Cynic (Cynic ) gurgled happily, sounding much like
they were saying:

A big problem with a single genny is that it must be large enough to
supply your peak load. A generator that size is very inefficient most
of the day when the load is low.

Yes, but given...

An on-site generator can also be used
to sell surpluss electricity back to the national grid.

....can the over-size generator not be providing income by selling that
surplus capacity back to the grid? I s'pose it would depend on the exact
cost of generating versus the price paid by the grid as to whether it
would be profitable...