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Old 07-08-2011, 10:01 AM   #1
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Inverter sizing

Just to run the (gas/120) refrigerator on the 120 when in tow, I did the math. When plugged into 120 the heater in the refr draws 2.4 amps. IxE=P 2.4x120=288. so i bought an 800 watt inverter. I thought over kill but the price was right.
Why then does it scream to much power draw? What am I missing here?
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Old 07-08-2011, 10:12 AM   #2
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Some cheap inverters use Surge power as their sales rating and not RMS

Also your amp draw was based on sine wave coming off the grid

What your inverter may be making is square wave. A resistive load like a element will run on square but draw more
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Old 07-08-2011, 10:12 AM   #3
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What inverter? Specs?
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Old 07-08-2011, 10:24 AM   #4
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Oftentimes inverters where the price is right can't deliver anything close to their advertised output.
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Old 07-08-2011, 11:09 AM   #5
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Any chance you bought a 50hz inverter???
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Old 07-08-2011, 05:04 PM   #6
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Most inverters are only 80% to 85% efficient and many aren't rated even that high.
Have you determined how many amps you are actually drawing from the battery when running the fridge on the inverter?
If you figure 85% efficiency the 2.4 amps when on shore power looks like
1.22 x 2.4 x 120 volts = 352 watts. So on the 12 volt side it is 352 / 12 = 29.3 amp draw on the battery.
If for example you were trying to run a 240 watt heating element with a 12 volt battery, you would require a continuous 20 amp draw at 100% efficiency. Running the fridge on 12 volt electric (battery) is really inefficient.
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Old 07-08-2011, 06:27 PM   #7
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Especially heating a resistive load to drive an absorption cycle system designed for propane. A good off grid dc fridge around 7cf should draw a real 55w for comparison
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Old 07-20-2011, 08:57 AM   #8
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Solved the problem

The inverter was large enough. It was my wire size between the battery and the inverter that was to small.
The fridge now works off the 120v through the inverter when in tow or just off the battery. Battery only I watched the 12.89vdc drain down to 12.24vdc in 10 hours. Now I know if I had to I could get through the night.
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Old 07-20-2011, 09:25 AM   #9
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Quote:
Originally Posted by tiger
The inverter was large enough. It was my wire size between the battery and the inverter that was to small.
The fridge now works off the 120v through the inverter when in tow or just off the battery. Battery only I watched the 12.89vdc drain down to 12.24vdc in 10 hours. Now I know if I had to I could get through the night.
Well that's cool! Glad you got it working
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