Nine of the deals which expire on 30 September will result in higher bills if customers fail to switch and are rolled onto standard tariffs. Two of the price rises will come from big six providers EDF Energy (£147.24) and Scottish Power (£183.72).
Independent supplier M&S Energy customers will see the largest average rise in the UK of £285.65.
Three of the big six suppliers, British Gas, Npower and Scottish Power have tariffs expiring that could see bills reduced by up to £53.54.
Gocompare.com energy spokesperson Ben Wilson said: “With darker nights and colder weather, September marks the start of ‘thermostat switch-on season’, where household energy usage starts to increase.
“Therefore, it’s even more important that energy customers avoid paying over the odds by allowing themselves to be rolled over onto expensive variable tariffs when their fixed deal comes to an end.”
Other incumbent supplier’s customers will also see large price increases if rolled onto the standard tariff including Extra Energy (up to £257.68) and Gnegry (£240.16).