No, an energy supplier cannot charge you based upon when you use your energy unless you specifically choose such a tariff (known as time-of-use tariffs). Time-of-use tariffs could mean lower bills if you use energy outside peak times. They will also help energy suppliers to better manage periods of peak demand. Time-of-use tariffs are not compulsory and if they suit your lifestyle, could save you money. There are already some time-of-use tariff such as Economy 7 and Economy 10, or even free energy at certain times of day, but smart meters mean that time-of-use tariffs can be more flexible.