A Lister Petter or another better built 1800 rpm diesel engine powered 4 pole 100% duty cycle head. It will be heavy but you can make it mobile if its on a cart or trailer. It will sip fuel compared to any 3600 rpm unit and in times of need.... I know I can get heating oil. Gasoline may be harder to find. I have a 500 gallon tank under my porch and a 275 gallon tank in the garage.
For most people, I don't think diesel generators make sense. Here's why. It's true that diesel generators are more reliable and fuel-efficient than gas generators. Because they run at lower RPMs, they are usually also quieter. But they're typically around double the cost of a gas generator for the same wattage.
I don't think that the average generator user is going to put enough hours on the generator to make up the difference in fuel cost. If you're running the generator day in and day out at a work site, that's one thing. But if you pull out the generator a few days a year when there is a power outage, or you go RV'ing somewhere, it's going to take longer than many people are likely to own the generator to make your money back in fuel--especially now that diesel is pretty comparably priced to gasoline. If you have a source of alternate fuel, such as off-road diesel, or better yet, waste vegetable oil or biofuel that you refine yourself from e.g. sunflowers (yes, people do this), that's another story. But if you're buying fuel at the pump and not using the genny pretty much on a daily basis, the fuel efficiency doesn't seem like it matters.
Diesel generators seem to use roughly half the fuel of gas generators, for a given wattage output, and are roughly twice as expensive. Consider a 7000 watt generator. You can get that for around $700 in gasoline, or around $1400 in diesel. Say that gasoline and diesel are priced the same, at about $3.50 per gallon, which is the case at gas stations around me. You need to make up $700 in gas to break even. For every hour you run the generator, you make up $3.50 * 50% fuel usage = $1.75 with the diesel generator. $700 / $1.75 = 400 hours you have to run the generator before you break even on the diesel. That doesn't sound like a lot of hours, but if your generator is intended for emergency usage, you're going to put, what, maybe 50 hours a year on it? So you'll break even in eight years.
The more expensive diesel and gasoline get, the lower the break-even becomes, so that may factor in. But if diesel gets more expensive than gasoline, as it was up until recently, the break-even gets longer. I'm not saying it's cut-and-dried either way, but you should at least do the math before you shell out for diesel.
