Inverter to run small chest freezer?

PlowmanPlowman Solar Expert Posts: 203 ✭✭✭✭✭
I've been living off grid for about a year and half now, but I'll soon be moving where I'll have access to grid power again. My little solar electric system is built onto a cargo trailer, so I can take it with me.

I'm thinking I'd like to run my chest freezer in the cargo trailer. It's a Kenmore 5.0 cf, which I currently keep in a friend's barn. I've only just begun to measure it's draw with a Kill-A-Watt, in the last 45.5 hours it's drawn 1.73 kWh. I think my little 12V 225 Ah battery bank can keep up with that, even accounting for inverter inefficiencies.

What I don't know is how big of an inverter I need. I have no idea what the start up draw of this freezer is and I'm not sure how to measure it. Kill-A-Watt doesn't measure it, and don't really have time to sit next to it with a clamp meter (not even sure that would capture it).

Will a 1000W pure-sine inverter do? Or do I need to go bigger? I probably wouldn't run anything else.

ETA: I'm looking at the Xantrex ProWatt 1000, continuous rating of 900W, surge of 2000W.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,189 admin
    The 24 hour kWH usage is:
    • 1.75 kWH * 1/45.5 hours measured * 24 hours per day = 0.92 kWH per day
    Battery sizing assuming 2 days of storage and 50% maximum discharge:
    • 920 WH per day * 1/0.85 inverter eff * 1/12 volt battery * 2 days * 1/0.50 max discharge = 360 AH @ 12 volt battery bank "recommended"
    The maximum inverter sizing I would suggest for a 12 volt @ 225 AH battery bank:
    • 12 volts * 225 AH * 1/5 hour max battery discharge rate * 0.85 AC inverter eff = 459 Watt maximum AC inverter continuous loading
    • 12 volts * 225 AH * 1/2.5 hour max battery surge rate * 0.85 AC inverter eff = 918 Watt maximum AC inverter continuous loading
    Realistically, you would need an AC inverter in the range of 1,200 to 1,500 Watt minimum rating--Although, if the freezer has relatively low surge current and your inverter has very good surge rating, a 1,000 Watt rated inverter MAY work.

    Solar panel wise:
    • 920 Watt*Hours per day * 1/0.52 off grid system eff * 1/3 hours a day minimum (winter may have "problems" running fridge) = 590 Watt solar array
    At this point, I would have to guess that your 12 volt @ 225 AH and a 1,000 Watt AC inverter is at the very minimum point of possibly supporting your freezer as a load.

    If you wanted a system that was more reliable--I would be suggesting a ~2x larger (Amp*Hour) battery bank.

    If you have a 450 AH battery bank and want a 5% to 13% rate of charge:
    • 14.5 volts charging * 450 AH battery bank * 1/0.77 panel+controller deratings * 0.05 rate of charge = 423 Watt solar array minimum
    • 14.5 volts charging * 450 AH battery bank * 1/0.77 panel+controller deratings * 0.10 rate of charge = 847 Watt array nominal
    • 14.5 volts charging * 450 AH battery bank * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,102 Watt array "cost effective" maximum
    For the larger battery bank, I would be suggesting a 10% rate of charge for a full time off grid freezer for a more reliable power system, longer battery life, and you don't have to have sleepless nights worrying that the freezer's power system went dead with a couple cloudy days.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,360 ✭✭✭✭
    You can measure startup current by getting a clamp-on current probe and wiring the output to the audio input jack on a computer. Then run a sound card oscilloscope such as http://www.zeitnitz.eu/scope_en

    It's too bad that such systems can't use thermal storage instead of batteries.

    I am available for custom hardware/firmware development

  • PlowmanPlowman Solar Expert Posts: 203 ✭✭✭✭✭
    Thanks Bill, I knew I'd be working at the edge of my system's capacity, but looks like it might be a bit over the edge. I'd have a hard time expanding my little system (currently 632W of panel), as you can see, very little room for an additional panel, though i could conceivably add a 100W panel up top (maybe two), but it'd be at a 90 degree angle. Plus I'd need a new charge controller....

    15464433152_7a8c13ef43.jpg

    Any idea if the start-up surge would be the same if I converted the freezer to a fridge with a Johnson controller? I assume it would be the same, but don't know.

    Jonr, thanks for the tip. Don't suppose I could get a regular clamp meter to measure start up surge? I have an Extech true RMS clamp meter (model MA640). No idea how I'd wire it into my audio jack, though (2 outputs vs. 1 input). Unlike some clamp meters, mine doesn't appear to have the capacity to measure inrush current on its own. Maybe I should just save up for one that does, never really liked my clamp meter anyway.
  • BB.BB. Super Moderators, Administrators Posts: 30,189 admin
    It is tough... Refrigerator/freezers are what usually push a small system to a medium sized PV system.

    Changing to a digital temperature controller would not change anything. You could go with a 12/24 VDC freezer--But those are not cheap either.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    BB. wrote: »
    You could go with a 12/24 VDC freezer--But those are not cheap either.

    They are not as cheap as a conventional freezer, but Plowman would not need an inverter at all if he had a DC freezer. He would have a simpler, more reliable system without the tare losses that go with an inverter.
    BB. wrote: »
    Refrigerator/freezers are what usually push a small system to a medium sized PV system

    Very true, and that is also true of well pumps. I usually recommend an expensive, low surge well pump to folks as an alternative to using a cheap conventional pump with an expensive, larger-than-otherwise-needed inverter.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    You could drastically cut the start surge of standard AC refrigeration compressors by installing a properly sized START CAPACITOR in series between the Start Winding and it's control circuit. I've done this with every freezer / fridge I've ever had, but personally know of no one else who has done so. One big problem is that almost every compressor design requires a different capacitor value to do the trick. In my case I have a good friend who owns an electric motor repair shop and has loaned me different value capacitors so I could find which one works best. My experience showed that back in the day, the old units required roughly 200 MFD, but new units are far more efficient and require only around 100 MFD. That has run anywhere from 88 to 150 MFD in my case with the new designs, depending on the compressor. But it's very important for the best results, to find what works best with yours. It's also demanded whoever does the change to the start circuit, knows their way around electricity, motors and wiring, otherwise a burned out compressor can easily result.
    If however you get it right, the results are amazing.
  • SaiproSaipro Solar Expert Posts: 74 ✭✭
    You could drastically cut the start surge of standard AC refrigeration compressors by installing a properly sized START CAPACITOR in series between the Start Winding and it's control circuit. I've done this with every freezer / fridge I've ever had, but personally know of no one else who has done so. One big problem is that almost every compressor design requires a different capacitor value to do the trick. In my case I have a good friend who owns an electric motor repair shop and has loaned me different value capacitors so I could find which one works best. My experience showed that back in the day, the old units required roughly 200 MFD, but new units are far more efficient and require only around 100 MFD. That has run anywhere from 88 to 150 MFD in my case with the new designs, depending on the compressor. But it's very important for the best results, to find what works best with yours. It's also demanded whoever does the change to the start circuit, knows their way around electricity, motors and wiring, otherwise a burned out compressor can easily result.
    If however you get it right, the results are amazing.


    Tell me more about this series-in wiring of capacitors to compressors. Having a large deep freezer, a fridge and 2 A/C units makes this more of a temptation than even I can resist. Or provide the very links that set you on the right path. I don't wanna go erroneously reading some junk post elsewhere.
    Semi off-grid

    255W Canadian Solar × 12, 200AH 48V US 185 XC2 bank, Victron Bluesolar MPPT 150/85, Victron CCGX, Victron MultiPlus 48V/5kVA/70A inverter (primary system) Victron Phoenix 48V/375VA inverter (backup for critical loads)

    300W Yingli × 2, Midnite Brat, 200AH 24V bank (powers DC LED security lights)

  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Saipro wrote: »


    Tell me more about this series-in wiring of capacitors to compressors. Having a large deep freezer, a fridge and 2 A/C units makes this more of a temptation than even I can resist. Or provide the very links that set you on the right path. I don't wanna go erroneously reading some junk post elsewhere.

    There was no link that set me on the right path. It came from my understanding of, and experience with electric motors. As I mentioned, whoever does the change to the start circuit, must know their way around electricity, motors and wiring, otherwise a burned out compressor or worse can easily result. There are just too many variables involved with the thousands of different motor/compressor designs to give exact instructions. What works for one, could easily smoke the next. If you have a local electric motor repair guy in your area, it's best to get him or her involved.
    As to the AC units, they will already have the proper capacitors right from the factory. It's a shame the manufacturers of domestic refrigerator and freezer don't do the same. But then it might add a dollar to the cost.
  • BB.BB. Super Moderators, Administrators Posts: 30,189 admin
    You need to look at the wiring diagram for your refrigerator--There are different ways of setting up the capacitor start circuit:

    Attachment not found.

    Basically, the smaller value of motor start capacitor, the less starting current (and starting torque) you will have.

    You will also need to check the capacitor voltage rating.

    These days, many of the refrigerators combine the Overload, "start" relay, and capacitor in one package (3 in 1 unit) or even as a plug in device from the mfg.:

    Attachment not found.

    Makes it more difficult to do what Wayne did--experiment with different value of capacitors (I think that is what he did) since everything is integrated in one box.

    Also, Wayne did something really neat--He used a transformer (one post he made) that dropped the input voltage to the motor a bit (transformer outputs higher current at lower voltage--Motors need high current to start, not high voltage)--That also reduces the starting load to the inverter. A really cool idea, but not something that 99% of the people here are going to attempt.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SaiproSaipro Solar Expert Posts: 74 ✭✭
    Thanks. I did a little reading in between and just now reverted back to this thread. Indeed the starting capacitors of A/Cs (which seems to be the most replaced component in my experiencewith them) are the very thing you've described. Fridges and freezers ae however a different deal. The fear of frying my compressors simply to save my inverter (which has been correspondingly oversized by the way) is both a daunting and scary tasks. As as Wayne has been able to succeed, I dare say so should I. And thanks for the diagram.

    And using a stepdown transformer to boost the starting current, that's simply genius on Wayne's part.
    Semi off-grid

    255W Canadian Solar × 12, 200AH 48V US 185 XC2 bank, Victron Bluesolar MPPT 150/85, Victron CCGX, Victron MultiPlus 48V/5kVA/70A inverter (primary system) Victron Phoenix 48V/375VA inverter (backup for critical loads)

    300W Yingli × 2, Midnite Brat, 200AH 24V bank (powers DC LED security lights)

  • westyd1982westyd1982 Solar Expert Posts: 85 ✭✭
    I've been working on this issue for a few years looking for a simple solution with no success yet. I have a Haier chest freezer that draws about 70 watts when it is running. I'd like to run it from my Morningstar SureSine-300, so I need the start surge under 600 watts.

    From some tests I've done, the freezer seems to draw 1800-2000 watts for the first few power cycles (sine waves) and drops to about 100 watts in less than a second. I've tried two different Supco units like BB has pictured above: Supco URCO810 (smaller motors) and Supco URCO410 (medium sized motors). These units replace the start and run circuits with an electronic controller and they add a start capacitor. My freezer has a run capacitor. The electronic controller reduces the average run power to about 65 watts.

    I don't have very good test gear for measuring the start surge, but the URCO810 doesn't seem to reduce the start surge much. The URCO410 seems to drop it to maybe 1200-900 watts and it seems to spread out the higher current draw over a longer time. Instead of a sharp spike it is flattening out the curve, which makes sense to me. Supco makes the URCO210 that is for bigger motors. I haven't tried it yet, but maybe it would work better.

    BTW I have a 12 year old Xantrex 1200W MSW inverter that starts the freezer without any modifications just fine.
  • jonrjonr Solar Expert Posts: 1,360 ✭✭✭✭
    I had some success using NTC thermistors but I think that getting a refrigerator that uses an inverter drive would be much more effective.

    I am available for custom hardware/firmware development

  • JoshKJoshK Solar Expert Posts: 232 ✭✭
    Before I thought about tampering with a perfectly good chest freezer, I would just get an inverter that is sized right. I have a Whistler Pro 1600w that runs my chest freezer great and is probably even oversized for startup. It was only $95.
  • micahmelnykmicahmelnyk Registered Users Posts: 1
    @waynefromnscanada  I am also from Canada, and would love to learn more about how to provide the startup inrush current for a chest freezer.  Is there a way I can contact you directly?
  • mike95490mike95490 Solar Expert Posts: 8,659 ✭✭✭✭✭
    Plowman said:
    I've been living off grid for about a year and half now, but I'll soon be moving where I'll have access to grid power again. My little solar electric system is built onto a cargo trailer, so I can take it with me.

    I'm thinking I'd like to run my chest freezer in the cargo trailer. It's a Kenmore 5.0 cf, which I currently keep in a friend's barn. I've only just begun to measure it's draw with a Kill-A-Watt, in the last 45.5 hours it's drawn 1.73 kWh. I think my little 12V 225 Ah battery bank can keep up with that, even accounting for inverter inefficiencies........
    There are better ways to store the bodies than in a freezer, in a trailer, run by solar power :o
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • softdownsoftdown Solar Expert Posts: 2,989 ✭✭✭✭
    With grid power it costs about 30/year to run a newish small chest freezer. Seems the op moved on?
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
  • littleharbor2littleharbor2 Solar Expert Posts: 1,492 ✭✭✭✭
    softdown said Seems the op moved on?
    About 30 months ago.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • softdownsoftdown Solar Expert Posts: 2,989 ✭✭✭✭
    18 months tops. Still somewhat "fresh" in my world.
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
  • stillchillinstillchillin Registered Users Posts: 44 ✭✭✭

    For those of you looking for start components for small appliance compressors Supco makes a universal 3 in 1 relay package.

    very simple 3 wires to the compressor, replaces the start relay, start capacitor and over load in on inexpensive package.

    18- 235 W Kyocera panel, 12- 4-KS-25PS Rolls 1350 Ah, Magnum MS4448PAE, ME RC50, ME AGS, Outback FM 80, Generac 8KW LP generator, 6.5 Honda Portable generator
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