I have googled enough 
I would guess that getting your electricity from the local grid and best priced supplier will be cheaper for a 415v supply. Depending how large a supply you want you could be charged 1 or 2 pence a unit cheaper than domestic prices (or as much as half if its a really huge supply you need). Cheaper becasue of economy of scale for the generators – they can produce electricity lots cheaper than you.
Generators might be cheaper to run if you can find a cheap source of fuel for them and use the generator over many years, 20 or so.
Answerer above had a good point, why not get a kiln that is heated by the fuel directly rather than electricity? Could be cheaper depending on the fuel of course
