Set the stage:

Say there is a massive grid disruption like what happened to Puerto Rico after Hurricane Maria. I live somewhere were air conditioning may be needed for some groups to remain healthy.

I have a 7 kWh PV solar install and a 6kWh stand alone generator(Its on wheels).

Say to run my house with air conditioning: I need 10kWh of production and demand for air conditioning matches the solar cycle.

Without air conditioning the generator can handle everything.

My idea is:

I could throw the main breaker and for redundancy cut the utility tamper lock and pop off the meter to prevent back feeding to the grid. Then I could wire the 240 output of the generator in to the distribution panel alongside the 240 out of the solar.

The solar will not run when the grid is out. I assume it knows when the grid is out by the lack of a AC sine wave. So connecting the generator will give it a wave to match.

The real question:

From 0 - 10, where 1 is about as dangerous as an open flame indoors and 10 being might as well try it if you're going to die anyway, how bad of an idea is this? Why?

  • \$\begingroup\$ Generator has overcurrent and undercurrent protections. Also assume: I do this at night, have input disconnects, take proper precautions when doing the wiring, and have the appropriate safety equipment. \$\endgroup\$ – user2115867 Jun 17 at 23:54
  • \$\begingroup\$ If you want safety, then don’t talk about bypassing the electrical supplier’s infrastructure. There are legal and safe methods to isolate yourself from the grid. Your solar and generator are rated in kW not kW/h. Given infinite fuel, your generator could generate way in excess of 7kW/h. \$\endgroup\$ – Kartman Jun 18 at 0:01
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    \$\begingroup\$ What you are proposing seems reasonable. However, please do it properly by installing a transfer switch between the utility meter and the panel. \$\endgroup\$ – Dwayne Reid Jun 18 at 0:10
  • \$\begingroup\$ @Kartman If the only objection is the disconnect method: I would of course throw the main breaker. The solar back feeds into the panel via a breaker. So throwing the main disconnect would create an island. I said pop off the meter was to show 100% I would not back feed onto the grid and create a safety issue for linemen. Actual solar system size is 8kW. Typical solar noon production is 7kWhs. So its very likely in any event I would make the 4 kWH difference or the over protection on the generator would trip and everything would shutdown. \$\endgroup\$ – user2115867 Jun 18 at 0:13
  • \$\begingroup\$ @DwayneReid basically I would buy the transfer switch and the generator connector but not hook it up except in a real life or death emergency. My utility wont let you install one of these and the manufacture don't recommend you use them with solar. Otherwise this would be trivial. Basically what I want to understand why this isn't a standard thing. They install solar all the time with batteries, almost never with transfer switches. Why? \$\endgroup\$ – user2115867 Jun 18 at 0:24

It won't work. A grid-tied PV inverter will go into an overvoltage error as soon as it produces more then the load.

If it even accepts the unstable frequency to start operation to begin with.

You may even damage the generator since you may not have negative kw/kvar protection.

It can be done, but you need to limit the PV load. Last month our entire suburb was on a generator (approx 500 kVA) for a week. All PV systems remained operational because there was more load than peak PV watts.


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