When the sun doesn't shine and the wind doesn't blow...
Backup power is needed for those times when your system's batteries are discharged, usually when you've consumed too much energy or when there has not been enough renewable energy coming into your system. This happens to photovoltaic (PV) systems during long periods of cloudiness, especially during the shorter days of winter. It can happen to hydro systems during the dry season, or to wind systems during calm. If you live outside of town in a remote area, a generator is vital in case you run out of power.
There are basically two kinds of generators used in RE systems. The most common is the standard 120/240 VAC generator you can buy at a hardware or department store. Since they produce alternating current, they require a battery charger to change it to DC for your batteries. This is not the most efficient way to do things. First, most battery chargers cannot take advantage of the full power available from an AC generator, causing them to run at less than their full efficiency. Second, the charger itself has built-in inefficiencies, especially if it has a large transformer.
The other kind of generator is low voltage DC. These generators are capable of putting nearly their full power into a battery bank, since no additional charger is required. The disadvantage of these is that they cannot be used independently of an inverter-based system as household 120 VAC backup.
If you have access to any combination of solar, hydrogen, and wind, you have the advantage of access to complimentary systems. Often when one isn't able to produce, the other is at its prime. Hydrogen can be used as a source of energy and has hardly any detrimental effects on the enivronment. Research a variety of renewable energy sources you can utilize to ensure minimum consequences including biomass and geothermal power.
Backup power can come either from a generator or from the utility grid. Most renewable energy users have the goal of not using backup power. In the case of a generator, it's a noisy, pollution-belching machine, and in the case of the utility grid... well, you're avoiding that for a reason, right?
Essentially, if you need back up power, your best option is a solar powered generator as they are more environmentally friendly. Any invention, whether it be a vehicle or a household appliance, that can benefit the environment should be the more appealing choice.