With Octopus you can go to an Agile tariff, where you're paying exactly what the market rate is, which means that frequently at night you are being paid for using electricity(the rate per kWh goes into negative). I know a few people who have it with an electric car, and it means that at certain times you are being paid to charge your car. It's crazy. But also it means that at peak times the cost can be as high as 25-30p/kWh