I would be very skeptical about the more optimistic posts above and echo those that are pointing out the difficulties. First thing any electrician will ask when surveying a new build garden building is 'How do you intend to heat it' even a poxy little fan heater draws significant amps/watts not sure which. My partner is an architect specializing in sustainable development and just happens to be having pvs installed on her house, garden office this week. If you are near Oxford you are welcome to come have a look, she will talk you senseless about the whole thing. One thing to point out- she has a ruled out battery storage after a lecture we both attended at Oxford University by industry and academic leaders in the field who trashed the idea of batteries in a domestic setting.