There was a neat exchange on this point in the House of Commons on Jan 18th:
The hon. Gentleman talks of how cheap onshore wind is as a renewable. Does he not accept that it must be backed up by fossil fuels, which are not so cheap? If the full system cost of onshore wind is taken into consideration, it is one of the least affordable renewable technologies that we have. Callum McCaig: So we are backing up the cheap renewables with fossil fuels that are not so cheap, and the solution to that is to use the fossil fuels that are not so cheap all the time? That sum does not quite add up. I am not sure that I have worked out the equation.