Mr Reasonable - that is why the excess supply goes back into the grid - so the daytime usage is not wasted. These are not like the pretty solar lights in the backgarden charging a battery!
Gavin - you have a PhD in physics, which is well used in your engineering career i'm sure, but sadly does not seem to help understand something that is not that difficult.
(i) Charge is generated, has a value, get paid for value, deduct from cost of generation including installation as a cost.
(ii) Individual panels and arrays produce small amounts of electricity.
(iii) Collectively, on a large scale produces more electricity.
Does your Phd follow all that, or should I make it simpler for you Gavin?
My advice to Dave - don't go there unless you have testimony signed by Faraday (and do not tell anyone if you are a member of the RI or Gavin will get to hear about it and try to trump it with his Phd - which cost how much on the internet these days?)