Pure Sine Wave Inverters

memorybabe
memorybabe Registered Users Posts: 7
The fridge we intend to purchase uses 92 watts when running. Our solar setup will consist of 4 - 480 amp hr., 6-volt batteries, connected to a 50 amp charge controller, 2 - 340 watt panels, and perhaps an additional 235-watt panel. On cloudy days we may have to supplement with a generator/charger.
The question I have is: should we purchase a 2000- watt inverter or a 3000-watt inverter? Will the 3000-watt inverter operate at a cooler temperature given that it is larger in size?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    More or less, for running a standard full size energy star rated refrigerator needs a 1,200 to 1,500 Watt rated AC inverter to reliably run (plus run a few LED lights and such).

    A compressor fridge typically takes around 100-120 Watts running, and at least 5x that to start (around 600 VA or more).

    Some folks have tried (over the years) to run a simple fridge/freezer on a 600 Watt AC inverter--And it would not reliably start them--One retired member here was able to set a "soft start" relay and transformer--But that is the exception.

    For an AC inverter... Suggest that the maximum AC inverter on a 12 VDC system be around 1,200 to 1,800 Watts maximum. The surge/run current is just too high for average copper wiring. Example:
    • 1,800 Watts * 1/0.85 inverter eff * 1/10.5 battery cutoff voltage = 202 Amps (and 2x that for starting surge of 3,600 Watts)
    Looking at a simplified NEC wire AWG vs Current capacity:

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    That is 3/0 to 4/0 AWG which is around 0.46 inches in diameter (just the copper).

    https://www.solaris-shop.com/content/American Wire Gauge Conductor Size Table.pdf

    And costs something like $1.20 per foot (from Home Depot).

    For >1,200-1,800 Watts, suggest 24 or 48 volt battery bank, and the wire is 1/2 or 1/4 the size/cost (Power=Voltage*Currint... 4x more voltage, 1/4 the current for the same power).

    If your only load is the refrigerator--Then a 1,200-1,500 Watt inverter is more than enough. Larger inverters also waste more power (idle current)--So over sizing costs you more battery and solar panels (small inverter may take 6 Watts just "turned on", a larger inverter can take 20-40 Watts--Or 50% more energy just to run that one fridge).

    Get a Kill-a-Watt meter and measure your daily energy usage too (hotter room, more energy used. Making Ice, more energy usage).

    https://www.amazon.com/s?k=kill+a+watt+meter

    For example, a mid-size efficient 120 VAC uses around 1,500 Watt*Hours per day--The battery bank would be:
    • 1,500 WH / 92 Watt running = 16.3 Hours per day "pump is on" out of 24 hours per day (just an example)
    • 1,500 WH per day fridge * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 max planned battery discharge * 1/12 volt bank = 588 AH @ 12 volt battery bank nominal
    Using 6 volt @ 480 AH -- Either 2x batteries in series or 2x series for 12 volts @ 480 AH or 2x parallel strings for 12 volts @ 960 AH. Your choice, either should work... But the larger battery bank needs more solar to charge well (10% rate of charge nominal suggested for full time off grid). Or use different sized batteries (voltage and AH).
    • 480 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 904 Watt nominal charging array rating
    And you need to also know how much sun you have for your location. Is this 12 month off grid or 9 months off grid? Say 9 months, no (or very little) genset usage:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Winnipeg
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 40° angle from vertical:
    (For best year-round performance)

    JanFebMarAprMayJun
    2.24
     
    3.37
     
    4.40
     
    5.08
     
    5.13
     
    5.05
     
    JulAugSepOctNovDec
    5.27
     
    5.04
     
    4.09
     
    3.26
     
    2.47
     
    1.87
     

    Toss bottom 3 months which leaves 3.26 Hours of sun per day for October:
    • 1,500 WH per day (average fridge) * 1/0.52 off grid AC system eff * 1/3.26 Hours of sun per day October = 885 Watt array "October" break even
    If you want little to no genset usage (except for a week of bad weather)--Plan on using 50% to 65% base load fudge factor of predicted harvest:
    • 884 Watt array "break even" * 1/0.65 fudge factor = 1,362 Watt array 65% harvest
    • 884 Watt array "break even" * 1/0.50 fudge factor = 1,770 Watt array 50% harvest
    So, based on almost no "real" information, the justifiable array for a "smaller" battery bank would be around 904 to 1,770 Watts... Depending on when you need the power, how much genset usage for charging during bad weather, etc...

    Lots of guesses here. And not a "real" design--Just a quick and dirty first pass estimate. Refrigerators take you from a "small system" (500 to 1,000 WH per day for lighting, cell phone charging, RV water pump, radio, laptop) to a medium size system (something like 3,300 WH per day for a "near normal" highly conservation cabin/home--using wood/propane/etc. for heat, cooking, hot water).

    You want to design a "balanced" system... A large inverter with a small battery bank and solar array--Is not "balanced" and not "useful" for the average cabin/home. You will take the battery bank "dead" if you try to use the 3,000 Watts for more than 30 minutes per day.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • memorybabe
    memorybabe Registered Users Posts: 7
    Bill, thank you for taking the time to respond to my question. Very good information. Much appreciated!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    You are very welcome MB,

    Also... Much of this stuff is varies with respect to what actual brand/model of appliance you purchase... For example, the standard frost free fridge/freezer has a 500-600 Watt heater inside that runs for a relatively short time every 12 to 24 hours--And there is a timer that needs to start the cycle... So, many newer frost free models can require 120 VAC 24 hours per day to keep the timer running (fancy refrigerators may have a digital timer and reset to start a defrost cycle every time AC mains are cycled back on).

    Again--The above is just an example of thought processes. You will need to figure out what will work best for you (money, time on site, seasons, local shading/trees/etc... Solar panels pretty much don't work if there is any shading on the panel). etc...

    Please feel free to ask more questions as needed... And you can keep it in this (or another) one thread/discussion--That way people and see earlier Q&A and other information as the discussion rolls on.

    If you have something different (solar at your city home, etc.), starting a different discussion keeps the subjects separated.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • memorybabe
    memorybabe Registered Users Posts: 7
    The fridge that we are considering purchasing is the Hisense 17 cubic feet, Model # RB17N6DSE. The specs. link is https://content.syndigo.com/asset/1dc798c3-b917-4e1d-a47f-72bd15ee116c/original.pdf, it is energy star rated and appears to be fairly energy efficient. The cost at Home Depot in Canada is $1049.00 vs a 14 cubic foot propane fridge at a price of $3599.00. We are located {Summer Cabin} in a somewhat remote wilderness area {no electricity}.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    Yes--It does make a viable option to get a "relatively inexpensive" 120 VAC fridge/freezer, and spend it on solar+batteries+inverter vs a very expensive propane fridge/freezer and the cost and hassles of bring out propane...

    From your link, it is rated at 449 kWH per year:
    • 449,000 WH per year / 365 days per year = 1,230 WH per day
    Which is not bad at all... HOWEVER, some refrigerator Mfg. have been known play fast an loose with the Energy Star numbers.

    If you can find a fridge and plug it into a Kill-A-Watt type meter to find out how it works for you (try at home on utility power for a week in similar temperature/usage).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • memorybabe
    memorybabe Registered Users Posts: 7
    !!!!!!
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    1500-2000 watt inverter should work for it.

    Other things to know about.   Startup surge current will be higher, driving the need for 1500-2000 watt inverter.  Don't unplug and restart until refrig equalizes and compressor cools down for about 30 minutes.  Most use PTC thermistor to disengage starter cap which has to cool off to get starter capacitor to have effect again. Otherwise startup surge will be a lot higher.

    There is also power for defrost cycles.  Some refrig use higher power heaters for shorter time, others low power heater for longer time.

    Be careful of inverter overhead/idle current.  Some cheap sinewave inverters can have high no load idle current that can significant consume battery capacity over time.  Don't get con'd by standby mode sold as idle current.  Standby mode shuts off AC output and periodically pulses inverter on to see if there is a load.  Light loads and some appliances with microprocessor controls don't work when inverter is in strobing standby mode.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    FWIW,  just picked up an inexpensive Samsung 18 cu ft top freezer fridge (no frills),  for about U$D 780,  plus tax.

    It IS an Inverter type fridge,  which usually means,  low surge, starting.  Presently have no way of measuring the peak AC current on starting.

    Just sayin'  this inverter type fridge,  is now more available in the low end of the market,  finally,  more common in the US.

    Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • 706jim
    706jim Solar Expert Posts: 514 ✭✭✭✭
    My old Trace DR1524 idles at 350ma. I leave it on 24/7 for 6 months at a time. It happily starts my fridge. I see no need for a 3kw inverter if this is going to be its main load.
    Island cottage solar system with 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 30th year.
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Samsung inverter refrig has lower startup surge current but has other issue when running on battery powered inverter.

    Because the U.S. doesn't have power factor regulations in most states, Samsung just uses a simple rectifier-filter cap for three phase inverter high voltage DC power supply.  Its power supply does have a NTC thermistor and bypass relay to avoid high surge current to charge HV filter cap when it is first plugged in.

    My Samsung inverter refrig has run PF between 0.60 and 0.65.  This causes greater inverter losses and since it runs almost continuously it ends up drawing more from batteries over a day than my older conventional GE refrig.  It actually has heater in door to keep outside of door from sweating.  Its 'high efficiency' mode just turns off the door heater.  I regret buying the Samsung.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    edited October 2021 #12
    Hi RC..,

    Thank you very much for the info  ...  all news to me.

    I do not need low start-surge.   Did notice that this fridge was not particularly furgal on AC power  --  448 kWh/year.

    When first plugged in,  the door seal area did seem hot-ish.   But,  have not noticed this heating,  since.

    As always,  thank you for your detailed info,  and analysis. 

    The only reason for the purchase of this fridge,  was,  as a backup for the old Whilrpoo fridge that has been in service for 16 years.   Lead-times for appliances became quite long.   this Summer,  so felt that a backup would be a good idea,  as the old fridge is probaby un-repairable.  This off-grid cabin is also not power frugal,  so,  can live with its excessive needs.

    Thanks again,   Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • MichaelK
    MichaelK Registered Users Posts: 234 ✭✭✭
    One of the things I am not reading in your post is the system voltage that you'll be working with?  Will it be 12V or 24V?  This sounds like a design for a cabin, so I would recommend you going with 24V.  At 12V you are already maxing out the amperage of your 50A controller with just two panels.  340W X 2 panels / 13V charging = 52.3A.  At 24V though the numbers are 340W X 2 panels / 25V charging = 27A, so you have the opportunity to double your panel strings.

    With a 480Ah battery, that's a low charging rate.  Assuming lead-acid batteries want to charge at about 1/8C, that means 480Ah/8 =60A.  At 27A, that's not even half.  If you assume your real-world production is only about 85%, you are actually looking at getting only about 22-23A around noon.

    This will help a lot in the winter, when I'd guestimate you are only getting about 2.5-3.0 sunhours per day?  Your location will determine the winter/summer sunhours you get.  Assuming 2.5SH, you'd make about 1.7kWh per day in the winter, and that's assuming a sunny day.  With an already low charging rate, those two panels might not keep up in December.  And they definately will not keep up in cloudy weather.

    The good news is that at 24V the sine-wave inverter choices are a lot better, with very high quality whole-house inverters designed to be hard-wired into your main electrical panel.  They may also include a built-in generator charging circuit.  You just wire your generator directly into the ACin terminals and run your generator on cloudy days to charge the batteries.

    System 1) 15 Renogy 300w + 4 250W Astronergy panels,  Midnight 200 CC, 8 Trojan L16 bat., Schneider XW6848 NA inverter, AC-Delco 6000w gen.
    System 2) 8 YingLi 250W panels, Midnight 200CC, three 8V Rolls batteries, Schneider Conext 4024 inverter (workshop)
  • Surfpath
    Surfpath Solar Expert Posts: 463 ✭✭✭
    Vic said:
    FWIW, ….
    this inverter type fridge,  is now more available in the low end of the market,  finally,  more common in the US.
    ——————-
    Amen Vic. We are super happy with our LG Inverter fridge, and envision it would allow us to have a much smaller inverter in the future.
    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.