Sorry for bringing up tedious facts and data, but that's what engineers like to do.Don't you just hate a smarty pants that's either been on the course or even worse-----------------------university
Here's another observation:
My little Kipor IG1000 generator, similar to a small Honda, rated at 900W continuous, is specified to produce 1 kWh using 550g of petrol that I can get anywhere, and weighs 14kg. That's about 0.75 litres, costing about 90p.
So the efoy fuel costs are five times higher, but it is a completely different thing.
The smallest efoy (though they all the same physical size and weight, just different fuel cell stacks inside) outputs 40W. Enough to deliver 80Ah per day (running continuously).
Consuming 0.87 l of fuel, costing £4.35
If I increased my charger capabilities to say 50A which the generator and even a single LiFePO4 should easily handle, with spare 240V to power other things as well, I could put in 80Ah in about 1.6 hours running. And do it as often as required. For a fifth of the fuel cost.
Put it another way, 5000 hours of efoy usage ~= 200 hours of generator time. During which the generator maintenance will have been one oil change and a check of the spark plug. It should be nicely run in by then, fit for many years more service.