Although sunlight is free, the equipment necessary to convert and use it is not. It is usually too expensive for PV to supply all household energy and power requirements. Most PV system owners use another energy source for cooking, space and water heating, and to run large loads such as air conditioners, power tools, and washing machines. The system owners sell their excess electricity to the utility company, and buy electricity when their PV system cannot meet their electricity demand. In effect, they use the utility grid for storage.
Inverters are available that produce electricity of the quality acceptable to electric utilities.
They incorporate safety features to automatically disconnect the PV system from the grid in case of a power outage on the grid. Some intertied systems can charge a small-capacity battery for emergency power if the grid goes down. Most utilities power purchase rates are much lower than the costs of PV-generated electricity. At current prices for modules and inverters, it may be difficult to justify a utility-integrated system on the basis of electricity sales to the utility alone. Several utilities are demonstrating grid-integrated residential PV systems.