Martin Lewis has busted a widespread myth - that it's always cheaper to use energy at night.
Many Brits believe using gas and electricity at night means paying less than using it during the day - and that is true for some.
But the MoneySavingExpert boss explained that it is all down to what sort of energy deal you have.
Some households will save money by using energy at off-peak times, but others won't.
Speaking on the Money Show Live programme on ITV yesterday, Martin was asked by a viewer: "Can you please clarify if there are peak and off peak energy rates?
"If I do my laundry at 10pm will it be cheaper than at 8.30am?"
Martin explained that it all depends on your energy deal.
He said you can save money for off-peak energy use "if you have a time of use tariff, like Eco 7, or Eco 10, or one of the Octopus time of use tariffs".
An Eco 7, or Economy 7, energy deal charges two rates for energy - day and night.
People with Eco 7 meters pay less for energy used in seven hours overnight. Those with Eco 10 deals pay less for 10 hours a day, again overnight.
Both require a special meter to be fitted.
Martin added: "If you don't have a time of use tariff, no, the vast majority of people pay the same rate all the time.
"So unless you do, it will be exactly the same cost.
"If you do have a time of use tariff, you'll want to use the majority of energy when it's cheaper. Don't put a tumble dryer on overnight though, that can be particularly dangerous."
Martin added that if a household has solar panels connected, its cheaper to use electricity at times when it is light.
He said: "If you happen to have solar panels, you will want to use energy during the day, because then you're utilising what goes on in the solar panels."
The MoneySavingExpert boss also had something to say on energy use over the colder months.
National Grid boss John Pettigrew has outlined the "worst case scenario" which could see three-hour blackouts from as soon as January if gas stocks get low this winter.
Martin said: "One thing I would say though, just as a nation, you'll have read there is a risk of energy blackouts this winter in an emergency situation.
"Peak time for energy use is 4pm to 9pm. If you are able to use some of the electricity you would use during that time outside of those hours, putting your washing machine on for example, that helps us all with energy security.
"So give it a go, even if it doesn't save you money."
