’s system offers five times that strength. It also assumes that solar panelling will cover an area no bigger than 3 square metres – Briggs has four times that.

The insulation in the loft was also overlooked, because it had been put in at rafter rather than joist level. As it couldn’t be seen when the energy assessor stuck his head into the attic, it was assumed the level of insulation was pre1900. This, Briggs says, was despite his documentation for the new roof and insulation he had installed.

“When the Hip company prepared the energy rating, my house didn’t fit the box,” he says. “But not everything fits into neat little boxes.”

Furious at the results, Briggs contacted Simply Hip, his Hip provider, who in turn passed his query onto National Energy Services (NES), Sava’s parent company. NES then agreed to carry out a second assessment to check the original had been performed correctly.

It had not: the assessor had missed the mechanical ventilation that Briggs had installed, and had also input details of the insulation wrongly. A revised certificate was issued, but it didn’t improve Tanners Farm’s low rating. Briggs’s only consolation was that if he installed low-energy lightbulbs inside the house and upgraded his boiler, his rating would rise to 66 points, or a D.

NES is sticking by its findings. “The initial assessor did miss the mechanical ventilation – but, to be fair, the unit was outside and normally they’re in the loft space,” says Lisa McNeil, one of its energy consultants. She admits, however, that Briggs’s case is not the first of its kind.

“There are quite a few properties we’ve come across where the model may not be assessing the energy efficiency of a house entirely fairly,” she says, adding: “Because energy assessment needs to be relatively quick and noninvasive, there has to be a fair number of assumptions.”

Briggs’s battle highlights growing concern over the reliability of the energy assessments. A quarter of people polled recently by ICM, a market research company, were confused about the certificates and the impact they have on their homes. The Royal Institute of Chartered Surveyors (Rics), meanwhile, has found homeowners acting on information provided in an EPC will have to wait 13 years to recoup the cost of new loft insulation; solar panelling typically has a 208-year payback time.

“The method being used is fine for an average property,” says James Scott-Lee, deputy chairman of the Rics residential faculty board. “But as soon as you step out of three-bedroom terraced house territory, it becomes difficult to stand by it.”

Stewart Lilly, president of the National Association of Estate Agents, says more time should be spent assessing a property. “It must be done properly,” he says. “It can’t be done in half an hour.”

Despite his travails, Briggs is still committed to saving energy. He has just bought a £2.9m property at Sandbanks, in Poole – but was shocked to see the house, stuffed with modern technology, has no eco-features. “Astonishingly, it has zero solar panelling,” he says. “But I won’t accept that lying down, not at all.”

Tanners Farm is for sale for £3.5m through Savills; 020 7499 8644, www.savills.com

The ABC of EPCs

What is it? An energy performance certificate (EPC), similar to the certification found on domestic appliances, shows the official energy rating of a property.

How is it measured? An energy assessor will visit your home to measure its energy performance, looking at how the property is heated and lit. The building will then be rated on a scale from A to G, based on points, up to a maximum of 100. A is the highest rating, indicating optimum efficiency and lower fuel bills; G is the lowest, most inefficient, with higher fuel bills. The certificate also shows the building’s environmental impact by indicating its carbon-dioxide emissions: the higher the rating, the less impact it has on the environment.

What is it supposed to do? An EPC records the energy efficiency of a property for the benefit of potential buyers. It will also give recommendations on how to improve the building’s energy performance.

Times recommends

• Fall in house sales hits builders, estate agents and mortgage brokers
• Why the property credit crunch favours cash buyers
• The blood-curdling history of Quarrelwood House