Domestic "microwind" turbines, recently championed as "power from the people" by opposition leader David Cameron, are about as useful as a chocolate teapot.

A study of domestic turbines was published by renewable energy consultants Encraft in December. According to the study, only one of the 15 household wind turbines generated enough to power a 75W light bulb. The average daily output was 393.3 Watt hours: an average of 17W.

In all, only three of the turbines generated over 400 Watt hours of electricity, with one generating 1,790 Watt hours.

Four of the turbines didn't even make it into three figures. By way of comparison, a washing machine consumes 4kW (4,000W), and a fridge-freezer 1.9kW. [PDF,1MB]

The average turbine also operates at only 1.84 per cent of capacity.

The carbon-obsessed BBC has suggested that a domestic turbine may contribute about "a fifth" of a household's electricity needs - but the reality is this is only true if the household's only electricity need is one fifth of a single crack-den-dim light bulb.

The numbers suggests that the turbines would take, at best, 15 years to pay for themselves.

So what's gone wrong?

Mind the bats

Encraft stresses it's early days, which is true - the first 13 sites only went live last January, with 13 more following in October.

However, it appears that the measured windspeed for many sites fell below the predicted figure. Turbulence in built-up areas makes for poor windflow. Or as SK Watson, of the Centre for Renewable Energy System Technology at Loughborough University, observes:

"Those areas with higher capacity factor are where urban areas tend not to be!"

Windspeeds in rural vs urban areas

Worse, the measured energy output from the domestic turbines was far below the "theoretical" energy predicted.

Er, quite.

The trial has suffered other problems. One turbine was stolen, another damaged, and a further one was beseiged by pro-bat protestors. Several needed their inverters replacing.

"We have had some reality checks," Encraft admits.

However, Encraft MD Matthew Rhodes, quoted in The Guardian found one "benefit" from the white elephants. Apparently, seven out of ten people who see a turbine say it reminds them to save energy.

The logic is, apparently, that when one sees one of these monuments to self-righteousness, one dashes back to turn the lights off.

But surely there must be cheaper ways of inducing feelings of guilt and low self-worth in the general population - such as availing oneself of the latest Radiohead album, perhaps? ®