NHS trust chiefs believe they could make as much as pounds 10m a year selling power from their stand-by generators to the privatised electricity industry.
The fund-raising energy plan will be put to health bosses in seminars in Birmingham, Bristol and Cardiff next month.
By running emergency generators that normally operate when there is a power cut, a hospital could bring an extra 3,000 kilowatts a day into the national grid, enough power to satisfy the needs of 600 families.
Larger hospitals have enough generating capacity to set up in business on their own, but smaller ones will join forces to sell their electricity.
The Royal Free Hospital in London is already generating its own cheap electricity, and new gas turbines are now being installed so that it can make sell power to London Electricity.
The energy seminars are being organised by the solicitors Bevan Ashford, which represents 120 NHS trusts.
"We want the trusts to know what they're sitting on," said the firm's energy expert, Gareth Dodds, former commercial contracts manager for South Western Electricity.
"At the moment their generators are dead money which can be put to better use to generate extra income."
But Labour is unconvinced about the idea. "We want hospitals to use resources efficiently as long as patients get the benefit," said Shadow health minister Tessa Jowell.
"But they should not be forced to strike up deals like this just to make ends meet. Hospitals should not lose sight of what they do best - which is making people better."
A Health Department spokesman added: "Income generation is actively encouraged, as it provides extra funds which can be ploughed back into patient care."Reuse content