6.5kw solar system with a 20kw briggs standby generator

Re: 6.5kw solar system with a 20kw briggs standby generator

Welcome to the forum John!

First question... Your 6.5 kWatt solar system is a Grid Tied Inverter based system (solar panels + GT Inverter, no battery bank).

These system work in parallel with the Utility Power... When you have 120/240 VAC and 60 Hz power, the GT inverter "syncs" with the AC mains and simply pumps energy into your home's power lines--Which goes to support local AC appliances, and if there is extra power, goes out the AC mains and turns your utility meter "backwards" (assuming you have a Utility Net Metering agreement and the correct utility meter).

If you lose utility power, the GT inverter system will shut down too. No AC utility power, no GT solar power.

Now--You toss in an AC generator. You have two places that the generator transfer switch can be installed relatively to the GT Inverter's connection.

The "standard" installation would be:

AC mains + GT Inverter -> Transfer Switch -> Generator + "protected loads" (loads that the generator runs.

A second method would be:

AC mains -> Transfer Switch -> Generator + GT Inverter + "protected loads" (loads that the generator runs.

The AC mains would go down (so with GT solar), then the transfer switch starts the genset and then connects the sub panel with the protected loads. Generator voltage/frequency, if withing the GT inverter's specification, will allow the GT inverter to sync and start feeding power to the grid... And then the fun begins...

First, the voltage/frequency/waveform needs to be qualified and stable for >5 minutes by the GT inverter before it turns on. The frequency is +/- 0.5 Hz or so--And many generators can drift outside of that frequency tolerance range... So, the GT inverter may never "turn on" if the generator frequency wanders too much (not all generators can hold these tight frequency tolerances).

Second, the GT inverter turns on and starts feeding ~5-6 kWatts or so into the protected sub panel. If your loads are greater than the GT inverter's output, all is "OK". However, if the loads are less than the GT inverter's power output, the extra power will try to "run backwards" through the generator. That is not a good thing.

The generator could over speed (and shut down the GT inverter for fine minutes/fault the genertor), could over voltage (shut down GT inverter for five minutes/fault generator), or start feeding current backwards through the alternator/regulator (and possibly let the magic smoke out of the generator).

At this point, I do not know if a good/safe/reliable method to make a pure GT Inverter + Generator play together unless you are going to somehow actively manage your loads to prevent the GT inverter from backfeeding your genset.

There are other methods that can be done to make solar+generator play together nicely... But that usually involves a "Hybrid" AC inverter (GT+off grid capable AC inverter) and battery bank. This can be expensive and may not be cost effective if your power failures are rare/short term.

-Bill