British Gas saw its profits rise to £574 million last year.
That’s a 31% increase from £439 between January and December in 2014.
The company said its profits were boosted as there was a 5% rise in household gas usage despite the warmest December on record and as a result of lower energy efficiency scheme costs.
However profits at its parent company Centrica fell by 12% to £1.46 billion due to a fall in wholesale gas prices.
The results come a week after the energy supplier became one of the last Big Six companies to lower gas prices.
Chief Executive Iain Conn insists the company is passing on the reductions.
He said: “We absolutely have passed on the low costs of commodity prices as they fell to our customers. The reason the profits went up in 2015 versus 2014 actually is very simple. It’s about the weather and about consumption and we saw a very mild 2014 and we saw a more normal 2015 and therefore the amount of energy that our customers used went up and therefore the actual total profit went up.
“But very importantly the profit only went up to a level that’s a little bit below the average of profit over the last six or seven years.”
Centrica is reducing its exploration and production capital spending to around £500 million this year and expects to cut it further in 2017 and 2018 “if current low price environment is sustained”.
British Gas Business also saw its energy supply points fall by 11% last year as it was affected by issues following the migration of customer accounts and related data onto a new billing system.
That had a “significant impact” on customer service, the company added.
The Competition and Markets Authority (CMA) found British households are paying £1.7 billion a year too much for gas and electricity.