Across France this is around 11gigawatts, or over the year almost 10 terawatt hours of electricity generation per year. This is a very sensible idea, as it is a second use of the land.
In the south of England, there are now 2000 panels placed over car parking spaces. These offer a capacity of almost 1MW hour. Each parking space covered, has a capacity of about 2kw. In the UK, the countries target of solar by 2030 is to have 40gw of solar capacity, up from 15gw currently. It is estimated, that almost half the remaining target would be met by carpark roofing – which suggests that we could hit far more than 40gw of solar.