Not only would rooftop turbines in built-up areas be an enormous money-pit (barring crippling electricity price rises), they would also be bad for the environment. More greenhouse gas would be emitted during their manufacture, shipping, installation and maintenance than would be saved at power stations. The Carbon Trust people considered the case of the SWIFT turbine, a representative model.
Assuming that the entire household population of UK urban areas has this machine installed ... in the majority of cases (over 80 per cent), yields are less than 500 kWh/year ... over 50 per cent of installations have a carbon payback of more than 20 years, which is beyond the expected life of the turbine ...
So an urban rooftop windmill normally produces well under 2 per cent of the energy requirements of an average UK household*, represents an enormous financial loss, and will normally increase the world's greenhouse gas emissions rather than reducing them. So you'd have to be mad to install one.
In fact, no you wouldn't - because the government will give you money or tax breaks for a roof turbine. Even though it's usually expensive and actually bad for the environment, thanks to the various kinds of eco-subsidies (particularly those built into the low- and zero-carbon buildings regs) it may make sound financial sense to buy a windmill.
Some government grants are not conditional on site wind speed and electricity generation potential. This carries the possible risk of grants being awarded to installations which save little carbon ...
Only certain sites amongst all places where small turbines could possibly be installed are actually suitable for installation ...
A range of policies is encouraging the growth of small-scale wind energy in the UK, including the Low Carbon Buildings Programme, Permitted Development Rights (PDRs) for domestic installations and the Code for Sustainable Homes.
It is recommended that ... In any future grant schemes, a criterion is used to measure likely carbon savings ...
You'd think that grant schemes with the phrases "low carbon" and "sustainable" in their titles would have such criteria, but this seems not to be the case.
This is all quite hard for the Trust to say - being eco-quangocrats, they are the kind of people who would love to believe in a Blighty powered largely - or even partly - by renewable microgen. But, through gritted teeth, they admit that "up to 1.5 TWh could be generated [annually] ... these figures are fairly low ... [if the cost of wind turbines halved, this] could change to 3.1 TWh ..."
The UK used 2,700 terawatt-hours of energy in 2006, actually, so we're talking about a thousandth of our energy supplies at the very outside. A better description would be "insignificant" or "microscopic".
It would be nice to think that this finally puts a merciful stake through the heart of the seemingly unkillable home-windmill idea, but it probably won't. Even the Carbon Trust didn't much like reading their own numbers, and it will be surprising if anyone else bothers to heed them. For a lot of people, off-grid living and microgeneration are religious/moral standpoints, not sets of engineering techniques; they won't be swayed by mere numbers, even ones from fairly right-on types like the Trust.
The rest of us, though - unless we happen to live on a windy hilltop out in the boonies - might reasonably file the roof-turbine plans in the bin and think of something else. ®
*According to the UK Office of National Statistics.