Half of the council's 300 machines are to be fitted with solar cells by the end of 2008, and already 50 machines have been converted to run on the renewable technology.

Around 30 machines are also being fitted with chip-and-pin debit card technology, so drivers can pay with credit or debit card as well as with coins.

The move follows last year's decision to allow motorists to pay via their mobile phone at the machines.

Parking manager Ken Ellcome said: 'Drivers want to have a choice of easy ways to pay, and we're bearing in mind that Portsmouth is growing as a tourist destination.

'We're also busy converting our machines to run on solar power. This will save council tax payers' money, because it's much cheaper than mains electricity.

'Unfortunately, the chip-and-pin machines can't run on solar power yet, but our supplier is working on a solution for us. Portsmouth is working hard for a greener parking service.'

Councillor Lynne Stagg, the council's cabinet member for traffic and transportation, said: 'Any way of lessening our impact on the environment has to be good. These solar-powered machines do that, but are cheaper to run as well.'