Running an a/c unit off a power inverter

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I am contemplating a dc generator as a power source for a 13.5 btu a/c. My thought is powering the a/c off an inverter connected to the batteries charged by a 120 amp dc generator. The batteries would supply the excess power for the compressor start and the dc generator would supply the running electricity which is 1500-2000 watts. Solar would fill the deficiency or reduce the dc generator load.

I am wondering if or how well a 3-4000 watt inverter would work with an ac unit. Any thoughts, experience or input would be appreciated.

Note: My experience with numerous ac generators has been poor at best.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
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    Re: Running an a/c unit off a power inverter

    Depending on your "pure" DC needs--I would seriously look at the modern "inverter style" Mini-split A/C systems (both cool only and even heat pump versions).

    In general, AC is lot easier to deal with and you can use DC locally to the battery bank/charge controllers.

    A 3-4 kW AC inverter is pretty big, and a good mini-split might be able use 1/2 or even less (some folks here using Mini-splits can set them on low and run ~300 watts all day long peak and average wattage to pre-cool/keep the house cooler during the day rather than having to run at 100% in the evening when everyone comes home).

    Search the forum on mini-split AC systems:

    https://www.google.com/search?q=mini-split+ac+site%3Awind-sun.com

    What ever you do, don't spend any money until you have your loads/power needs defined and have a firm design on paper first.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Running an a/c unit off a power inverter

    Welcome to the forum.

    The basic plan is sound: essentially running the A/C from an inverter powered by batteries and then recharging the batteries as needed via DC or AC gen or solar.

    The size of the inverter and batteries will depend on the size of the A/C unit and its duty cycle. That last part is a bit difficult to determine in advance as it will change according to the weather (hotter = A/C runs more). You will definitely want a pure sine inverter.

    We have in the Energy & Conservation section a few threads discussing A/C use, including the very long and involved one on mini-split A/C units and some tests of window units and portables. They all have a pretty good kW hour appetite.

    DC generators tend not to be as efficient as most people think. This is because all generators actually produce AC which is then rectified to create DC output on some. For the most part, using a quality inverter-generator powering a stand-alone battery charger designed for deep cycle batteries (none are perfect for this) is the best bet.

    Solar can be added to supplement or even supplant generator charging.

    But be advised this is a fairly expensive way of running A/C as opposed to connecting it to the grid in most instances.
  • silvertop
    silvertop Solar Expert Posts: 155 ✭✭✭✭✭
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    Re: Running an a/c unit off a power inverter

    What you are contemplating is similar to what I do. I run a 10,200BTU off my 24volt 3000 watt inverter. I run this inverter in search mode to conserve power. I did run this last year off my 1500 watt inverter but I needed to be carefull what else was turned on at the time.
  • Dragonlady5
    Dragonlady5 Registered Users Posts: 19 ✭✭
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    Re: Running an a/c unit off a power inverter

    Silvertop,
    Is your 10,200 BTU AC unit a mini split (evaporator and condenser separate) or is it all in one? Reason for asking is AC unit typically demand a high start-up power. In a test with a current probe and oscilloscope a surge of 38 Amps for 150 msec was measured on a 6500 BTU Danby AC unit, (4370 Watts). One generator manufacturer (inverters will need the same) recommends a surge of 6000 Watts for a 10,500 BTU AC unit. Not sure if the mini splits have the same issue or not.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Running an a/c unit off a power inverter

    The mini splits don't have the surge problem due to their built-in VFD. Greatest invention since sliced bread as far as off-grid A/C is concerned; eliminates the biggest problem. Wish they'd build refrigerators like that! You could run one off a 300 Watt Morningstar if they did. :D
  • Dragonlady5
    Dragonlady5 Registered Users Posts: 19 ✭✭
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    Re: Running an a/c unit off a power inverter

    Good to know based on the mini split discussions I suspected this but was not sure. Agreed, this is a game changer!
  • H2SO4_guy
    H2SO4_guy Solar Expert Posts: 213 ✭✭✭
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    Re: Running an a/c unit off a power inverter

    I have a Mitsubishi mini split Mr. Slim model that is 9000 BTU heat pump that is 26 SEER. Last night I ran it from a 12 volt Exeltech XP-1100 inverter that is 110 volts. That powers an Outback PSX-240 transformer which gives me 220 VAC. On full-tilt-boogie it draws about 930 watts, but it only does that for 15 minutes, then it goes to a bit less cooling mode.

    This is for the LQ (Living Quarters) which is 175 sq ft, inside of a barn, and pretty well insulated. It cools the room down in about 15 minutes and if I leave it on, like when the batteries are full and the sun is shining, I can get the room to 62 degrees! It is pretty quiet, works great, and my two gripes are the remote is not backlit, so at night you need a light of some kind to adjust controls more than just the basic temp up or down, and it does not ever seem to ever shut off, just keeps running drawing power. I usually run it for 25 - 35 minutes and shut it off, or at night I'll set the timer to shut down after 20 or 30 minutes.

    On med fan and thermostat at 59 degrees, it draws about 320 watts and just pours ice-cold air into the room. If the temp is set a bit higher, it can draw as little as 230 watts, so it is pretty efficient. I am reworking batteries to run it longer, so hit me up in a few weeks and I'll give you an update if you're interested.

    Skip

    P.S. I bought mine off of evil-bay for $1520 delivered and it was factory-sealed in a box. Had a buddy of mine with an EPA cert for AC run the lines and it didn't even need any Freon, comes precharged, but we ran less than 25' of line. Uses 1/4" and 3/8" lines, 220 volts to the outside unit, and 5 wires from the outside unit to the inside unit.
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: Running an a/c unit off a power inverter
    803M wrote: »
    I am contemplating a dc generator as a power source for a 13.5 btu a/c. My thought is powering the a/c off an inverter connected to the batteries charged by a 120 amp dc generator. The batteries would supply the excess power for the compressor start and the dc generator would supply the running electricity which is 1500-2000 watts. Solar would fill the deficiency or reduce the dc generator load.

    We put in a new Trane 2-ton AC unit this year and I had visions of running it off solar power (which only works when the sun shines). I had to regroup (AC units use unbelievable amounts of power) and come up with a new theory on how to run it. We put in a small prime power diesel generator coupled to our inverter using Generator Support to handle the starting amps of the compressor. The small diesel provides continuous 2.4 - 3.0 kVA prime power on about .22 gallon of diesel fuel/hr.

    It's my thinking that an AC genset powering an AC load, and using solar to fill in the gaps for compressor starting and loads beyond what the genset can supply is going to be more fuel efficient than running genset DC power into an inverter and then converting it back to AC to run the load. You're talking about a prime power application, just like we use, and the less conversion steps there are the more efficient (and affordable) it is.

    Just to give you an idea of how bad even efficient AC units are - we typically use ~25 kWh/day in our off-grid home. Running the AC increases our daily consumption to at least 40 kWh/day, and it can go over 50 kWh/day if it's humid and the compressor in the AC runs more steady. We got $50 Grand invested in equipment to generate the first 25 kWh/day reliably, day in and day out. No way we're going to spend another $50 for a seasonal load. The small diesel generator will produce that other 25 kWh/day for a cost of about $7 bucks in fuel.
    --
    Chris
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
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    Re: Running an a/c unit off a power inverter

    It is hot here today (car said 119F and patio thermometer said 118F). From 3PM to 7PM the pair of Trane 3 ton 17 seer units ran at near 100% duty cycle. The 3 ton mini-split on my office with a 9000 BTU wall unit duty cycled about 1200 watts every few minutes. (the other 2 wall units are off)

    BTW the low tonight is predicted to be 90F and even hotter tomorrow.

    So far we are 52 kWh in the hole today with about 65.9 kWh generated off the solar. Still 5 hours till midnight and I bet there is another 25-30 kWh yet to be consumed. Come on off peak and thank goodness for the net metering bank of kWh.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Running an a/c unit off a power inverter

    you can only do what you can do dave. when it's an exceptional time what real choices do you have? all i can say is i'm glad it isn't me dealing with that kind of heat as i can't take it that well. i know this first hand when pittsburgh had a hot spell back in the 90s with temps into the 100+ area and lows about 90. you can keep it.

    with the low humidity levels going out there i'll bet the swamp coolers are doing fairly well.
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
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    Re: Running an a/c unit off a power inverter
    niel wrote: »
    you can only do what you can do dave. when it's an exceptional time what real choices do you have? all i can say is i'm glad it isn't me dealing with that kind of heat as i can't take it that well. i know this first hand when pittsburgh had a hot spell back in the 90s with temps into the 100+ area and lows about 90. you can keep it.

    with the low humidity levels going out there i'll bet the swamp coolers are doing fairly well.

    Actually we right at the monsoon set points of a dew point of 55 degrees ( saw 40% humidity a day or so ago). It tried to rain on Monday with thunder and a few drops and the dark clouds were here yesterday as well. The swampies are probably running out of cooling now and the temps still peak above 110F. My AC units are spitting out water now.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Running an a/c unit off a power inverter

    yes, humidity is a problem here lately as it has been raining quite a bit for about a month or so now. very few days without rain.:grr
  • Ol'Dude
    Ol'Dude Registered Users Posts: 11
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    Re: Running an a/c unit off a power inverter

    Howdy

    I've run across something interesting ..... somebody called Daiken has DC geothermal heat pumps. They use a slow-start, keeping in-rush peak loading to a minimum. Apparently, they rectify the usual AC grid power, so they should be a natural for a DC based off-grid system. Might have to do some DC-DC conversion, though.

    We went to geothermal five years ago, and even at the mid-20s SEER we get from single stage, it cut energy use (mostly, heating and cooling) by more than half, comparing KWhours+NatGas cubic feet (heating went from Nat Gas to electric so total gas went way down)

    If you can provide water around 5gpm and 55-60 degrees, those Daiken units claim EER nearly 60, and come in 3 and 4 ton capacity.

    I bought a 5500 watt generator for backup, along with a natgas/LPgas conversion kit, but haven't needed it yet.

    I'm in contact with a Chinese MPPT controller manufacturer, but he's reluctant to sell me a spare parts kit and service manual and I'll be damned if I'll be hostage to a chinese repair facility .... buying a spare controller or two makes some sense, but I like to repair things when I can. Too many people are anticipating solar flares or EMP strikes to simply ignore. Not when maybe a hundred bucks of spare parts could get you up and running again. And not compared to a thousand bucks next-flight airfare back to ....and return from ..... or days turning into weeks, broken down ....

    I haven't found much discussion of the mono- VS poly-crystalline output differential, ie output VS offset degrees from normal to the surface. One type seems better suited to fixed position, the other better suited to use with a tracker .... from some back-of-the-envelope calculations, it looks like a totally tracked array might produce almost 3X the daily KwH from the same number and size non-movable panels and the funny thing is, they cost about the same. Panels, that is, not the tracking thingy.

    To be honest, it's the batteries been holding me back. Hydrogen out-gassing is dangerous ... I once blew the top off a battery from a static spark ... good thing I was well off to the side, never the less I got splashed with acid, and the bang was deafening. Prior to that I had several years experience with Silver Oxide cells, even small ones could produce prodigious amperage. Mistakes there turned number 8 awg wire into tiny metal balls deposited along the split-open former teflon jacket. So, been there, done that.

    Guess I'll go with 6 volt floor sweeper/Zamboni's, unless I can score some nice single-cell Exides or something. 24 volts looks good.

    My (present) neighbor said he's getting a Tesla ... I asked if he was planning to park it in his (built-in) garage .... should be here around Thanksgiving. Gotta get another fire extinguisher to keep at the side door ... Lithiums seem nice, but they are sorta scary.

    Nice to see pre-built panels in the 3 foot by 5 foot range approaching a half-buck a watt. Just can't find any test results for EMP survival of PV cells ...is that EMP blaster in Albuquerque still functional? I know they must have tested satellite PV stuff.

    Gonna put me together something 'real soon now'

    Thought I'd stick this here, as heating/cooling seems to be our biggest 'use' (or 'need' ?)

    Ol'Dude, his actual self
  • Hogan
    Hogan Registered Users Posts: 4
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    Re: Running an a/c unit off a power inverter

    Small and efficient window unit A/C, probably no problem; if you have the battery and charging capacity and current delivery to handle the surge and continued running draw.

    Poster above has the right idea: heat pump. Not sure, unless you already have a well with large casing or are planning one, if geo-thermal is practical.

    Looked at running our homes Lennox heatpump from the Trace 4024 stack. The compressor took 20-25kw to actuate and that was about all we had. Really didn't need the
    cooling function except at night in dead of Summer, so we kept that circuit on the grid.

    Small efficient window unit seems the answer, or size your pump's compressor to work within your parameters.