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Topic starter
October 6, 2025 6:32 am
FlexBoss21 and GridBoss: Pre sales engineering question(s)
Only have room for 10 Kw of panels. Want to generate with solar to offset bill. During summer months would like to use battery charged by solar and off peak power to reduce bill. Typical peak demand according to utility on a monthly basis is 11 Kw to 14 Kw, demand is measured on a 60 min window. Typical consumption between 2,400 KwH and 3,800 KwH per month over the year. So a GridBoss and FlexBoss21 with the 200 AMP (same as SE rating) passthrough works fine. Inverters can't do the full load but that is Ok because the grid makes up for what the inverters can't supply. Any flaws in this?
Here is the problem. We have had an 18 Kw Kohler Nat Gas whole house generator for the last 8 years. Everyone expects everything to work during a gird down situation and it does. If I put the Kohler on the GridBoss generator lugs this is what I think happens. The inverter carries the load until the batteries need charging, the generator starts, carries the house directly and charges the batteries connected to the FlexBoss21. When the batteries are charged to the set-point the generator is shut down. If the load exceeds the inverter rating while the generator is not running then the FlexBoss21 overloads and shuts down. Hose is dark, no way to run the generator. From the one line drawings it appears that when the generator is running it will be carrying the house and the but it will only be running when the batteries are not fully charged. When the generator is running the system capacity will be that of the generator less the battery charging load. When the generator is shut down the system is limited to what the inverter(s) can carry.
The most obvious solution is to have two FlexBoss21 inverters but that is buying a second inverter just for grid down when the loads exceed one FlexBoss21, and we already have the generator for that.
If these presumptions are correct could I set something so the generator runs the whole time regardless of battery charge state? Set the generator cutoff battery charge level to 105 % which would never be reachable.
Is there a setting that I could limit the battery charge rate when the generator is running to something so low that it would take days to fully charge the battery?
Could I put the GridBoss upstream of the ATS and set the GridBoss not to provide power during a grid outage, then the ATS would see the grid failure and start the generator?
What is the right way to handle this or do I just have to get a second FlexBoss21 if I want to carry the whole load during a grid down?
Topic starter
October 16, 2025 5:49 am
Here is the solution I have come up with unless I am breaking a rule.
What I did not realized was that the non backed up lugs were on the inverter until the grid was lost. Was thinking that they were isolated from the inverter(s). Since that is not the case will feed the grid lugs on my ATS from the GridBoss non backed up lugs. In a grid failure the inverter will not be overloaded because the larger loads will be dropped since they are on the non backed up lugs. With Grid failure the ATS will loose its grid feed from the non backed up lugs and start the generator. Once the generator is up to speed the ATS will switch so it is feeding the non backed up panel and all my loads will be powered. The output of the generator will also feed the FlexGrid generator lugs which will keep the batteries charged and support the load.
The only potential problem with this is if having the generator running for days during a grid down situation will present a problem for the FlexBoss21 or GirdBoss. Obvious concern would be the batteries over charging.
Thoughts?
October 16, 2025 3:44 pm
I don’t have a flex boss but I’m assuming its programming is close to the 18kpv. I would do two things in a grid down situation. First manage my household amps. Second I would lower the charge amp settings to the battery to its lowest setting. That will let the most power from your gen pass through to the house. In a grid down situation managing amps is probably more important because you don’t know how long the grid will be down and you want to conserve as much gen fuel as possible.
Topic starter
October 16, 2025 9:17 pm
@pawnee My generator is natural gas, at least one thing that is not an issue for me. 😀
Thanks for the feedback.
October 17, 2025 2:21 pm
@customertobe natural gas is probably the best source for gen fuel, but I would still conserve fuel just in case it’s a wide spread long term grid failure. After a week or so without the compressor stations running to push the gas it will run out. That’s probably never going to happen just food for thought. Good luck
