What could be more environmentally friendly than harvesting electricity from the wind and what better ethical badge of honour than a turbine spinning on my roof?

That's certainly what I thought three years ago, when my family and I were challenged by the Newsnight editor to try to make our lifestyle greener.

Unfortunately, I wasn't the only wannabe ethical man to want to reap the wind. As I began exploring the possibility of erecting a turbine on my terraced London home the Tory leader David Cameron announced his ambition to do the same.

The question was, who would get theirs up first?

Three years on and neither I nor the Tory party leader have a turbine on our roof.
Why?

The answer is very simple. In most urban locations in Britain wind turbines simply do not work.

Yes, they spin, but they do not generate significant amounts of power. Why not?

Here's the science bit... (don't worry, you will be able to follow it).

A simple equation gives the power of the wind. Power = 0.5 x collection area x the wind speed cubed.

What it tells us is that the power of a turbine is related to two factors: the size of the turbine and the strength of the wind.

Take the micro turbine I was planning. Its blades were 1.75m long, giving a collection area of just under 10sq m. Tiny.

Compare that to the wind turbines I visited in Texas earlier this year. Some had turbine blades 45m long, giving a collection area of 6,358sq m. Huge.

The message is clear from the maths - small turbines have disproportionately smaller collection areas and therefore generate dramatically less power.

And what about wind speed?

The key here is that cube function on the wind speed. The power of the wind is related to the cube of the wind speed. So, at low wind speeds you get virtually nothing. When it really blows it you get a lot of power.