Our mate Glort is right , as usual, (very irritating)
The amount of fuel used is totally dependent on the load. Be it a diesel or petrol engine the governor will try to maintain a steady RPM by increasing/decreasing the fuel pumped into the ICE. A heavy load burns more fuel, a light load less. Diesel engines run better under a heavy load and suffer from bore glazing and carbon build up under light loads. Your Lister L will be a lot less economical than a diesel (and noisier) but shouldn`t suffer from any of the problems associated with diesels at low loads.
Diesel engines have the advantage that they will run on a multitude of fuels: regular diesel, waste oils and vegetable oils, depending on where you live and what is available for free. Petrol is only available from the local, government taxed outlet, depending on your occupation some of the cost may be tax deductible.
Batteries are a PITA, they never last as long as they claim in the sales blurb and when you really need them they let you down. They are also very susceptible to damage from cold weather. I have two starter batteries for my back up generator that have always let me down, I now have both coupled to permanent maintenance chargers which come on every evening as the temperature drops.
The beauty of Lister engines is that they come with a crank handle, when every thing else has failed you will still have power and be able to maintain a comfortable living environment while everyone else sits in the cold and dark.
Bob