And a growing proportion relate to households that have changed to one of the scores of newer, smaller suppliers to have entered the market.
So if you're fed up with your current supplier but are worried about switching, what should you look out for?
Why should I switch?
If you think you're paying too much for your energy, then you should definitely consider switching suppliers.
If you haven't switched before or for a long time, it's likely you will be on your current supplier's default standard variable tariff - and that's likely to be way more expensive than the cheapest tariffs on the market.
But, be warned, that's not the maximum you will pay if you're on an SVT - use more energy, and you'll pay more.
How do I go about switching then? What will I need?
Before logging on, make sure you have to hand your postcode, the name of your current supplier and the name of your current tariff.
You can find your tariff and supplier details on a recent energy bill.
What company and tariff should I go for?
There are scores of energy companies, with dozens launching in the past four or five years.
So, while going for the cheapest tariff on the market might save you money, it may be better to go for a more recognisable, longer established name, with better feedback from customers.
Getting a fixed deal at a competitive price will give you peace of mind. You can lock in your rates for 12 months, 18 months or longer.
Alternatively, some suppliers offer cheaper, variable deals way under the market average - over £350 in some cases - but as they are variable, the price can rise at any time.
The supplier has to give you at least 30 days' notice - and you can switch penalty-free.
And, should the worst happen and the company goes bust, your energy supply is protected by Ofgem, so the lights won't go out.
It's also worth bearing in mind that often your current supplier will have cheaper tariffs available than its SVT, so check.
How long should the switch take?
Once you've identified your new supplier and tariff, you need to decide how to pay. Usually paying by direct debit works out cheapest.
The switching process will then start, and your new supplier will be in touch to confirm your switch-over date.
The switching process can take up to 21 days. In most cases, it’s around 17 days.
If you change your mind, you have 14 days to cancel from the date you agree a contract.
What if the switch goes wrong?
If your switch is delayed you will soon be entitled to automatic compensation.
Under Ofgem plans set to come into force from next month, suppliers will be required to pay at least £30 in compensation for each switching problem - such as being moved to the wrong company or undue delays in being refunded credit.