Generator and battery: who carries which hour.

Two sources, one plan

The battery takes the first second. The generator takes the third day.

The argument people have in showrooms, generator or battery, is the wrong argument for a house in a shutoff zone. A home battery of the size that fits on a garage wall holds enough energy to run a modest set of circuits overnight; it does not run a well pump, an air conditioner and a kitchen for three days. A standby generator does, for as long as the gas main holds pressure or the propane tank holds fuel, but it takes ten to thirty seconds to start, it makes noise, and it burns fuel at night to carry a refrigerator. Put together under one electrical design, each covers the other's weakness: the battery carries the house through the gap before the generator starts and through the quiet hours, the generator carries the heavy loads and the long days, and if there is solar on the roof it refills the battery each morning.

Sizing is done as one calculation, not two. County Line adds up the critical loads with their starting currents, which for a well pump or an air conditioning compressor can be several times the running current, and decides which source has to be able to start which motor. A generator is sized to start and run the full critical list; a battery is sized to the overnight energy and the instant power the house needs before the generator is online. A battery that is too small drops out the moment the pump starts; a generator sized without the battery in mind runs all night at a fraction of its load, which is hard on the engine and loud for the neighbors.

The wiring is where most combined systems go wrong when they are installed by separate companies. A generator transfer switch ahead of the main panel and a battery inverter with its own critical-loads subpanel were each designed assuming it was the only backup in the house. Put both in and, during a shutoff, the generator can be feeding a panel the battery inverter has just disconnected from, or the inverter can be trying to charge the battery from the generator while the generator is at its limit. The residential microgrid design settles this once: one point where the house islands from the grid, a clear order of operations, inverter settings that recognize generator power as a source, and a panel layout where every critical circuit is on the protected side of both devices.

Fuel and chemistry both need a plan. Natural gas is the simplest generator fuel in a shutoff because the gas system is not what the utility turned off; propane needs a tank sized for the days the design assumes, with the supplier told the house has a generator. Lithium batteries have a temperature range and a cycle life, which is why the design lets the generator carry the daytime heavy loads instead of cycling the battery flat every afternoon. Both pieces get the twice-yearly service County Line already performs on its generators, with the battery system checked on the same visit.

There is a version of this house that needs no battery at all, and a version that needs no generator, and the honest page in this series says which is which. For a large property in Malibu, Hidden Hills or Montecito with pumps, gates and a family at home during fire season, the pair is usually the right answer, and designing them together is the whole point.

Back to the overview: Residential Microgrid.