Some one I know has been offered a solar panel deal

The information I have been given is that the panel installation would
normally cost 10k

At that rate it would pay for itself in ten years due to a combination of
electricity not paid for and inconme from the excess going into the grid
they say

They are offering the panels for 250ukp and say that they make their money
by taking the money from the excess going into the grid

My current electric bill is about 80ukp per month

How big would the panels need to be to generate in excess of this usage?

I cannot see how this could ever work so can some one please explain

Regards