Solar is far cheaper than that in my location - especially if you put the panels up yourself, and storing 100 gallons of gas (400 litres) to run for 3 weeks is far more expensive.
I realize this isn't the case for everyone, but I already have a large propane tank that supplies the house. My generator is tied in to that. So I'm not storing fuel specifically for the generator, but if it came down to it I could probably power the house for a couple months at least. And our geography and flora are pretty unfavorable for solar. So as with most things, it just depends.
