Williams is unlikely to run with KERS regularly this season because of concerns about the weight of its in-house system.
The team had originally targeted next week's Turkish Grand Prix for its KERS debut. But despite making good progress with developing its flywheel system, director of engineering Patrick Head has admitted that it will be difficult to integrate it into the Williams FW31 without having a negative impact on its handling.
Head admitted that the nature of a flywheel system means that it is significantly heavier than the battery systems used by the rest of the field. At 35kg, its in-house KERS is understood to be around 10kg heavier than the McLaren-Mercedes system.
Although this means that the team will struggle to gain any performance benefit out of the system at most tracks, it could still give it a run later in the season at a low-speed track, where the need to get the weight distribution as far forward as possible is less critical. This raises the possibility of Williams racing its KERS system at, for example, Singapore.
"The fundamental problem is, and this was raised early on, that even though our system has got a lot of capacity, it weights 35kg," said Head. "We can't get the optimum weight distribution if we're running the system.
"We're keeping working on our KERS. I'm not saying we won't run it this year but it will be difficult to run it on the car and have the car set in its optimum performance window.
"But if you go to a slower speed track, your optimum weight distribution moves rearwards anyway."
Head believes that the flywheel KERS system will offer a bigger advantage next year, when the imperative for a forward weight distribution is eased by the introduction of a narrower front tyre. However, it is not yet clear whether teams will be able to use their own systems, or the mooted common KERS unit.
He also added that he was happy with the progress the team has made on its KERS system along with its in-house company Williams Hybrid Power.
"KERS is certainly still under development," said Head. "We've developed our own motor and they are amazing things. Quite small - about the size of a part-used lavatory roll and it puts out 80bhp. That's been running on our rig back in the factory both absorbing and feeding back in power.
"The inverter is all done and that's running well. The flywheel has been running on the rig and we had a few development problems associated with mounting a flywheel within a car, but we've solved them now."