The energy bills of more than 11m households will be cut by around £75 a year on average under a price cap promised by Theresa May in the 2017 election, amid concerns that loyal customers are being ripped off. From the end of the year, customers who are on their supplier’s standard variable tariff will see their prices capped at £1,136 on average. It mostly affects those have never switched supplier, or not done so recently, as customers coming off a fixed deal are automatically dumped onto these tariffs, which are almost always the most expensive.

But the savings fall short of the £100 that Theresa May originally promised for consumers. The average standard tariff (from a big supplier) is currently £1,220 – hence the typical £75 saving. Those on the UK’s priciest tariff, from Scottish Power, will save the most – £121 a year – while Ovo customers will save just £14. Ofgem, the regulator, claims 3m homes will save £100 a year or more as a result of the cap.

Does this mean I will only pay £1,136 a year?

No, all the quoted prices are very much averages, based on Ofgem’s consumption figures. What your household will actually pay will still be calculated according to usage – it is the gas and electricity unit prices that are now capped. The £1,136 cap is for direct debit customers. For those who pay by cash or cheque, the cap is £1,219. Ofgem will review the cap every six months – in April and October – and adjust it according to wholesale and other retail prices. Consumers on it can expect the first rise (or fall) in April 2019.

The cap is set to expire in December 2020, but could be extended for three years.

Haven’t the big suppliers all just put up their prices?

Yes. All the big firms have pushed through price rises since April – in most cases twice – adding a joint £100 a year to bills. E.ON and British Gas started in April adding £50 and £60 to bills respectively. Both have since made another increase. EDF has made two, SSE, npower and Scottish Power one – and further rises are expected. It allows the firms to charge more from now until the cap begins at the end of the year.

Who is the cap aimed at?

The millions of people who fail to shop around for a deal, and overpay as a result, should benefit. Many feel that “life’s just too short” to faff around with tariffs or don’t feel capable of using comparison sites.

Does this mean I no longer need to keep switching supplier each year?

No, no, and no. All that the price cap means is that the non-switching households don’t get overcharged by quite as much as they used to. If this group have any sense, they will still look for a better deal now, and should save around £250-£300 off their bill. Families who use a lot of energy could easily save £500 a year by switching.

Smart consumers who always shop around should continue as before. But they may end up paying a little more relatively, as the power firms alter their tariffs to make up income lost to the cap.

The two cheapest suppliers are currently OutFoxtheMarket and the Powershop, which are charging about £850 a year on average. Bulb is also cheap at about £890 a year. Prices vary according to where you live, but remember, the bigger the usage, the greater the saving.

I have a pre-payment meter. Does this affect me?

No, your prices have already been capped – although you may not have noticed. On 1 October the capped fees the suppliers can charge you rises from £1,089 a to £1,136 a year on average. It follows a £57 a year increase in April.

I have never switched supplier – how does it work?

It is really easy, so there really is no excuse. First, try to dig out an old bill that shows your projected or actual usage.

Go to Energyhelpline.com, input your postcode, your existing supplier, and house size or consumption figures. Click on the box that says “Show me all generally available plans”. Take the claimed saving with a slight pinch of salt, but be happy that you have found the cheapest tariff for your area. To switch just follow the links. It’s worth noting the customer service ratings and choose accordingly.

You don’t even need to tell your existing supplier. You should take meter readings on the switching date, and keep them somewhere safe.

Non-internet users can phone Energyhelpline on 0800 074 0745 every day, but be aware that you may not get the cheapest deal if they carry out the switch on your behalf.

If the above is too much, simply call your existing supplier and ask to be put on to the cheapest tariff.