Why the solar on your roof goes dark when the grid does.

The roof in a shutoff

Solar is a source only when the house is designed to let it be one.

The most common surprise in the first shutoff is that the house with solar panels is as dark as the house next door. That is not a defect. A standard grid-tied solar inverter is required to stop producing the moment it loses the utility, so that it never pushes power into a line a utility crew believes is dead. The panels are fine, the sun is out, and the inverter is sitting in its mandated safe state. Unless the house has a battery and an inverter capable of forming its own grid behind an islanding device, solar contributes nothing to an outage.

Inside a residential microgrid the roof has a job. When the gateway or transfer device islands the house, a battery inverter that can grid-form provides the reference the solar inverter needs to keep operating, and the solar recharges the battery through the daylight hours of a shutoff that runs for days. The generator then carries the heavy loads and the nights, the battery carries the quiet hours, and the roof refills the battery each morning. On a long shutoff that combination uses a fraction of the generator fuel a generator-only house burns, which matters on propane in Malibu and Topanga, where the tank is the limit.

The electrical design has to make the sources agree. A solar inverter, a battery inverter and a generator each think they are in charge of voltage and frequency, and only one can be. In a well-designed system the battery inverter forms the island, the solar follows it, and the generator is brought in as a source the battery inverter manages, with the solar held back or shed when the generator is running if the equipment requires it. Those are settings, interlocks and sometimes specific hardware, and they are the reason County Line treats a house with existing solar as a design problem rather than a generator sale. The existing solar is surveyed first: inverter model, capacity, how it is interconnected, whether it can be integrated or has to be isolated behind the transfer switch.

There is also the economic side, which has changed. Under California's current net billing rules, exported solar is credited at far less than it once was, and the equipment that makes solar useful in a shutoff, a battery, is also the equipment that makes it useful on an ordinary evening by storing the afternoon's production for the night. A homeowner designing a residential microgrid for the outages often finds that the battery pays its way on the weekday bills too. County Line does the electrical side of that; the tariff decision belongs with the owner and the utility.

What solar does not do: it does not carry a house at night, it does not start a well pump on its own, and it does not run in a shutoff without a battery and the right inverter. A solar company that promised backup without a battery has promised something the equipment cannot deliver. The honest page in this series says when solar should stay a bill-reduction investment and nothing more.

Back to the overview: Residential Microgrid.