How to connect solar Inverter system to a permanent generator disconnect ,grid power

House has grid power and a permanent generator that has a disconnect switch.  Grid comes in, connects to generator disconnect switch, which connects through to the panels.
Am installing a solar system with Siemens microinverters.  These come together into the solar 6/3 wire.
Question is:  Do I connect the solar system directly into the incoming grid power before the generator disconnect
OR can I go directly to the main breaker panel. In this case it will be on the "inside" of the generator service disconnect.

Going to the panel:
1. is easier,
2.  might provide power (along with the generator) during power outages,
3. does this affect the generator or disconnect switch? does it work ?
Going on the line side:
1.  have to break the line power and add a big connection to the incoming wires
2.  doesn't let solar power work when line power is out
3.  separates generator and solar inverters, so might be "safer" or not let interactions between the two happen.



  • BB.BB. Super Moderators Posts: 26,980 admin
    Welcome to the forum MLB.

    From the generator point of view--Most will not "like" having power driven backwards into the altenrator (i.e., you have 5 kW of solar, and 2 kW of loads--A GT inverter will try to pump 5 kW of power into the AC "local grid"--Where will that power go? Possibly backwards into the genset. Genset would not be happy).

    In practice, GT inverters "qualify" the AC mains voltage (say 212 to 264 VAC for US) and 60 Hz +/- 0.5 Hz (again for US). If you have a "typical" mechanically governed genset, many will have have frequency regulation withing +/- 2 Hz or more... So, the GT micro inverters will never "qualify" the utility power (must be within specs for 5 minutes before GT inverters will "turn on").

    If you have a genset with very good frequency and voltage regulation, or have an "inverter genset" (which usually have very good regulation), then the GT inverter would back drive energy into the load+genset local grid and probably, at the very least, cause the local voltage to exceed 264 VAC and fault the GT inverters (shut down for 5 minutes, re-qualify, and restart).

    So--The standard answer would be to put the GT inverter AC panel connection on the Grid Side of your setup--And not attempt to use the GT solar power as part of your "local grid". It will be safer for your hardware. But, as you say, you will not have the use of your solar array during this time.

    If you have a genset with very good regulation and AC loads that are always larger than the GT inverters' output--It could work as you need and reduce your genset loads. However...

    If you have a gasoline or propane powered genset, they typically tend to be most efficient at 50% or greater load... Reducing electrical loads to less than 50% of rated output, and fuel flow does not drop that much (inverter genset may be more fuel efficient down towards 25% of rated load before you hit a fuel flow plateau). So, "solar support" may not save you fuel costs.

    If you have a diesel cycle genset, they will reduce fuel flow below 50% of rated output (more efficient Diesel cycle vs Otto cycle gasoline engines)--But lightly loaded diesel engines (especially older ones?) have other issues (wet stacking, carbon buildup, cylinder wall glazing if operated long term at less than 60-40% of loads--I am not any sort of diesel expert). Solar support may save you fuel costs--But could increase maintenance costs over time.

    Your thoughts/comments?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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