Load does vary, yes, but with ones I've worked with it isn't very much, they run pretty much full bore all the time. How much change in fuel consumption is there if you are just powering a couple of 50W lightbulbs vs full on household use? I'm thinking something that when you have no draw, you burn no gas. For commercial purposes, this isn't as effective as a normal generator and I accept that, just like I accept that hybrid cars have no advantage if you spend all your time on the highway other than the extra mass means a Prius wins if it gets attacked by a Hummer. (There are photos of that accident online.)

What got me thinking about this is my folk's generator- they need a few lights, a pump for the well, and every so often the fridge kicks in, but most of the time when it's running, it's only feeding that couple of 50W bulbs and sometimes a water pump. It's the smallest that they could find that didn't seem like it would die if it was run for more than six hours, but it still feels like it's wasting gas most of the time.

And yes, I know that charging a battery isn't as efficent as putting the power in. But at some point, you are under the minimum of what the generator's output is. And this is the kind of thing where if you put a small waterwheel generator under your storm gutter, or a couple of solar panels...