I've no special insight into this other than reading exactly what it says in the relevant section of the Finance Act - which is:
20 Income tax exemption for domestic microgeneration
(1)In ITTOIA 2005, after section 782 insert—
“782A Domestic microgeneration
(1)No liability to income tax arises in respect of income arising to an individual from the sale of electricity generated by a microgeneration system if—
(a)the system is installed at or near domestic premises occupied by the individual, and
(b)the individual intends that the amount of electricity generated by it will not significantly exceed the amount of electricity consumed in those premises.
(2)In subsection (1)—
“domestic premises” means premises used wholly or mainly as a separate private dwelling, and
“microgeneration system” has the same meaning as in section 4 of the Climate Change and Sustainable Energy Act 2006.”
(2)The amendment made by subsection (1) has effect for the tax year 2007-08 and subsequent tax years.
Until HMRC actually look at this in individual cases then I don't think anyone is going to know for certain how the "not significantly exceed" part is going to be interpreted.
But at least you should draw your customers attention to the possibility that this could happen.