David MacKay's one per cent rule

David MacKay - author of the influential book Sustainable Energy: Without Hot Air - proposes a smart idea in the Guardian today: that a gadget should only really be discussed if it can lead to energy savings of 1 per cent or greater.

MacKay suggests this in discussing 'kinetic energy plates' in a supermarket car park, designed to capture energy as cars pass over them. MacKay writes: 

Let's guess that the kinetic road plates extract one fifth of the kinetic energy of the arriving car. For a car weighing one tonne travelling at 20mph when it hits the road plates, the extracted energy comes to 0.002 kilowatt-hours (kWh). Now, the energy used by the car, assuming it is driven three miles to and three miles from the supermarket with a fuel efficiency of 33 miles per gallon, is about 8 kWh. The savings from parking at the green car park thus amount to one four-thousandth of the energy used by the trip to the supermarket.

That's much less than 1%. So this "green energy system" is just eco-bling, creating a delusion of happy progress while distracting people from serious change.