In my experience, very few sites actually charge for electricity, simply including it in the pitch fee - so a fuel cell breakeven point is infinite as there's no reduction in site charge by being self-sufficient.
I stayed on a CL in July, ie peak summer, which had meters fitted but not used for charging - over the whole fortnight we were there, the average electricity cost logged by the meter was just 83p per day, which I guess was about 5 kwh/day - so what's the price of a fuel cell capable of 5 kwh/day for summer use - I hate to think how expensive a unit would be that copes with winter usage, say 50 kwh/day !