Energy giant npower has had its knuckles rapped by the advertising watchdog this week over a newsletter that it ruled had overstated the carbon savings attained from one of its wind farm developments.

Npower's newsletter had claimed that a planned wind farm in north Devon would help prevent the release of 33,000 tonnes of CO2 a year, but following complaints from an anti-wind farm group, the Advertising Standards Authority (ASA) ruled the figure was out of date and npower's claims broke rules relating to "truthfulness", "environmental claims" and "substantiation".

The watchdog's investigation found that npower's calculations had been based on the assumption that wind power would largely displace coal-fired energy generation and that the figures were "no longer representative" of a UK energy mix increasingly reliant on cleaner gas-fired power.

The ASA warned npower to ensure future carbon saving claims were based on a "more representative and rigorous carbon emissions factor".

In its defence, npower said that its carbon emission reduction calculations had been made using industry standard figures recommended by the British Wind Energy Association (BWEA).

"This figure has previously been upheld by the ASA as the correct one to use for carbon dioxide offsets by wind farms," the company said in a statement. " The BWEA is already liaising with the ASA to arrive at a mechanism that's updated and universally accepted. The last thing we want is to confuse people. We'll follow a new accepted industry standard when we have it."

The ruling highlights the close eye the ASA is keeping on firms' environmental claims. Earlier this year the watchdog released a checklist for firms looking to promote their green credentials, reminding them to get their facts right, avoid using "pseudo-science" and curb sweeping claims such as " environmentally-friendly".

Speaking at the launch of the checklist, ASA director general Christopher Graham warned that the organisation would be keeping a close eye on those that overstate their environmental benefits.

“Advertisers have every right to promote their green credentials and many have been quick to reassure consumers about the efforts they are making to be greener," he said. "However, the ASA needs to see robust evidence to back up any eco-friendly claims."