British Gas owner Centrica swung back into profit this year despite losing thousands of customers to rivals.
Bosses at the company also revealed they are starting to see a slight rise in the number of employees being told to self-isolate via the NHS Covid-19 app but said it was too early to say what impact it would have.
Chief executive Chris O’Shea added: “We have seen an increase over the past 10 days, in sickness absence, primarily in our engineering force.
“We have probably slightly more reschedules … there’s no material impact but it’s early days.”
But overall sickness levels remain below previous highs at the height of the pandemic.
He said: “We had a high of just under 20% around the end of our industrial action.
“It’s been down below 10% recently and is now back up at about 11%. We have seen an increase in the past week and a half but I think it’s too early to tell (what impact it will have).”
Customer numbers have dropped by 114,000 at British Gas, yet profits soared by 121% to £172 million due to the cold weather and a reduction in Covid-related costs for the division, versus the entire Centrica group.
Several customers quit because a recent rise in the energy price cap led to households shopping around for better deals, the company added.
Around 144,000 customers also left its services business – down 4% on the same period a year ago.
Mr O’Shea explained the profit boost was also due to a rise in business customers increasing their usage.
He said: “The small business community was impacted far more by Covid in 2020, when everything was shut down than in 2021. A £20 million increase in the profits relates to that.”
The Covid-19 crisis and industrial action taken by staff cost the Centrica business £80 million in the first six months of the year – an increase of £58 million in the same period a year ago.
But pre-tax profits soared to £907 million in the first six months of the year, compared to a £462 million pre-tax loss in the same period last year, following higher energy prices benefitting its energy generation division.
Mr O’Shea warned it was too early to say whether Covid could have further impacts on the business and said the unwinding of Government support would be key.
He said there had not been a material increase in non-payments of bills, but warned: “What we’ve got to remember is Government support schemes for jobs are still in place just now and it remains to be seen what the long-term impact Covid is on employment and on individuals.
“I think we’ll only know the true scale of this… when the Government funding and other support schemes start to taper off, and we find out the true state of the labour market.”
The rising costs of energy helped Centrica’s commodity division as it could sell the units of energy produced at a higher price due to demand increasing from businesses as the global economy opened up again.
And bosses said they still intend to exit oil and gas production, having made progress towards pursuing alternative sale options of their assets.
But they said they are reconsidering whether to sell a 20% stake in nuclear power stations, having first announced plans to leave the division in 2018.
Earlier this month, a long-running dispute at British Gas ended after workers accepted an improved pay deal.
Around 7,000 British Gas engineers staged 44 days of strike action stretching over several months.
The GMB union declared an end to the so-called “fire and rehire” dispute after its members backed an agreement by three to one.