The UK only currently generates 4% of its electricity supply from solar, and only a fraction of that is from domestic sources (basically panels on roofs).

The capacity to generate a certain amount of KWH varies depending on where you live in the U.K, but the most populated areas in the South East get the most sun and are so the most viable.

The key equation when calculating how many KWH can be produced per domestic solar installation is KWH/KWP (Kilowatt hours over Kilowatt potential), which ranges from 2 to 3 in the United Kingdom:

So if you were to install a 3KW system in the South East you would generate, on average, 9KW hours a day, further north average generation would steadily decrease to 6KW a day.

6 KW hours is sufficient for a two person household if yer careful...

The easy way to look at this is this: 6KW hours is equivalent to 6 1000W appliances running for 1 hour a day, that is A LOT of energy for basic household needs.

Around 2.5K of that is going to be taken up by the fridge and doing one load of washing a day, but then you've got 3.5 KW hours left for other appliance, most of which don't drain that much electricity - televisions, computers, lights, there's not much else draining.

Of course I am assuming here that you're using gas or something else for water and space heating, and don't want a dishwashwer or microwave.

If you want a lot of modcons, then a 5KW system should be all you need, and should power a typical family home, probably even if you have an electric shower.

Limitations offset by grid-feed in...

The most obvious limitation is, in one word, winter. Especially in December and January there is a lot less sunlight falling on those panels than in July, for example!

The variation is ENORMOUS

However, this variation could be offset by a system which is not entirely off-grid, so in summer households with panels can sell their excess energy to the grid, and in winter just pay for it.

How much roof space do you need...?

For a 5K system you need around 32 square metres of roof space, which you just about get with a family sized home, so for a 3KW system, you'd need around 22 Square metres or 4 by 5, which is doable with a small roof.

Probably best not to rely totally on solar

I'm not suggesting here that the UK should rely entirely on solar, as we all know it is unreliable, but the more we have, the less we rely on nuclear and other sources of electricity, and there is A HUGE untapped amount of roof space out there that could be used to generate more from the sun!

More details on KWH and KWP here.