Grid-tied systems are the most common type of solar PV system. Grid-tied systems are connected to the electrical grid, and allow residents of a building to use solar energy as well as electricity from the grid. Grid-tied systems do not need to produce 100% of the electricity demand for a home or business. When there is no demand for energy, the solar panels send excess electricity back out into the grid for use elsewhere. When a home or business is using energy, but the solar panels aren’t producing enough energy (at night, or on a stormy day), electricity from the grid supplements or replaces electricity from the panels. Owners of a grid-tied system complete a net metering agreement with their utility. This agreement allows utility customers to receive credit for the excess energy they generate, typically credited as a kilowatt-hour credit on the next month’s bill. Net metering policies and agreements are different for each utility. Living with a grid-tied solar PV system is no different than living with utility electricity, except that some or all of the electricity you use comes from the sun. Grid-tied systems do not provide protection from power outages. When the electrical grid fails, grid-tied systems will not continue to operate. This allows utility employees to fix the power lines safely without wasting time identifying solar energy systems that are still feeding electricity into the power lines.
It is possible to install a solar system that is independent of the electrical grid. This is called an off-grid system, and it requires that the solar panels are able to produce enough electricity to cover 100% of the energy needs of the building. Most homes have higher electricity demand in the evening or at night, so off-grid systems usually incorporate either a battery (to store energy produced during the day), a backup source of energy (like a generator), or both. Off-grid systems are more complex and less flexible than grid-tied systems.
Off-grid systems are most common in remote locations without utility service. Off-grid solar-electric systems operate independently from the local utility grid to provide electricity to a home, building, boat, or RV (or remote agricultural pumps, gates, traffic signs, etc.). These systems typically require either a battery bank (to store solar electricity for use during nighttime or cloudy weather) a backup source of energy (like a generator), or both. An off-grid solar system must be large enough to produce enough electricity to cover 100% of the energy needs of the building. In all off-grid scenarios, electrical usage must be monitored and kept below the maximum output of the panels and batteries as there is no grid-source to supply excess power.
Grid-tied systems can be upgraded to include a battery backup: a bank of deep-cycle batteries. The backup battery is charged by both the grid and the solar panels. In the event of an outage, the backup battery will need to be switched on either manually or with an automatic system to provide backup power to the building.