Money is tight for many people at the moment, plus those staying at home more may see bigger energy bills. So if you find that your energy company is holding onto large amounts of your cash, it's time to get it back.
More than four in 10 households have £136 worth of credit with their energy supplier, according to new research from price comparison website uSwitch, while one in 10 say they are owed more than £200.
Similarly, our research last year found that two thirds of Which? members who paid by direct debit were in credit to their energy firm, with a handful having built up a balance of more than £1,000.
Large amounts of credit can accumulate if you pay by direct debit and your payments and usage don't match up over a year. However, it's worth taking into account how your energy bills might change if you're spending more time at home owing to the coronavirus lockdown.
You're allowed to request a refund of excess credit at any time, and your supplier should review your account periodically. But it's more straight-forward to get your money back from some companies than others.
If you pay for gas and electricity by direct debit, check your online account or your latest statement. This should show your payments and usage - and whether you're in credit or debt overall.
If you pay when you receive a bill, or pay-as-you-go for energy (via a prepayment meter), you only pay for what you use and won't build up credit.
When you agree to pay your energy bills by direct debit, your energy company will usually calculate your monthly payments by taking the total expected cost of the gas and electricity you will use over a year and dividing it by 12.
So your payments are the same amount each month, even though your usage changes throughout the year. This means you avoid shock bills in winter.
In winter you tend to use more energy than you're paying for, while in summer you pay for more than you use. Over a year, the total payments and total usage should match up.
But if your direct debit payments are set higher than they need to be, you will end up with a credit balance after a year. Equally, if your payments were set too low, you would end up in debt.
Often when you switch energy supplier it will estimate how much gas and electricity it expects you will use in a year. This happens if you don't provide accurate annual usage figures when you sign up. If it has overestimated, you could build up credit.
Energy companies should review your direct debit payments at least once a year, but you can build up significant credit in this time if your payments are inaccurate.
During the coronavirus lockdown, many homes will use more gas and electricity thanks to occupants being there more often.
If you have a small amount of credit, perhaps the equivalent of one or two months' worth of energy use, it's worth considering leaving this as a buffer to help pay the higher gas and electricity bills you'll face while spending more time at home.
Check how much credit you have built up in comparison with how much you spend on gas and electricity in a year. If your credit would pay for many months' energy use, it's an indication that your direct debit is set too high.
If you feel that you have built up excessive credit (like our case study below) you can request a refund, ask your energy company to lower your direct debit to gradually use up the credit, or a combination of both.
Check whether your energy company pays interest on your balance - for example, pays between 3% and 5% interest on balances of up to £1,000, and pays £1 for every £33 credit above £100 (to a maximum of £12).
'When doing a Which? survey, I went online and found out how much credit I was in with my provider (now called Shell Energy): £728.36!
'Previously, I didn't keep an eye on my direct debits: they just go out of my bank account. It was the fact I wasn't told how much credit I had that annoyed me.
'But getting a refund was easy. I used the live chat function on the website and they suggested I took a refund of £550 and kept the rest as credit for winter. I received the money within a week.'
Many energy firms have an online form you can complete to request a refund, while others will refund automatically at certain times in the year. Check the table below to find your supplier's policy.
Energy companies' phone lines are likely to be busy at the moment as they deal with customer enquiries relating to coronavirus, including difficulty paying bills, cancelled smart meter appointments, and staff illness or self-isolation. So if you can request a refund online, it's likely to be a smoother process.
When you request a refund, your supplier must give it promptly unless there are reasonable grounds not to. It will probably check whether your payments and credit will cover your predicted use over the next year. It will also usually require an up-to-date meter reading for both fuels.
This helps make sure that giving you a refund won't mean that you end up paying more or getting into debt in future.
uSwitch's research is from an Opinium survey of 2,008 UK energy bill payers between 20 and 24 March 2020.
Which? ran an online survey of 3,950 Which? Connect panel members in July 2019 who pay their energy bills by direct debit.