Any experience with a DC-powered mini-split air conditioner?

When in heating the indoor unit is mostly gas and it will have more volume due to the line sets (Not saying the volume changes... just pointing out the fact that in reality the two side are not equal because of the line set Volume). When in cooling the indoor unit is mostly liquid. If the 'Indoor' and 'outdoor' sides are different volumes the amount of refrigerant in use will vary depending on the mode of operation. More refrigerant is needed for cooling than heating because the indoor side has more volume is filled with liquid in cooling, gas in heating and liquid is more dense than gas.

Regardless. The whole point of this tangent was whether or not it was necessary to be precise with the refrigerant charge. EVEN IF it was true that the same amount of refrigerant was required for cooling or heating there would certainly be different amounts of refrigerant in use with multi-splits depending on the number of indoor units in operation. I have a 3 split 2 ton. Right now I'm only using the 1 ton in my living room... I could turn on the other two units and that would fill those with liquid refrigerant for cooling using a lot more... so obviously there's a lot of surplus refrigerant in the outdoor unit.

Most split units come pre-charged with XX#s of refrigerant for up to XX' of line set. If your lineset is <XX' you don't need to add or remove any refrigerant. Super Easy to DIY. Connect the lines, pressure and vacuum test for leaks, evacuate the lines, open the valves in the outdoor unit to release the refrigerant. Super Easy. I've installed 4 with no problems... they're all working great... and I don't know the difference between an evaporator and a condenser or what does which when and what it's called. Just knew to connect the small lines together and the big lines together... and make sure there were no leaks or air in the lines. Super Easy.

Not a DC system, but I thought I would show you how I do it. My system has been running for a few weeks now without a problem and I'm writing this in 76 degree temp while it's over 100 outside.
To begin I would like to answer the inevitable questions.

1. Doing this make no economic sense whatsoever. I have spent about $2000 on the system, excluding the split A/C unit and only shaved about $15 off my power bill so far. But for me, it's a hobby and a chance to thumb my nose at the power company.

2. I have an ideal setup. The roof of my house is at a perfect summer angle and faces dead south with absolutely nothing in the way to shade the panels. I also live in south eastern New Mexico with about 6.5 hours of sun each day. When I remodeled my house I made the walls an extra 6 inches thick to give about 12" of insulation. There is about a 2 foot thick layer of insulation in the attic. So it's about as well insulated as I can get it.

3. It involves a change of attitude. By that I mean I get up at dawn to open the house windows and cool the house to outside ambient as much as possible. As the sun comes up the heat goes up but my panels start generating power. When the outside temperature matches the inside the windows are closed and the A/C turned on to one or two degrees lower than ambient. As the ambient goes down the A/C is adjusted to be just a degree or two lower. When the panels are generating full power I drop the thermostat about 10 degrees to get the house as cool as possible. When the sun drops and the panels output drops the A/C is turned off and the remaining power goes into the batteries. During the evening the temperature will slowly rise and sometimes I need to run the A/C for 10 minutes or so to bring the temperature back down. In the morning my batteries are at around 25.5 volts.
This is what I mean by living around the A/C, you don't just turn it on and walk away, you adjust your life around it.

My system is comprised or six roof mounted Canadian Solar 235 watt panels. Two panels are wired in series to give about 67 volts at about 9A. 10 AWG wire in metal conduit brings the power down to a porch which I enclosed as my battery room. I wanted to run 3 panels in series but the charge controllers maximum voltage was 100, which was cutting it a bit close for my liking.
So three banks of two panels in series into my battery room. First stop is a 15A DC fuse/panel shutoff for each bank. From there two of the banks are wired into a 40A MPPT charger then out through a 40A DC fuse. The other bank goes into another 40A MPPT charger with it's own 40A DC fuse on the output. From the fuses it goes into four Sam's Club 6 volt series wired golf cart batteries. Yes I know the way to go is lithium but 5KWh total and about 2KWh usable capacity for under $400 vs. 2.4KWh and about 2KWh usable for $2,000 didn't make sense for what started as essentially an experiment.
From my 24 volt battery bank through a 250A breaker/cutoff into a 2500 watt pure sine wave inverter which supplies a regular Homeline breaker box with 15A AC fuses and then onto the 12,000 BTU inverter mini split A/C unit.
It can be done but it's not cheap or easy and it takes a lot of work.

I would point out that this applies to 'off-grid' mini-split. For most use cases upgrading to a mini-split is even more cost effective than solar... especially DIY. I paid <$3k for my 2 ton LG multi-split and it's easily saving me >$50/mo (If I paid for electricity). The biggest benefit IMO is the fact you can cool only the room you're in. At night instead of cooling the entire house I only cool my bedroom. During the day I only cool the living room.

As I often write, PV is about the most expensive way to reduce an electric bill.

Today I finally commissioned my 4th mini split, in the 1080 sq ft car shop. In 88F, in between
the nightly severe thunder storms. I set it down to 72 F just to give it some work, it had that
building cooled and it shut down in no time. Of course heating in the dead of winter will
likely be more of a challenge. Bruce Roe

Solar thermal is usually more expensive But yes... I agree, PV is ~'about' the most expensive way

For more than a few folks, except probably those in colder/cloudier climates, solar thermal for domestic water heating was, and still may be more cost effective than electric resistance heating of water. It was for me once I left Buffalo.

I'd also think that for such cold(er) climates, nat. gas would still be the choice when compared to a HPWH, both in terms of fuel and equipment costs and maintenance/reliability.

But, heating water with nat. gas, if available, is probably still the best way to do it in terms of fuel and equipment costs in most places, in the U.S. anyway.

Some of us do solar as an investment to be recouped...others (like me) do it just to do it. And we laugh all the way to the 'fridge when the grid goes down.
NeilTheCop I really like your ideology; it's actually very similar to what I am planning to do--but being an electronics geek, I plan to have all the A/C stuff handled automatically, i.e. the goal is to get the humidity down, then regulate the temperature....but that'll be subject to the power available from the solar panels, less the current loads. If it's cloudy, the A/C will either be shut off, or run at a low speed.

Just out of curiosity, do you mind sharing what make/model of mini-split you're using, rated BTUs, etc.?

This from 1 Jun Building Reserve and Using KWH, continues about the Fujitsu. Others are Mitsubishi.


Meantime the car shop is well along in the installation of a 16,000 BTU mini-split heat pump, pictures
soon. This RLS3H has a -25F degree operational capability, achieved with some internal supplemental
resistive heat at the very lowest outside temps. This luxury will attempt to keep inside temps at least
40 F year around, as opposed to just being really cold in Feb. This will make quick jobs (oil change)
easy and a quick blast from the propane furnace will get me 65 F for a transmission
overhaul. Bruce Roe



15RLS3H9.JPG

So after a good bit of back-and-forth with a seller on Alibaba, I got the following very informative Chinglish quote:
That's just like 'em to rate the unit at 48v, 888W (maximum 1500W)...then tell me that full power is 1580W (so what's the maximum???), and it only drops to 888W when the room is already cooled down. Umm...I don't think that's worth $1,900 (and told them as much). The EER on the spec sheet is 5.18 (W/W, not BTU/W), but that works out to 1,062W input at full power--not 1,580. Basically, ignore the specs--they don't mean anything. At least I got that answer before buying it.

Adding insult to injury, it doesn't seem that those units have anything to do with 48vDC internally--it's basically a decent 220vAC inverter air conditioner with a boost board to convert 48vDC to 360vDC to run the inverter compressor driver, as well as the 360vDC outside fan. Oh, and it idles at 10W to do nothing (=boost converter losses).
I managed to find a service manual for a 220vAC inverter Daikin mini-split unit, and found that EVERYTHING (except perhaps the reversing valve) ran off of 360vDC. For that matter, I can find a 23.5 SEER inverter mini-split 18,000btu A/C that runs under 1,500W at full power. For less than $1,900. (Comes with a warranty much longer than 2 years, to boot.)

So there, I've (hopefully) saved someone else from chasing a pointless rabbit trail. I've found 48vDC compressors (and 48vDC fan motors) for sale on Alibaba, but that's evidently not what's being used on all of the "48vDC" air conditioners I found on Alibaba.

If EER divided by 3.41214 = COP, you are at 1.5. Certainly not all minis run near 4, and
I stick with the best performers.

I just expect the current best units to all use the technology of 360VDC inverter driven
variable speed motors. It does not matter what the input form is, internal conversion is
not very difficult today. What does matter is the COP. Bruce Roe