The company, which already announced 2,000 job cuts, also plans to cut 3,000 jobs in 2016.
Adjusted earnings in the year to December 31, 2015, fell 4% to £863mln on a 5% drop in revenue to £28bn.
Adjusted operating profit fell 12% to about £1.5bn and adjusted basic earnings per share dipped 4% to 17.2p. Centrica cut its full-year dividend per share by 11% to 12p.
The group said post-tax one-off items of about £1.8bn due mainly to falling commodity prices but operating cash flow was strong, allowing it to reduce net debt by 9% to £4.7bn.
Chief executive Iain Conn said: “Centrica has achieved a resilient financial performance, with solid 2015 adjusted earnings despite the challenge of falling wholesale oil and gas prices.
"We remain confident that our plans and underlying performance momentum will allow us to more than balance cash flows and deliver at least 3-5% per annum underlying operating cash flow growth to 2020, even in the current environment, so underpinning a progressive dividend policy.”
Centrica is likely to attract controversy for boosting residential energy supply operating profits by 31% to £574mln, given criticism of the industry that reductions in household bills have not kept pace with falls in wholesale oil & gas prices.
But the company said residential services operating profit dropped 5% to £257mln while business energy supply and services fell into the red, leaving total British Gas operating profit down 2% at £809mln.
Centrica said it planned to save £200mln in 2016 as part of a drive to cut costs by £750mln a year by 2020.
It added that it was working on assumed flat real prices of $35/bbl Brent oil, 35p/th UK NBP gas and £35/MWh UK power.