What Does a Grid-Tied Solar System Do With Extra Power, and Why Does It Shut Off in a Blackout?
Summary
Extra power flows out to the grid, and where net metering applies, the customer is credited for what the system sends. Solar used inside the home never passes through the utility's meter, so the household does not buy it. In an outage, a standard grid-tied system disconnects and stops for safety, and it will not power the house unless it was designed with storage and an inverter that can run without the grid.
Where the power goes during the day
According to the Department of Energy, distributed solar sits "behind" the meter, so customers do not pay the utility for the solar power they generate and use. When production exceeds what the house is drawing, the surplus goes onto the distribution grid, and under net metering, customers are credited for the amount of power they supply. DOE puts small-scale solar at about one-third of U.S. solar energy production.
Picture a roof array making 4 kW at noon while the house uses 1.5 kW: the other 2.5 kW heads out to the grid. At 6 p.m., with the array down to 1 kW and the house pulling 3 kW, the missing 2 kW comes from the utility.
Riding through small glitches
The inverter converts the panels' DC into the AC the grid uses. DOE describes the standard for small inverters, like those on household systems, as staying on through small disruptions in voltage or frequency, and disconnecting and shutting down if a disruption lasts a long time or is larger than normal.
Why it goes dark in a blackout
An islanded section of grid could damage connected equipment or injure workers who believe the line is dead. That is why, DOE says, many solar energy systems are programmed to detect islanding and disconnect. Solar-plus-battery systems can keep a home running during an outage only if they are designed to, and they rely on advanced inverters to operate without any support from the grid.
Sources
- U.S. Department of Energy — Solar Integration: Distributed Energy Resources and Microgrids Basics
- U.S. Department of Energy — Solar Integration: Inverters and Grid Services Basics
- U.S. Department of Energy — Solar and Resilience Basics
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