As Britain heads into a heatwave next week, the last thing on your mind might be your gas bills. But experts say the message to millions of standard tariff customers is that now is the time to switch suppliers to beat recent price hikes.

This week, E.ON became the last of the big six energy companies to hike its prices, blaming higher wholesale costs for its latest 4.8% increase. It follows a raft of price rises from the other big suppliers this year.

From 16 August the 1.8 million customers on E.ON’s standard tariff will see their dual-fuel bills rise by an average of £55 a year. Electricity-only customers will see a 6.2% average increase, or £36 a year. Meanwhile, its gas-only customers will see a typical 3.3% hike, or £19 a year.

The changes mean the average E.ON customer will now pay about £1,208 a year. It comes a few months after the company was accused of a backdoor price hike when it scrapped a number of discounts resulting in an increase of £22 a year for its standard tariff customers.

Peter Earl, the head of energy at CompareTheMarket, says there are still big savings to be had by changing suppliers. “This move will most likely drive thousands of customers to switch to a cheaper deal. The average saving for someone on a standard variable rate is around £350 – a huge windfall for those who take the time to shop around for a better deal.”

A Guardian Money search this week using EnergyHelpline found that a standard E.ON customer could save about £400 by switching to the cheapest firm – Utility Point, which is based in Poole, Dorset. Its “flexi online tracker 18” tariff is a variable deal, meaning prices could go up at any point, but there are no cancellation fees.

There are a host of small suppliers offering to supply a year’s energy for around £800 a year compared with the £1,200-odd charged by one of the big six providers.

• See if you can switch supplier and save money with the Guardian