This question is based on the premise that using "cheap" biofuel in a diesel engine to power a generator (and not using batteries) is better than the alternatives. My understanding is that if you must grow crops to generate electricity that burning more of the plant is better - the cogen/trigen units I've seen mostly run of primarily-cellulose pellets rather than plant oil.
A 25kW generator will use about 2 gallons an hour at full power, perhaps 0.5 at idle, so I'm guessing 20 gallons a day. I suspect using a battery instead of idling could drop that but exact numbers are hard to find because RAPS using backup generators dominate the search results. Oilseed crops run as high as 240 gallons/acre/year (gross yield, not net!) and at a societal level we can't both eat and drive biodiesels.
20 gallons of fuel a day is about 7000 gallons/year requiring somewhere between 30 and 300 acres of arable land to grow. Alternatively, the average deep-fryer shop should need to dispose of about 5 gallons of oil a week but in practice it seems to be less than half that. So 140 gallons a week will mean visiting about 30 fast food outlets, plus a few extra to supply the vehicle fuel for those pickups.
Is it really better to burn plant oil than the whole plant?
As secondary questions:
- is it more efficient to use a battery pack instead of idling the generator at low-demand times?
- are my numbers for fuel consumption and DIY oil cropping right?
- is it practical to run a generator off scrounged vege oil like this?