Do you have a washing machine? If it's British model, chances are that it's connected up to the hot and cold water systems. I've heard a rumour that new washing machines will be cold-fill only, as is standard practice in Europe. On the face of it, that's a good thing. Only one water valve, less wiring, simpler plumbing in the kitchen. But what if you have a lot of hot water, from your solar panel or from an older boiler with a hot-water cylinder? You can't use that. Your machine will fill up from the cold supply - and then use electricity to heat the water.
If you're buying a new washing machine this weekend check how green it really is. And if you're planning to install a solar panel to heat your water, make sure you can use all the hot water you generate!