Hi @missussimoa , unless you have a multi-rate tariff (Economy 7 or similar), then you are charged the same rate for energy use whenever you use it. 
If you are thinking more about the greenest time to use, then that changes daily but it is generally overnight (midnight -7am)

You can see the latest forecast beneath your usage on the app 

It’s also different in different parts of the country. For example right now 

The north of the country has significantly greener energy at the moment