Using two solar inverters on a grid tie setup

[df0rster]

I currently have an Outback Skybox with around 5kw of panels. It is working great, but it is about at its limit as far as PV input.

I am thinking of installing a second inverter out in my shop with another 3 or 4kw of panels that I picked up for a good deal. Are there problems with this arrangement? I wondered about the safety settings on the Skybox that detect a grid outage and immediately stop any grid interaction. During an outage my 2nd inverter would continue supplying power and the skybox might not know the grid was down and would continue to try to backfeed.

Any ideas are apprecaited.

[Ampster]

How do you have the two inverters connected? Do you have a single line diagram? I had to give my building department a single line diagram when I pulled my permit.

Where are they in relationship to your main panel? Don't worry about the Skybox backfeeding the grid, if it is connected directly to the grid. It is UL1741SA compliant and will disconnect from the grid when the grid goes down.

[peakbagger]

I have two grid tied inverters and 4 enphase micros that all tie into a solar subpanel and the sub panel ties to my main panel. I used the subpanel to give me a bit more backfeed capability as if I connected them direct to my main panel I would have the add up the nameplate capacity of each inverter. By going to subpanel I can hook up to breaker sized for the actual PV output, not hot the nameplate output of the inverter. I also meter the combined output with production meter for SREC sales. Everything works well together.

BTW when I hook a Sunny Island to my main panel and shut the utility off, the other inverters seem to be quite happy synching to the Sunny Islands output.

[df0rster]

How do you have the two inverters connected? Do you have a single line diagram? I had to give my building department a single line diagram when I pulled my permit.

Where are they in relationship to your main panel? Don't worry about the Skybox backfeeding the grid.if it is connected directly to the grid. It is UL1741SA compliant and will disconnect from the grid when the grid goes down.

I was just thinking if I have another inverter tied into my main panel, would it keep feeding power if the grid went down and "trick" the skybox into thinking the grid is up? Just wanting to make sure no weird interactions would occur.

I don't have a diagram right now. I haven't decided to actually do this project yet.

So I could set it up however I need to. The 2nd inverter would be in my shop. Which is currently on a 60 amp breaker that is located in my critical loads panel. Although it doesn't pull 60 amps, probably 20amps at most at any given time. With this setup the 2nd inverter would be backfeeding through my CLP, which might be an issue itself, I'm not sure.

[Mike 134]

Following..........

not answering but an observation, 6 houses all on the same block can have solar with no issues back feeding with a power shutdown so .........??

[Ampster]

........ The 2nd inverter would be in my shop. Which is currently on a 60 amp breaker that is located in my critical loads panel. Although it doesn't pull 60 amps, probably 20amps at most at any given time. With this setup the 2nd inverter would be backfeeding through my CLP, which might be an issue itself, I'm not sure.

The most cost effective way to add capacity would be to use a GT inverter in your shop. Since it would be fed from your Skybox through your critical loads panel the Skybox would be the grid forming inverter which would AC couple to the GT inverter through the sub panel. Presumably the grid is connected to the AC In port of the Skybox so the Skybox would drop that connection when the grid goes down but continue to feed the subpanel through the AC Out port. I presume you have batteries connected to your Skybox?. AC coupling works best with some batteries to give you a buffer.

That is a classic AC coupling scenerio and saves the cost of buying a more expensive battery powered inverter for your shop. That is essentially what peakbagger described above and what I am doing with my Skybox. The Skybox can AC couple up to 8kW of GT solar so no issues there.The other thing that the Sktbox can do is limit the amount sold to the grid by both systems so you do not have to worry about exceeding your NEM limit if that is an issue.

[Mike90250]

Adding a second GT inverter is fine, till you approach the limit of the power company feeder wires. If you are "lucky" the perceived Grid voltage will rise on your house side and the inverters will start shutting off at their safety limits.

If you loose the Grid in daylight, and your internal loads are light, things may run for a while but at some point there will be a glitch, overvoltage and all shuts down.

[Ampster]

Adding a second GT inverter is fine,.........

If you loose the Grid in daylight, and your internal loads are light, things may run for a while but at some point there will be a glitch, overvoltage and all shuts down.

Fortunately in the OP's case his existing inverter is a hybrid so it should be able to manage production with charging and household loads. AC coupling will also shut down the proposed GT inverter before there is an issue with over voltage.

[df0rster]

The most cost effective way to add capacity would be to use a GT inverter in your shop. Since it would be fed from your Skybox through your critical loads panel the Skybox would be the grid forming inverter which would AC couple to the GT inverter through the sub panel. Presumably the grid is connected to the AC In port of the Skybox so the Skybox would drop that connection when the grid goes down but continue to feed the subpanel through the AC out port.

That is a classic AC coupling scenerio and saves the cost of buying a more expensive battery powered inverter for your shop. That is essentially what peakbagger described above and what I am doing with my Skybox. The Skybox can AC couple up to 8kW of GT solar so no issues there.The other thing that the Sktbox can do is limit the amount sold to the grid by both systems so you do not have to worry about exceeding your NEM limit if that is an issue.

The way you are explaining it makes sense. One of my goals is to be able to run my higher load devices completely from the solar inverters w/ solar + battery power (eventually), to prepare to be completely off grid. like the HVAC which is on a 40 amp circuit I believe, typically I think it will pull around 3kw in AC mode, maybe 5kw on startup. and then I think the dryer could pull over 5kw. Using the AC coupling on the skybox, does it allow you to exceed the 5kw limits on loads? From what I remember it has a 5kw load limit, as well as input and backfeed to grid as well. The input I understand can exceed the 5kw with AC coupling, but it is still going to be limited to 5kw load and/or 5kw grid, etc.

I also have another goal in mind. I would like the second system to use a cheaper hybrid inverter so I can relocate some of my batteries out there. I have seen a couple cheaper hybrid inverters in the $800-$1500 range. Haven't done much research on these yet but I plan to.

[Ampster]

The way you are explaining it makes sense. One of my goals is to be able to run my higher load devices completely from the solar inverters w/ solar + battery power (eventually), to prepare to be completely off grid. like the HVAC which is on a 40 amp circuit I believe, typically I think it will pull around 3kw in AC mode, maybe 5kw on startup. and then I think the dryer could pull over 5kw. Using the AC coupling on the skybox, does it allow you to exceed the 5kw limits on loads? From what I remember it has a 5kw load limit, as well as input and backfeed to grid as well. The input I understand can exceed the 5kw with AC coupling, but it is still going to be limited to 5kw load and/or 5kw grid, etc.

The Skybox is a high frequency inverter so it cannot handle much start up load beyond its native 5kW inverter capacity. Look on your AC to see if it shows LRA and that might give you a clue. You may be able to find a soft start device that makes that manageable. The dryer has a small startup load from the motor and its load is intermittent as it cycles the heating element off and on. The amount of AC coupled solar you can connect to the Skybox may depend on how big your battery capacity is. I think the limit is 8kW of AC coupled solar in addition to the native 5kW from the Skyox. That is a total of 13 kW. When connected to the grid you are limited to 60 Amps (14 kW) . I have 42kWh of batteries and 6.6kW of micro inverters coupled to my Skybox. The Skybox also has 2 kW of solar connected. I plan on adding another 2 kW of solar on a west facing barn. it has no shade but micros are economical until you get over a dozen panels. Also the wiring the AC is easier on a remote building because running DC back to the Skybox over that distance would mean another conduit and heavier wiring.

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I also have another goal in mind. I would like the second system to use a cheaper hybrid inverter so I can relocate some of my batteries out there. I have seen a couple cheaper hybrid inverters in the $800-$1500 range. Haven't done much research on these yet but I plan to.

Less expensive hybrids may not be UL1741SA compliant so I do not know how they would work in parallel with your Skybox. Some of those also are only 120 volts but that may be okay in a shop if you do not have 240 volt loads. I looked at that idea with my barn but decided I only wanted to manage one battery pack. Keep us informed as your thinking progresses. Have I seen your posts on other forums? I know we have had some conversations on the Outbackpower forum/

[df0rster]

Right now I have a small battery connected just for backup, I think around 5kwh. But I have a larger 15kwh setup in the garage waiting to be installed. I had hoped to use both, using the smaller pack in the shop with another hybrid inverter (if it is UL1741SA compliant and will work this way), so I'd have about 20kwh. Which is probably only 1/3 of what I'd need to be off grid. And those are just guesses for now, and dependent on converting my appliances to gas (dryer and water heater).

So what you are saying is that with AC coupling, I can have loads up to 14kw instead of the 5kw limit of the skybox? If I have the capacity from the 2nd system that is.

Yes I have discussed my initial system setup quite a bit with you on this forum and the Outback forum. You were very helpful in getting it setup right so thanks for that! I took a break after the initial setup and everything has been working flawless since then. With my 5kw of panels, my electric bill has been very low, paying for around 200kwh/month, or about $25 a month on average probably. Of course with the coop fees of around $50 that eats up a lot of the advantage of doing grid tie. If I could go completely off grid that $50/month would pay for some nice battery packs!

[solardreamer]

If you have a traditional central A/C then you may want to consider replacing it with a variable speed heat pump that is more efficient and requires much lower startup and running power instead of getting more batteries and inverters.

[bcroe]

Also the wiring the AC is easier on a remote building because running DC back to the Skybox over that distance would mean another conduit and heavier wiring.

Is it written, that DC wiring must be in conduit? There is plenty of direct burial wire
2 feet down here.

2 Net Metering inverters have worked well together for 8 years, but that type will
always shut down with loss of the grid. NM inverters expect an infinite energy sink
(the grid) to stabilize their operation, that will not happen with an outage. Bruce Roe

[df0rster]

If you have a traditional central A/C then you may want to consider replacing it with a variable speed heat pump that is more efficient and requires much lower startup and running power instead of getting more batteries and inverters.

I actually have a climatemaster geo 4 ton geo unit. I can measure its startup with my meter sometime, but I dont' think it is really that high.

I will need more batteries to go completely off grid just to account for cloudy days, not just to run the AC. The reason I will probably need another inverter is because the Skybox is limited to 5kw by itself and is designed to be AC coupled or used in parallel with another skybox. I will probalby need more than 5kw at times if the AC is running along with a microwave and fridge, etc. my peak load will most likely exceed 5kw.

[solardreamer]

I actually have a climatemaster geo 4 ton geo unit. I can measure its startup with my meter sometime, but I dont' think it is really that high.

I will need more batteries to go completely off grid just to account for cloudy days, not just to run the AC. The reason I will probably need another inverter is because the Skybox is limited to 5kw by itself and is designed to be AC coupled or used in parallel with another skybox. I will probalby need more than 5kw at times if the AC is running along with a microwave and fridge, etc. my peak load will most likely exceed 5kw.

That's nice you have a geothermal heat pump. Haven't see many in use. Regarding your unit, it appears that Climatemaster offers 2-stage and variable speed 4 ton capable models. The 2-stage model specs show 104A LRA so if that's what you have then even 2 5kW inverters may not reliably start it especially with other loads as the surge amps can be 150%+ of LRA. OTOH, if you have the variable speed model then LRA is only 32A which is much lower than the 2-stage model but it still seems significantly higher compared to the air based variable speed heat pumps. I guess there are more power consuming parts in a geothermal heat pump system.

Since your goal is to go off grid then it's probably best to first make sure your big loads have minimal starting and running power requirements.