Grid-tied solar Kits are connected to the utility company’s power lines. If the home or business needs more electricity than it can produce it draws energy from the grid and if it is producing excess electricity, it injects it into the electrical grid. Electricity added to the grid is credited to the homeowner or business’ electricity bill. When power is drawn from the grid, this electricity credit is reduced. This process is called “net-metering” and is accomplished with a bi-directional or smart meter.
There are also grid-tied installations that reserve power in a battery backup that is used during power outages. The solar panels charge the batteries so that continuous power is available, even if the utility grid is down. When the outage is fixed, net-metering resumes.
Off-grid systems are usually implemented in locations that are too remote to receive service from a utility. These systems can generate AC power that can run regular appliances and electric devices. They store power in batteries that are used to supply power when sunlight is not available. Those that generate DC power are used to power remote telecommunications gear, appliances used in boats and recreational vehicles as well as farm equipment. DC is less expensive than AC because it does not require an inverter. AC systems can power common home appliances.
Source: Peak Solar