It's not going to be really possible to guess at what size you need without a real good idea of your loads and tolerance for inconvenience. IE, if you're going to be running for a week after a hurricane, and you only want to use a 10kw unit, can you tolerate shutting off the AC for 2 hours to do laundry (assuming electric water heater and dryer)? If you can't, you need a bigger unit.
I had pretty good success with a MEP-002a (5kw) running my house, but it definitely felt the load when my water heater and pump were both on. Throw in 3 window shaker AC units in summertime, and it would trip the breaker. I switched to a MEP-802a, and found the same thing. I've run the 002a several days on very little fuel (5-10 gallons), 802a hasn't been needed long enough to measure yet.
Then I got a MEP-803a. That ran at 75% load with everything on in the house except AC. So for my house, we'll be just fine with a 10kw rated unit. Most likely using 1 gallon of fuel per hour, but again, haven't run long enough to measure.
A year ago, I bought a 23.5kw 1-ph/35kw 3-ph Onan DGBB with a Cummins 4bt that has 2 hours on it (one of which is mine just from running it to check it out). I got it literally brand new, it had been put in storage after being installed in a building that had no exhaust riser or rights (talk about expensive poor planning).
It's way too big for my house, so I'm most likely going to sell it, and will probably ask the same price as a privately sold MEP-803a. It's a commercial standby unit, much better designed and built than most any MEP. It also uses less fuel. And it does not wet stack with long no-load periods. I have experience with several units like this, wet stacking has never been a concern until I got into MEP's.
Good unit, too big for me, and might end up being the new motor for my Jeep...