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Old 10-12-2016, 08:04 AM   #29
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I think you are getting caught up in the non sinusoidal wave form coming out of the MSW Inverter. I will go back to my first post, put a True RMS Amp meter in series with the power cord. Most DMM's will do 10 A so that should not be more than a bit of wiring to make a way to connect the meter. Put it in a small box if you are worried.
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The P3 Kill-A-Watt meter IS a True RMS meter.

Kill A Watt Meter - Electricity Usage Monitor | P3
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Old 10-12-2016, 08:32 AM   #30
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OK, I think everything is working fine, and there is some sort of irregularity with the Kill A Watt when using the MSW inverter running the fridge.

I have done two more tests using only the Kwh on the KAW.

8 hours on shore power and the usage was 320 watts. This was my base line.

8 hours on inverter power and the usage was 280 watts. This was overnight, so the fridge doors were never opened.

Based on these readings, I think I can conclude that both the MSW inverter and the refrigerator are working fine together. Since I am expecting about 900-1,000 watts usage per day, I think I am on target, and everything should be fine.

Although I still cannot explain why the Kill A Watt readings for Amps are so different for shore vs inverter power, it does appear over time that the Kwh readings match and are right in line for what I was expecting.

Thanks to all who helped me understand this problem better.
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Old 10-12-2016, 08:43 AM   #31
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Originally Posted by nothermark View Post
I think you are getting caught up in the non sinusoidal wave form coming out of the MSW Inverter. I will go back to my first post, put a True RMS Amp meter in series with the power cord. Most DMM's will do 10 A so that should not be more than a bit of wiring to make a way to connect the meter. Put it in a small box if you are worried.
Exactly! You need a true rms meter to read MSW correctly. I would imagine for the price the kill-a-watt is not a true rms meter.
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Old 10-13-2016, 07:15 AM   #32
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Exactly. Even if it is there may be a calibration problem. That is why the answer to bad readings is to check the meter with another one thought to be good.
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Old 10-13-2016, 10:23 AM   #33
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There is another parameter not being reported: phase angle (or KVA versus Watts). With AC power, Volts X Amps DOES NOT NECESSARILY equal Watts. There is a third term called "power factor" or PF for short. In an AC circuit:

V X A X PF = Watts.

The KAW will also show Apparent Power which is VA or volts X amps. My guess is that the PF of the refrigerator changes significantly with the MSW versus the true sine wave. In my case, the Samsung sounds different (as does the microwave) when running on the MSW inverter, indicating that something is responding differently to the MSW power.

By the way, both your generator and inverter while they say they are good for some many watts, in reality they are really only rated for that many VA so running a load with a low power factor means they can't really deliver that much power (watts). Motors and a lot of electronic devices actually have pretty low power factors, as low as 60% in many cases.
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Old 10-29-2016, 04:03 PM   #34
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Just a quick update.

After using the new refrigerator for a few weeks, and monitoring its usage while on the inverter, everything seems to be fine. The usage has been about 800 watts a day, and the new fridge is nice and cold. So, I am very relieved that my MSW seems to run it just fine. I don't know how it will work over the long run, but for now all seems good.
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Old 10-30-2016, 05:51 AM   #35
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My inverter only draws off 4 house batteries. Volt meter at outlet while on invert power shows 110 to 113 volts.2000 watt msw. Most residential fridge require 115 vote min to run properly. If we had 2500 watt plus 2 more house batteries I'm certain we would get 115 m to 120 volts
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Old 10-30-2016, 06:23 AM   #36
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My inverter only draws off 4 house batteries. Volt meter at outlet while on invert power shows 110 to 113 volts.2000 watt msw. Most residential fridge require 115 vote min to run properly. If we had 2500 watt plus 2 more house batteries I'm certain we would get 115 m to 120 volts
That's really not how it works.

Any size inverter will output a steady usable AC line voltage regardless of the the amount of batteries in the bank.

If the battery bank is run down or not large enough, to handle the load, the inverter will sustain that usable voltage until it shuts down.

Many volt meters are inaccurate in use. There are also many that, while accurate on shore power, are not accurate measuring MSW inverter power.

IMO, a 1000 watt inverter will run most modern refrigerators. The size of the battery bank will determine how long it will run before the bank is depleted to the inverters cut off voltage.
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Old 10-31-2016, 07:29 AM   #37
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+1 on what twinboat said. Modern electronics are mostly closed loop controllers working against a reference. The internals know what the output should be and adjust to get there. That accommodates battery sag as it discharges.
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Old 10-31-2016, 08:15 AM   #38
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First: I am told the Kill-a-Watt does not like MSW inverters and can be damaged by them. That said I ahve never tested that theory and do not plan on doing it.. The person reporting said even TSW inverters can damage them but I have tested that theory.. NO DAMAGE.


Now. Fact: Both readings may be accurate.

Modern Residential FROST FREE Refrigerators contain not only the cooling system (Compressor and coils and such) But an electric heater that "Defrosts" after the cooling cycle.. You may have measured during a Defrost period. and gotten the 1.x Amp when on Mains power, and just during a cooling period on inverter power.

On some the defroster is controlled by a timer.
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Old 10-31-2016, 09:22 AM   #39
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I have a Kill-a-Watt. Works fine with multiple MSW inverters I have tried. It identified a low frequency problem that was impacting the cooling of the freezer on my RR.
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Old 11-01-2016, 06:30 AM   #40
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Off hand I can easily conceive of how a Kill A Watt can have an error reading the output of an MSW Inverter but I cannot think of a reason the Inverter output would hurt them. Two different issues. OTOH there are some folks that would have us believe any MSW will kill everything. Sort of makes me wonder why so many are sold. ;-)
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