In solar electric systems, generators are useful backup systems, especially in rural areas. There may also be times when you need to run very large loads that are beyond what your energy system was designed for, such as large power tools or a clothes dryer. Clearly, you want to use the generator as little as possible, because it costs you money for fuel, and is noisy, and emits pollution. You'll want to try to use the generator at peak capacity for the shortest practical period, say drying a load of clothes while recharging your batteries.
You have a lot of choices for the generator type, with diesel and propane being the most practical, and gasoline being the most convenient. You can get generators that will automatically cut off when the batteries are full, and you'll probably want a separate enclosure for the generator.