Obviously, any fuel-based backup for your generator is confined to expedient/emergency use only for charging your batteries. A fuel powered generator is a good thing, but you don't want to rely on fuel for a variety of reasons.
1. Fuel will run out
2. In all likelihood there will be a severe risk from carbon monoxide or other hazardous fumes. You will always have to be worried about monitoring the exhaust scheme at all times.
3. Generators are complicated. They will someday break down. Maybe you'll repair them. They'll break down again. Eventually, without a supply chain from civilization, it is likely your generator will break down finally and never start again.
4. They can be expensive.
Now don't get me wrong - generators are a must have in any shelter. I don't know if it is in your best interest to rely on them as your primary energy source. Especially during a twenty year nuclear winter.
I do not think solar power should be your main preference for your shelter. I'll list the reasons, feedback is welcome.
1. Solar panels wear out quickly, lose efficiency and are easily damaged in a variety of situations.
2. They are complex and can be hard to replace and maintain.
3. They can be expensive.
4. Obviously, in a lot of situations they can be useless if there is no light. Like during an extended nuclear winter or after several supervolcanos have blown and clouded the atmosphere with ash for a couple of decades.
I have come to the conclusion that the single most sensible primary energy system for a shelter should come from wind generation. I'll tell you why.
1. A 12-24 volt DC wind generator can literally be built from junk that other people throw away. If destroyed or damaged, a new one can be built in hours. Several can be built and stored in the shelter as spares. A wind generator can be demounted during an attack preparation phase.
2. In almost any situation, there will always be some wind. Especially enough to power a 12-24 volt DC generator motor.
3. A wind generator is simple. If it breaks, the cause is nearly always obvious.
4. A wind generator is durable. Short of gale force winds, most wind generators can stand up to awesome weather - and still keep generating electricity!
5. Wind generators are flexible. They can be the size of a shoebox or a ten story building. They can use a number of designs and all of them will generate useful amounts of electricity, be it from rotors, vanes or various other innovative uses of wind force.
6. Wind generators are good long-long-term strategies. The wreckage of TEOTWAWKI would yield the raw materials to build a billion wind generators for anybody who could gather the scrap.
Vault-Co plans to build an expedient wind generator very soon for less than $100.00 to prove it can be done, as soon as Sparkgap gets a roof on it.