help with power inverter

[gm4106]

I have a compact house frig it is a Magic Chef 10 cubic ft model MCBR1010S.  I have looked and can not find the electrical specs.  I want to find an inverter which will allow me to run the frig on occasion without having to run the generator.  Does anyone know how big or any suggestions on an inverter?  I am planning on using two large rv batteries.  How long could I expect the batteries to keep the frig running? As far as quality goes I am looking for the middle of the line as I am only needing to run the frig.  I may decide to go with a bigger and better inverter at a later date. 
Thanks

[JackConrad]

   You really need to find how many amps/watts the refrigerator uses. It should bo on a label somewhere on the refrigerator. It may be near the compressor, requiring removal of the panel on the lower part of the rear of the unit.  If the rating is in amps, multiply that times 120 volt to determine watts. Add the watts of TV or anything else that will be running on the inverter and add a little for a cushion. That will be MINIMUM inverter size. Also note the motors/compressors run more effeciently on a true sine wave inverter.
   What is the capacity of your RV batteries? If you connect them to a 12 volt inverter, multiply the amps determined above by 10 (12 volt insted of 120 volt power source). That is how many amps you will remove from the batteries each hour you are running the refrigerator, TV, etc.  I seem to recall our last refrigerator manual said to expect the refrigerator to run approx. 8 hours out of each 24. Of course this will vary with ambient temps, times door is opened and how long kept open, how much warm food is placed in the refrigerator, etc. If you refrigerator is auto-defrost, add the power consumption of the defrost heating element.  Jack

[Sean]

Ratings plates on refrigerators are notoriously inaccurate.

I recommend you get a Kill-a-Watt meter and let it monitor the fridge over a period of a few days.  The meter will tell you the average consumption as well as peak consumption.  It will also give instantaneous draw at any given time -- take three or four readings of this number when you notice the compressor is running and average them.  You want all three numbers; the overall average will help you size the batteries, and the other two will help you size the inverter.

Remember that average power consumption, which has to do both with the amount of power the compressor draws and the amount of time the compressor is running vs. stopped (duty cycle), will vary with ambient temperature and the amount of air flow past the coils.  Take your measurements under the conditions you will be most likely to encounter while in use, and preferably with the unit installed in its intended location (or a good mock-up).

When shopping for an inverter, you will need one whose surge rating is at least as much as 120% of the peak reading from the Kill-a-Watt; that reading should represent the maximum draw as the compressor is starting up.  You will also need one with a continuous rating of 120% of the steady draw while the compressor is running.  Lastly, for refrigerator use, you will want a true sine wave model.

For battery size, take the overall average consumption reading from the meter in watts, and divide by 5 (for 12-volt systems) or 10 (for 24-volt systems), then multiply that number by the number of hours you need to run the fridge between charges.  That will be the required battery capacity in amp-hours for a 50% DoD. 

Kill-a-Watt meters are widely available on-line and in stores, and some electric utilities and municipalities will lend you one.

Lastly, if the fridge has a frost-free feature and/or door seal heaters, you will want to disable them, at least while you are running it on the inverter, so do this before measuring power consumption with the meter.

-Sean
http://OurOdyssey.BlogSpot.com

[gm4106]

Thanks  Jack
I do need to get more information. I'm going out to look on the back of the frig. I seen a sticker on the compresser but only shows 115-120 volts 60hz. Checking again and looking at the batteries.

[buswarrior]

Some of us got a Kill-o-Watt here: $20 or so...

Canada:  http://www.newegg.ca/Product/Product.aspx?Item=N82E16882715001&Tpk=kill%20a%20watt

USA: http://www.newegg.com/Product/Product.aspx?Item=N82E16882715001&Tpk=kill%20a%20watt

happy coaching!
buswarrior

[TomC]

For instance- I'm looking at the Summit FFBF280W 13cu/ft refrigerator.  While they don't list the running amps, they do list the yearly consumption as 440kw (or 440,000 watts).  Quite simply, if you divide the 440,000 by 365 days a year, you'll have 1205 watts a day.  Divide that by 12v for the inverter, you'll have about 100amp/24 hour. Add to that that most inverters are about 85% efficient, and you have (round figures) 120amp/hours per 24 hours.  For battery longevity, double the amp/hours-and you have a battery of at least 240 amp/hours to JUST run the refrigerator for 24 hours.  Just an example.  Good Luck, TomC

[boogiethecat]

My experience with my summit refrig-

It draws 180 watts when running

My first inverter was a pure sine, 500w with 1000w surge. No way it would start the compressor.
So I sold it and bought a pure sine 1000w / 2000w surge inverter, and it works, but it's always on the edge. If the compressor has been running and turned off, then an attempt is made to run it again within 10 minutes,  it'll fault the inverter, and unless I manually reset it, the fridge will warm up and I loose my food.  Fortunately this never happens unless it's me screwing around with the fridges' temperature knob, which I don't often do.  So though it's "on the edge", it has been reliable for the last two years. 

FWIW I recommend a pure sine inverter for this.  You can certainly use a modified sine inverter but a lot of energy will be wasted.  Refrigerator compressors basically take all the energy that's not part of a perfect sine wave and turn it into heat. So with a modified sine inverter your compressor motor will run, but it will run a lot hotter than normal because all of the voltage waveform out of that inverter that isnt the actual sine will be turned into excess compressor heat,
and you'll probably be wasting 20-30% of your battery power.

To figure what size inverter you really need, the spec you really want to find is "LRA", or "locked rotor amps".  It's nothing to do with the average operating power, but if you want to do it right, that's the spec you would need.  Unfortunately that spec isn't usually given on domestic fridges.
The way to figure it out is, buy the fridge, put an AC ammeter in series with it.  Run it for about 5 minutes, unplug it for about 15 seconds and plug it back in.  The compressor will be temporarily stalled due to high internal refrigerant pressures, and you'll see the "locked rotor amps" on the meter for a few seconds until the compressor's internal thermal cutout drops it out for another try later on.  That amperage reading is ideally what your inverter should be capable of supplying if you want to be 100% reliable.  Using ohms law and 120 volts you can then calculate the minimum wattage of inverter you'll need. (amps x 120 = wattage)

For the batteries, you'd simply measure the normal running amps, figure out the wattage from there, then figure out how long per day the compressor runs (a stopwatch would be a boring but practical method) and you can figure out the watt/hours used per day, which you can then back into amp-hours at 12 volts and see how long your batteries will last, keeping in mind that you really don't want to use the full discharge capability of your batteries or you'll kill em early on...

Cheers
Gary

[Sean]

To figure what size inverter you really need, the spec you really want to find is "LRA", or "locked rotor amps".  It's nothing to do with the average operating power, but if you want to do it right, that's the spec you would need.  Unfortunately that spec isn't usually given on domestic fridges.
The way to figure it out is, buy the fridge, put an AC ammeter in series with it.  Run it for about 5 minutes, unplug it for about 15 seconds and plug it back in.  The compressor will be temporarily stalled due to high internal refrigerant pressures, and you'll see the "locked rotor amps" on the meter for a few seconds until the compressor's internal thermal cutout drops it out for another try later on.  That amperage reading is ideally what your inverter should be capable of supplying if you want to be 100% reliable.

Gary, excellent advice, and this is the reading I was suggesting should be used for the surge rating of the inverter.  I was under the impression that the Kill-a-Watt meter kept a history of peak draw and could supply this number after-the-fact.

I just went and checked, though, and I see I was mistaken.  The Kill-a-Watt provides only instantaneous draw and total consumption since power-up.  So you will still need to do the test Gary suggests.  That said. no need for a separate ammeter, as the Kill-a-Watt will give you this reading -- you just need to be actually looking at it during compressor start-up.  The Kill-a-Watt is still the best way to determine average consumption over time for the purpose of sizing the battery bank.

My experience mirrors Gary's: once you find an inverter with enough surge capacity to start a nearly stalled compressor, the continuous-duty rating will be more than enough to run the compressor indefinitely in steady-state.

-Sean
http://OurOdyssey.BlogSpot.com