This component is primarily used in off-grid systems, and can provide backup electricity during times where the solar arrays cannot produce enough to match the household loads, such as a few weeks of overcast weather during the winter. There are many different types and models of these, and they can run off of a variety of energy sources, such as gasoline, diesel, propane, natural gas, etc. Ideally they will only have to run 50-200 hours per year to help the house get through the worst stretches.