Martin Lewis is urging energy bill payers with a direct debit to see if they are owed hundreds - and today is the best day to do it.

In total he estimates energy firms are sitting on £3bn of energy bill credit, which customers can ask for back.

"This is specifically for people who are on monthly direct debit," he explained, on Good Morning Britain. "Before I start anything, you need to make sure either you're on a working smart meter, or that you've done meter readings before you look at your account. This is all about the energy direct debit cycle.

He elaborated: "Your monthly direct debit is paid at an even amount across the year, so that in the high use months, you're paying less than you're actually using, and in the low months use months, you're paying more than you're using. So it's all spread out."

He explained how it works with a series of graphs, showing that - in order to avoid big payments in winter and potential for people to run out of money - the firms take a bit extra through summer. But, by doing that, they are also earning interest, that could be in customers' banks.

He concluded: "The point where you either have the most debt or the minimum credit is today, which means, if you go and check your in credit and you have substantial credit on your monthly direct debit energy account right now, you have too much and you should be asking for it back. That's why we're doing it today."

How do customers claim that energy credit back?

"Here's my rough rule of thumb, because there are vagaries here, and the whole point of monthly direct debit is to it's a budgeting job to spread out a year's usage," he said. 

"Let's say your direct debit is £200and you're £500 in credit. I'd be asking for £200 back. I'd be getting in touch with my energy firm and saying I've done a meter reading.

"I'd be saying to them, you have too much of my credit, and I would like that cash back in my bank account, not in your bank account."

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What if they say no?

"Well, under condition 27 of the gas and electricity Licensing Act, you have a right for a fair direct debit that they should justify to you if it's too high, and they should not keep an excess amount of your credit," he said.

"So if you have done your research properly and you have checked all this out, and they have too much of your credit, then I would make a formal complaint to them for the money back. And if they refuse I would take them to the free energy ombudsman service and ask for your credit back.

"And I'd also ask for a bit of compensation for the time and hassle because they're not giving you your money back."

Is there a downside to this?

If you find budgeting tricky and you're likely to overspend if you have too much sitting in your account, then it might be a better idea to keep it in your energy account.

Another option would be to take it out, put it in its own 'pot' - a high interest savings account for example - and then pay it back in winter when your bills are higher.

It can also be an issue for some DWP benefits and means-tested pensions  if you have too much in the bank, so check it's best for your circumstances before you ask for the money back.