Solar DC mini split— thoughts

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EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
This came up an another thread. Solar powered mini splits are available. From my understanding they run on DC but can be grid tied, the mini split not the PV panels.
One option for AC that i was not aware or until recently is 48 volt minisplit that runs directly off solar panels (no batteries). It adjusts its load on the amount of generation from the panels. That would keep the AC loads from not needing to be shifted to the night period. https://www.hotspotenergy.com/solar-air-conditioner/ The pricing is pretty close to a regular minisplit (not including the PV panels). It makes sense in a balanced climate area, when the sun is out more AC is needed.
So what’s the advantage of the solar mini split? let’s say for this discussion you are limited to 4-8 panels. Would a small grid tied system make more since? Or would an off grid system be a better choice. Let’s also assume that going forward one can add the net metering rider and utilize the time of use rate schedule or the standard rate schedule.

In a world of infinite resources one could pay in cash for a system that was sized to provide 80-100+ % of your usage. I don’t see that as realistic.
I could afford today a 10-13k$ install.

So I want to limit this discussion upfront system costs under 15k$.

It was a hot day yesterday we used 53 kWh. Here is the graph. I cooled the house off in the morning and set the thermostat to 79.

It seems to me that load shifting really negates the DC mini split. And I would be better to go with a small grid tied system and time of use billing (baking during peak times might become a no-no)

Lastly I full expect that our house will add a second and or third EV in the next 10 years. Do EVs mean net metering is better than a DC mini split? Do they change the decision in anyway?

When I get some time I will run some numbers.

EFEEA089-5A2B-468E-9B62-0766FD2C8D77.png
 

peakbagger

Minister of Fire
Jul 11, 2008
7,206
Northern NH
IMO, its all wrapped up in the utility rate plan a person has. I brought it up with regards to someone who has a time of use rate where they pay a lot more for day time power than night time. Many big loads can be shifted but unless some one is making ice blocks at night to cool their house during the day (which is done on occasion on commercial buildings), then they need to run AC during day time. In this case a 48 volt DC solar minisplit may make sense as it has power from the panels when the sun is out and in theory the sensible heat load is highest. Note latent load does not follow the sun as much but if the house is reasonably tight, once the latent load from humidity is knocked down it tends to stay down with sensible load taking over as the sun heats the house. The device I linked to does have AC backup and my guess is run it during the low rate period off of utility power to keep the humidity low and then run it off the panels during the high rate period during the day.

I have one rate 24/7 and generate more than I use with no annual reset so I run mine when I need it and not worry.
 
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begreen

Mooderator
Staff member
Nov 18, 2005
93,219
South Puget Sound, WA
My interest was also piqued by the 48v minisplit system. I like the setup. One gets a high-efficiency mini-split along with the potential to be solar-powered. I am wondering who is making the mini-split for them and whether that manufacturer's other components, like a second wall unit, could be adapted.
 

woodgeek

Minister of Fire
Jan 27, 2008
4,557
SE PA
With all due respect, I wonder if envelope improvements could have a better ROI than a DC minisplit. Ofc, if you got better insulation (and airsealing against latent humidity loads) it would work synergistically with your cold banking TOU strategy.

Do you know your BTU/h°F per sq ft (insulation) and ACH_50 (blower door)? Can you get this info from a utility survey?

ETA: is your attic thoroughly air-sealed? The upper stories should NOT be much hotter than the lower if this is the case.
 
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EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
IMO, its all wrapped up in the utility rate plan a person has. I brought it up with regards to someone who has a time of use rate where they pay a lot more for day time power than night time. Many big loads can be shifted but unless some one is making ice blocks at night to cool their house during the day (which is done on occasion on commercial buildings), then they need to run AC during day time. In this case a 48 volt DC solar minisplit may make sense as it has power from the panels when the sun is out and in theory the sensible heat load is highest. Note latent load does not follow the sun as much but if the house is reasonably tight, once the latent load from humidity is knocked down it tends to stay down with sensible load taking over as the sun heats the house. The device I linked to does have AC backup and my guess is run it during the low rate period off of utility power to keep the humidity low and then run it off the panels during the high rate period during the day.

I have one rate 24/7 and generate more than I use with no annual reset so I run mine when I need it and not worry.
I think you hit it right on the head. It’s all rate dependent. And are electric rates predictable for the next 10 years?
 

EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
With all due respect, I wonder if envelope improvements could have a better ROI than a DC minisplit. Ofc, if you got better insulation (and airsealing against latent humidity loads) it would work synergistically with your cold banking TOU strategy.

Do you know your BTU/h°F per sq ft (insulation) and ACH_50 (blower door)? Can you get this info from a utility survey?
Very possible and more probable if I didn’t heat with free wood.

Really I can only guess load. The whole house is cooled by a 3 ton unit. 22k BTU serviceable at 95 outdoor and 60% RH indoor. You can’t pay anyone down here to do a manual J. We tried.

Never seen any energy audit do a door test. I’m leaky for sure.

First pic is ceiling. Second is north wall on a 90 degree afternoon. Attic insulation is on my list. But not for it’s ROI but for a way to feel greener without putting up solar. Right now we have what was r14 in the walls in 1968. About r14-20 of loose fiberglass in the attic.

I’m guessing we will see a 10-20% rate increase next time the utilities commission takes it up.

46D964DE-9768-441F-A067-7ED4B5B2CB38.jpeg E465E056-945F-45DA-94A9-E983DF4B6FF5.jpeg
 

EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
My interest was also piqued by the 48v minisplit system. I like the setup. One gets a high-efficiency mini-split along with the potential to be solar-powered. I am wondering who is making the mini-split for them and whether that manufacturer's other components, like a second wall unit, could be adapted.
For me though at least 3-4 months a year it wouldn’t be needed to heat or cool. Would it be better just to install a grid tied solar, especially if you already have a heatpump?
 

woodgeek

Minister of Fire
Jan 27, 2008
4,557
SE PA
First reaction is 60% indoor RH seems high if you are running AC steadily, suggesting a lot of air infiltration. In the summer, the cool air in the house falls, depressurizing the upper story and sucking heated attic air into the house (often largely around the top plates of interior framed walls, but also many other openings). This suction draws the hot air right through the insulation as if it weren't there. Adding more insulation doesn't help.

Attic airsealing is very DIY-able (if you can operate up there when it is cool). Lot's of guides on line. You poke around under the insulation, looking for openings. A major clue is where the fiberglass is discolored due to filtering dust in the air for the last few decades of summers. In a heating climate, I dropped my heating bill by 25% myself in a similar vintage house just airsealing the attic, and then had pros come in and so the stuff I couldn't reach. And THEN I had more insulation blown in.

I added it up, and found more than 10 sq ft of openings between the attic and the top story. And that sort of thing is TYPICAL for 1960s construction. Imagine a 3x3' window between the air in your attic and your house, open 24/7 !!

And BTW, attic airsealing is WAY greener than putting in solar. And the payback is often like 1 year or less.

Try the FLIR on the top of interior walls... if you see a lot of heat there... you have infiltration.
 
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peakbagger

Minister of Fire
Jul 11, 2008
7,206
Northern NH
For me though at least 3-4 months a year it wouldn’t be needed to heat or cool. Would it be better just to install a grid tied solar, especially if you already have a heatpump?
Some utilities have really have no grid tied options or the options they have are very poor. Thus a solar powered DC heat pump just cuts house load and keeps the utility out of the picture.
 
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begreen

Mooderator
Staff member
Nov 18, 2005
93,219
South Puget Sound, WA
Would it be better just to install a grid tied solar, especially if you already have a heatpump?
That's the current system. This is thinking toward the future. The heat pump is going on 17 yrs old and the solar system is 12 yrs old with a 9 yrs old second array addition.
 
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EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
That's the current system. This is thinking toward the future. The heat pump is going on 17 yrs old and the solar system is 12 yrs old with a 9 yrs old second array addition.

I’m always a bit torn between multiple independent systems and a single integrated system. I see advantages of both. I could totally see using old/used panels to power the minisplit.

But if I were to start from scratch. With a limited budget it seems to me that if you could skip the mini split you could install more capacity or spend that money on batteries.

Now if I were wanting to add heating and cooling Capacity, and I am considering it. (3 tons works ok 90% if the time.) I could make the argument that the DC minisplit is a good choice. My issue is I need cooling on the first floor and heat in the basement. I could.

I’m also torn torn between saving money and spending it to reduce my carbon footprint.
First reaction is 60% indoor RH seems high if you are running AC steadily, suggesting a lot of air infiltration. In the summer, the cool air in the house falls, depressurizing the upper story and sucking heated attic air into the house (often largely around the top plates of interior framed walls, but also many other openings). This suction draws the hot air right through the insulation as if it weren't there. Adding more insulation doesn't help.

Attic airsealing is very DIY-able (if you can operate up there when it is cool). Lot's of guides on line. You poke around under the insulation, looking for openings. A major clue is where the fiberglass is discolored due to filtering dust in the air for the last few decades of summers. In a heating climate, I dropped my heating bill by 25% myself in a similar vintage house just airsealing the attic, and then had pros come in and so the stuff I couldn't reach. And THEN I had more insulation blown in.

I added it up, and found more than 10 sq ft of openings between the attic and the top story. And that sort of thing is TYPICAL for 1960s construction. Imagine a 3x3' window between the air in your attic and your house, open 24/7 !!

And BTW, attic airsealing is WAY greener than putting in solar. And the payback is often like 1 year or less.

Try the FLIR on the top of interior walls... if you see a lot of heat there... you have infiltration.
Humidity is just high here. 3 miles from the ocean. Most items left outside grow mildew and lots of it. Basement makes it worse. Kids and dog means are doors always opening and closing. I’m two cans of great stuff into my air sealing project. Took me 11 years to properly seal an old sewer line hole. I’m making progress. Bathroom renovations added a lot of poorly sealed penetrations. Attic has a lot of junk, and some seasonal storage.

I don’t think latent load properly accounted for by many hvac contractors. Lack of maintenance which I’m guilty of too, compounds the issue. I’m about to go clean my coils my self. 120$ for an hours worth of work is about my rate. Just had to shop vac out my condensate drain. What’s plugged with lots of slime.
 

woodgeek

Minister of Fire
Jan 27, 2008
4,557
SE PA
I"m telling you that airsealing doesn't work the way most people think. They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid square feet of openings in their framing 50 years ago, on purpose. And then put insulation over the openings and the rest is history. Most of that infiltrating air flows down interior wall cavities into places like the basement.

When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise. I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings. In a normal 1960 split level house.

The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH. 60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both. What was the SEER on that thing? Is it really old? You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.
 
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Brian26

Minister of Fire
Sep 20, 2013
652
Branford, CT
I"m telling you that airsealing doesn't work the way most people think. They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid square feet of openings in their framing 50 years ago, on purpose. And then put insulation over the openings and the rest is history. Most of that infiltrating air flows down interior wall cavities into places like the basement.

When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise. I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings. In a normal 1960 split level house.

The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH. 60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both. What was the SEER on that thing? Is it really old? You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.
This is spot on. During Covid when I was on unemployment for 3 months I spent those months gutting, air sealing and installing mineral wool in all my exterior walls. My house is a 1958 1800 sq ft split level that had some fiberglass insulation in some walls. Some walls had none. The amount of gaps and holes I found was insane. Every piece of the exterior homesote sheathing had big gaps between them. I could feel the air just rushing in. These corners in the picture below were the worst offenders. The difference when I finished was astounding.



You said you used 53 kwh in one day for cooling? I just checked my Emporia energy monitor and I used 58.5 kwh so far for the entire month of June for cooling. It hasn't been exceptionally warm here but plenty of days in the 80s. I'm on net meter solar here but 53 kwh in New England would cost you close to $13-14 a day.

20200404_102031.jpg
 
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EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
I"m telling you that airsealing doesn't work the way most people think. They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid square feet of openings in their framing 50 years ago, on purpose. And then put insulation over the openings and the rest is history. Most of that infiltrating air flows down interior wall cavities into places like the basement.

When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise. I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings. In a normal 1960 split level house.

The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH. 60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both. What was the SEER on that thing? Is it really old? You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.
No doubt about high air infiltration. Heatpump is a trane 16 seer package unit that is 12 years old. It’s got lots of duct leakage in the crawl space. That’s really hurting things too. Not using it much in the winter really increases Breakeven time on a new more efficient unit. Last time I looked no package units have a SEER greater than 16. Not sure I could get a new separate evaporator in the crawspace.

Had the tech out gauges in it last time it was serviced. It wasn’t hot or cold out but he thought it all checked out ok. One thing I can’t quite figure out is the low high stage and the variable speed blower. Which was replaced 3-5 years ago. The only way to figure out what stage is running is based on fan speed. And that can be confusing as the fan has an extra speed period so the fan sounds like it has 3 stages of cooling. If you don’t listen from startup it can be hard to tell if it’s high stage or not. The high stage is just a bypass solenoid that closes to utilize the second stage. Compressor and condenser fan never change speed.
This is spot on. During Covid when I was on unemployment for 3 months I spent those months gutting, air sealing and installing mineral wool in all my exterior walls. My house is a 1958 1800 sq ft split level that had some fiberglass insulation in some walls. Some walls had none. The amount of gaps and holes I found was insane. Every piece of the exterior homesote sheathing had big gaps between them. I could feel the air just rushing in. These corners in the picture below were the worst offenders. The difference when I finished was astounding.



You said you used 53 kwh in one day for cooling? I just checked my Emporia energy monitor and I used 58.5 kwh so far for the entire month of June for cooling. It hasn't been exceptionally warm here but plenty of days in the 80s. I'm on net meter solar here but 53 kwh in New England would cost you close to $13-14 a day.

View attachment 296277
54 kWh was my total usage for the house and car charging. I only have a whole house monitor. AC uses 3 kw. I’m guessing it didn’t use more than 15 kw but don’t know really.

Adding insulation down here really has a long pay off period. It just doesn’t get cold enough. This past winter the high was always above freezing and that’s pretty normal. Lows probably only drop below freezing 10-15 days a year. Summers aren’t oppressive triple digits happens but not regularly.

I commend you for taking on that project. I’ll take on the attic an maybe pay for the crawl space but I’m not pulling drywall.


But back to topic would you rather install the 48v mini split or a 120/240.
 

Brian26

Minister of Fire
Sep 20, 2013
652
Branford, CT

EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
This company also has dc solar assisted mini splits. They even sell one that can run completely off solar and batteries.

It looks like the units are just rebadged Mideas based off the spec sheet and Toshiba compressor.

Midea was my guess too.

If forced into time use billing and peak rates are really high I could see batteries as helpful but why not just go with a 240 volt minisplit. Are the inverter losses something that makes a DC system more efficient?
 

begreen

Mooderator
Staff member
Nov 18, 2005
93,219
South Puget Sound, WA
This company also has dc solar assisted mini splits. They even sell one that can run completely off solar and batteries.

It looks like the units are just rebadged Mideas based off the spec sheet and Toshiba compressor.

That was my suspicion. Midea is behind many rebranded products.
 

EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
I"m telling you that airsealing doesn't work the way most people think. They put weather stripping on tiny cracks on their windows and think they did something, when the builders hid square feet of openings in their framing 50 years ago, on purpose. And then put insulation over the openings and the rest is history. Most of that infiltrating air flows down interior wall cavities into places like the basement.

When I finally attic airsealed (for real), while it was a PITA moving all the crap around up there, and working when I could, it was like I got a whole new house energy-wise. I kept track and it took several foam board panels and sheets of sheet metal, and 20 tubes of caulking to close all the openings. In a normal 1960 split level house.

The thing is, the indoor coil temp is supposed to be low enough versus CFM to maintain below 50% indoor RH. 60% indoor RH, and a 3-ton not keeping up all the time means either very high air infiltration, or an AC system on its last legs, or both. What was the SEER on that thing? Is it really old? You might make an economic case for replacement with an inverter drive unit, maybe even a ASHP version.
I had to look it up but our average July RH is 75%. That’s a couple points higher than New Orleans. Average 21 days of precipitation in July to the total of 100mm. We set the thermostat at 78 if it gets 95 or or hotter we set to 80. Basement is always cooler.

Giving this some more thought. Not having an inverter is much cheaper. Small inverter is probably 4-600$ and you need two to get 240v ( So car charging is probably out, but if it’s a 4 panel install it really wouldn’t amount to much) . So if you get a 12k btu unit with 4 panels. Adding an inverter and battery could make sense for an off grid backup 120 V system. Enough battery to run a fridge.

Now if I could dual head this unit I would consider it. One head upstairs one downstairs. I’d probably stick with the 12k btu unit. It’s probably not saving a lot of money. Say 1kw for 5 hours a day 180 days a year. Works out to be $120 a year. Best case it’s a factor of 2 higher.

So if you were to get this unit would you choose DC only with batteries, or the AC/DC grid tied unit? It’s heating efficiency is not anymore efficient than my 13 year old heatpump.
 

ABMax24

Minister of Fire
Sep 18, 2019
1,481
Grande Prairie, Alberta, Canada
What about the loss in revenue by not having the solar panels grid tied? Surely there will be times when the panels produce more energy than the heat pump can utilize, greatly lowering the ROI on them. If not there will be times the heat pump can't run because there isn't enough energy coming from the solar panels.

If maximizing self consumption is the goal then it seems a grid battery is the most logical option, and covers all devices in the home, not just the heat pump.

I've thought about this myself in relation to an offgrid house. My conclusion has always been it is better to operate on 120/240VAC or 120/208 3 phase AC for the house wiring and all appliances, because of the vastly greater selection of AC appliances and in many cases better operating efficiencies. The larger inverters are more costly to accommodate this design, but in the end overall efficiency and cost seem to be a wash.

My only concern would be DC ripple in the batteries/DC wiring when operating higher wattage devices like a heat pump on a single/split phase inverter. Although this would vary by inverter, and could also be solved to some extent by operating the house on 3 phase AC and balancing the loads as best as possible.
 
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EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
I think the loss of use for a small array is offset at least partially by not having to purchase an inverter. I don’t see the advantage of the dc mini split if you have a large array. Then I agree 120/240 is the way to go. If you go off grid and have the money to spend the storage solutions are available.
What about the loss in revenue by not having the solar panels grid tied? Surely there will be times when the panels produce more energy than the heat pump can utilize, greatly lowering the ROI on them. If not there will be times the heat pump can't run because there isn't enough energy coming from the solar panels.

If maximizing self consumption is the goal then it seems a grid battery is the most logical option, and covers all devices in the home, not just the heat pump.

I've thought about this myself in relation to an offgrid house. My conclusion has always been it is better to operate on 120/240VAC or 120/208 3 phase AC for the house wiring and all appliances, because of the vastly greater selection of AC appliances and in many cases better operating efficiencies. The larger inverters are more costly to accommodate this design, but in the end overall efficiency and cost seem to be a wash.

My only concern would be DC ripple in the batteries when operating higher wattage devices like a heat pump on a single/split phase inverter. Although this would vary by inverter, and could also be solved to some extent by operating the house on 3 phase AC and balancing the loads as best as possible.
 

ABMax24

Minister of Fire
Sep 18, 2019
1,481
Grande Prairie, Alberta, Canada
I think the loss of use for a small array is offset at least partially by not having to purchase an inverter. I don’t see the advantage of the dc mini split if you have a large array. Then I agree 120/240 is the way to go. If you go off grid and have the money to spend the storage solutions are available.

The cost of the batteries would offset the cost of the inverter though. I know some of the units are marketed as not needing batteries, but I don't accept the premise that these heatpumps are going to be effective when shutting down and starting up every time a cloud goes by, not to mention what it would do to compressor/motor longevity.

An APSystems QS1 inverter is $400 and will run 4 panels for a total output of 1200 watts, that cost would buy 2-3 12 volt batteries. I'd still be going grid tied.
 
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EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
The cost of the batteries would offset the cost of the inverter though. I know some of the units are marketed as not needing batteries, but I don't accept the premise that these heatpumps are going to be effective when shutting down and starting up every time a cloud goes by, not to mention what it would do to compressor/motor longevity.
Good points.
 

Poindexter

Minister of Fire
Jun 28, 2014
2,622
Fairbanks, Alaska
Appreciate this discussion. I do agree electricity rates kinda trump all here. But, if there is a split or differential utility rate, day time rates are almost always higher. Having a standalone heat pump/AC unit that can run on 48 volts would allow someone in a hot enough climate to just use solar to decrease there grid consumption, even if they are still pulling some juice off the grid to meet their total AC needs.

The main alternative, it looks like to me, is to store solar in an expensive battery today, so I can run the meter backwards tomorrow when rates are highest. Or use today's solar to run whatever size AC today and not buy the battery.

So much math and so many variables.

I do advise caution with air sealing alone. A very tight house, like mine was, without adequate ventilation, is an opportunity for mold. While I do plan for my next house to be tight like tupperware, the tightness will be tempered with an HRV system (heat recovery ventilation) to get some air turnovers for mold abatement without trying to condition all the air in the back garden.

What works in one zip code for an efficient healthy home may not work one zip code over.
 

woodgeek

Minister of Fire
Jan 27, 2008
4,557
SE PA
When was your house built @Poindexter ? Do you have central AC? In this case, airsealing would REDUCE indoor humidity.
 

EbS-P

Minister of Fire
Jan 19, 2019
2,424
SE North Carolina
More thinking on this topic. Wouldn’t it make more sense if doing a small system, 3-4 panels install, to have this hooked to your heatpump hot water heater. Thermal storage makes the most of the full rated output all year long. We just had two cool but sunny days right around solstice didn’t need much AC.

If you already have a heatpump this makes no sense. The mini split unit cost is more than the inverter so all that’s left is the automatic disconnect and you will get 100% utilization of your rated output.

I think it has its place but ROI is probably better spent elsewhere unless live somewhere hot. Much hotter than coastal NC.