It will be a combination of the British climate, and the inefficiency of American buildings (large, poor insulation), equipment (little incentive to reduce energy use with better appliances, timers to turn off heating/cooling when no one is home etc) and lifestyle (clothes must be dried in a machine, not on a line, t-shirt-warmth indoors even in winter).

British homes are terribly heat inefficient as well.

> the inefficiency of American buildings (large, poor insulation)

American dwellings being less well-insulated than British ones in aggregate absolutely does not pass the sniff test: British houses are older, and weather in the UK is much milder than in most of the US