26KW Solar and Sol-Ark 15K Evaluation/Grid Sell Limit

[RobND]

Hello all!

Been lurking quite a bit and wondering if I could get some thoughts.

We have a 26KW Roof-Mounted Solar installed (late 2023), 66 395w panels, 48 are DC (19KW) and 18 (7KW) have AC micro-inverters on them running into the Gen port on the Sol-Ark. We also have just one Sol-Ark 15K-LV inverter. Naively, I'm seeing the Sol-Ark is "grid sell" limited to 16KW and we keep getting clipped at 15.5KW nominal power selling back to the grid at peak times during the day.

Is this normal? Should we have had two Sol-Ark's installed for this size of system? Anyway to increase the grid sell over 16KW? Feels like the array was too large for the inverter.

And I'm not seeing us hit any more than 19KW nominal power on the peak of day, with full sun. Granted it is Jan-Feb, but is that to be expected this time of the year with 30 degree angled panels at 47 deg lat, no shading? PV Watts has it listed as being able to hit 23KW with my assumptions. I know getting 395w per panel is not realistic, and 295w is their real world optimal number (PMax?) and I think I'm getting topped at close to that (275w peak power output). Sound about right for panel output before we consider Sol-Ark limits?

Thanks! I could really use some advice!

Info:
Minnesota (47 deg lat)
Roof Mounted 30 deg angle
No trees or shading
Professionally installed
Heliscope calc was 35MWh/yr
PVWatts hourly can't hit that 35MWh number with grid-sell limitation clipping on every sunny day year-round

Specs:
26.1KW
48 395w (19KW) DC connected
18 395w (7KW) DC with micro-inverters AC Coupled to Gen port on Sol-Ark
Sol-Ark 15-LV
Edit: 10KWh Battery (5KWh x2)

 

[BentleyJ]

If Pmax is 295W then you would expect 66 x 295W = 19,470W seems like you are in fact hitting very close to that value based on the above. As far as PV Watts, you answered your own question with the acknowledgement of "assumptions" being part of that calculation.

Datasheet for Sol-Ark states that 15kW is the maximum available AC power from the 19kW PV array for all purposes. Pmax for the DC coupled array is 14,160W. Theoretically then 100% should be available with no clipping.

The microinverter output is hitting the AC bus via the Gen Input. Since the Sol-Ark has 200A pass-thru capability it seems like AC Coupled power would only have to be limited to about 15,000W which is the capacity of the battery charger to divert AC coupled power with no other loads present. So not sure why or how the Sol-Ark would need to limit total sell back to 16kW, didn't see that in the datasheet? In any case, Pmax of the AC Coupled array is 5,310W.

Sell back is only going to be the excess Watts minus the loads. This implies loads would be 3.97kW when selling back 15.5kW. If loads are less than this, it does sound like some clipping is going on or more likely the panels are indeed producing a bit less than Pmax. EDIT: There are also efficiency losses in the conversion process.

My question is this: If the single Sol-Ark is limited to 12kW from battery alone and was sized based on your nightime power reqirements, why is the solar array so large? Perhaps because the peak load occurs during the day. If so then based on my calculations your daytime loads are way under what would be expected?
Point being, the system seems like its way out of balance. If the loads are so low that sell back is being clipped you didn't need that much PV. If the loads are large then you wouldn't have more than 15.5kW left over to sell back anyway.

 

[RobND]

Thanks BentleyJ,

Sounds like panels are fine, just too many of them for the inverter.

In the Sol-Ark Limiter settings it's got "Grid Sell 16000W" at max setting. The DC power and micro-inverters are both grid sell limited. The Micro-Inverted panels are always 100% and the Sol-Ark throttles the DC panels to not sell back to the grid more than about 15.5KW at a time. If we put say a 8KW load (washer/dryer), then the DC panels produce more as we go beneath the 15.5KW grid sell limit, usually the DC panels topping out at 14KW during peak sun, but with no load/no boiler on, they throttle back down immediately to hit that grid sell limit, usually 5KW for Micro and 10KW for DC panels.

We are all electric and have a 23KW boiler for hydronic floor heat (on a separate 200A panel). So when floor heat is not on, we don't have much load. But when we do, we can hit 30KW with boiler and say washer/dryer. So we can easily run 4000-5000KWh/mn in winter months up in MN. So the (maybe faulty) thinking was make it big enough throughout the year to cover everything on a year over year basis (buy power winter, sell power summer). Overall we'll be consuming around 30-35MWh. We are net metered, on-grid tied with just a small battery back-up (10KWh).

Running Helioscope as well, 26KW was supposed to do 35MWh/year with no inverter limits, but with the inverter at Sol-Ark 15K it's showing only 31MHh per year. Not the end of the world, but still clipping a full 12%. That's in addition to the usual losses and panel angle/latitude.

So better option would've been?

1) Build a smaller system so the clipping wouldn't happen? Live with higher electrical bills overall but save a ton on the install of solar.
2) Install a second inverter so it can grid sell above 16KW? Two inverters could grid sell 32KW max?
*3) To accept the 12% loss in addition to all the typical losses. And in 20 years, they'll still be net metering at the same level as now due to panel degradation, lol.

 

[BentleyJ]

If the AC Coupled system is producing 5kW peak, that is close to the Pmax calculated to be 5,310W. Sounds like everything is working as per the documentation.

Not sure what Brand the Microinverters are. If Enphase, there are at least 2 strings of 9 into the combiner box. Probably would be a PITA to move one string to feed directly into a different breaker panel so the inverter doesn't see those Watts and wouldn't cut back the DC coupled output trying to stay under the limit. Of course that string of microinverter panels would not contribute during an outage.

 

[RobND]

You can run the inverted panels directly into another panel? Is a combiner box necessary? Would those panels still be able to go back out to the grid through our consumption/net meter? I thought all that was handled with the Sol-Ark. I don't think we have a combiner box, just the Sol-Ark and batteries inside. That would seem like the best solution if that limit can't be changed in the Sol-Ark. Outage isn't really a concern. I just would prefer not to have all that power clipped every sunny day at 15.5K.

 

[SunDave]

Hi Rob,
Interesting real world info. I have a 15k but have not fully paneled up yet. I was guessing PV sell would be limited to the 16k it said under the online "system work mode" settings.

In my initial read, it seems you have the option to add another inverter, or deal with the clipping. I hope you don't take this the wrong way, but that sounds like a good problem to have. If it were me, I would likely see how the system performs in the summer, to see how much you actually clip. A good summer day may not be as high of peak production as the best part of a winter day.

For example, if your expected 12% loss turns into a real world loss of just a few % (especially considering the # of full sun days you actually lose anything at all), then your system may be sized perfectly. Over-paneling is really just using the expensive parts of the rest of your system to their maximum more often.

Congrats on the awesome system.

 

[BentleyJ]

Yes, Grid Tie microinverters are set up to feed directly into a breaker panel. That said, Enphase for example uses Power Line Modulation to communicate with their network gateway and their string length is limited to what can fit on a 20A breaker. Typically multiple strings are put into an Enphase combiner box which includes the gateway. So yes it would be direct connection to the breaker panel but via a combiner box.
I believe its still possible to connect an Enphase system directly to a breaker without the combiner box, the stand alone gateway just has to have way to connect to L1 & L2 to collect data.

I have not used any other brands of microinverters so could speak to what other auxiliary equipment they require.

 

[DIYrich]

In the Sol-Ark Limiter settings it's got "Grid Sell 16000W" at max setting.

See if you can increase the number to 30,000 (basically no limit). If it is a hard 15,000 limit, then talk with Sol-Ark. The limit should either exclude AC PV, or be adjustable as high as you want. I'm thinking the latter since some people have a hard limit on export imposed by the Utility. Sol-Ark should allow something around 30kW limit (combination of 15kW of AC PV limit on Smart Load port plus 15kW inverter capability).

 

[RobND]

See if you can increase the number to 30,000 (basically no limit). If it is a hard 15,000 limit, then talk with Sol-Ark. The limit should either exclude AC PV, or be adjustable as high as you want. I'm thinking the latter since some people have a hard limit on export imposed by the Utility. Sol-Ark should allow something around 30kW limit (combination of 15kW of AC PV limit on Smart Load port plus 15kW inverter capability).

Thanks! I talked to Sol-Ark and they just ran a firmware update and now I can set the "sell grid" limit as high as 32,000/32KW. I didn't think it would be that easy and Sol-Ark support was pretty amazing. I guess this isn't a usual problem and my loads are somewhat unique.

My PVWatts summary hourly nominal power data was listing quite a large amount of hours we'd hit over 22KW output and that would've clipped 4-5 MWh each year. The update will end up saving me quite a large amount over the life of the system.

 

[Dave911]

Thanks! I talked to Sol-Ark and they just ran a firmware update and now I can set the "sell grid" limit as high as 32,000/32KW. I didn't think it would be that easy and Sol-Ark support was pretty amazing. I guess this isn't a usual problem and my loads are somewhat unique.

My PVWatts summary hourly nominal power data was listing quite a large amount of hours we'd hit over 22KW output and that would've clipped 4-5 MWh each year. The update will end up saving me quite a large amount over the life of the system.

Sol Ark is very impressive. If I wanted to drop $6k on a 15kva inverter, it would be a Solark/Deye.
Doesnt your utility have a cap on your sell amount?