AC and DC Coupling
Backup for preexisting PV systems
The most common solar photo-voltaic system is the grid-tie-only system. Owners of these systems are often surprised by the fact that they have no power during a grid outage even if the sun is going strong. Grid-tied-only inverters can not and are not allowed to function without the power grid. In order to benefit from your existing solar photo voltaic system during a grid outage, additional equipment including a battery bank needs to be installed. AC coupling and DC coupling are two common solutions.
With DC coupling, a battery bank gets charged from the PV panels. During a power outage a second, smaller inverter powers critical appliances like a water pump or furnace around the clock. DC coupled systems can be efficiently scaled for smaller needs, to support just 1 or 2 critical appliances and therefore tend to be less expensive. This solution is compatible with most grid tied inverters.
With AC coupling a battery bank gets charged from the power grid. During a power outage a second inverter will fool the grid-tied inverter into ‘thinking’ the grid is present so that it can power your house and charge the batteries. During the no-sun hours the second inverter will power critical appliances like sump pumps, well pumps, furnaces and freezers. Depending on battery bank size this solution can cost $5,000 - $25,000. This solution is compatible with most grid-tied inverters including micro inverters.