What does a Fridge amp rating mean?

chstoyer · June 2017

I am going to help some off-grid neighbors set up a fridge to replace their propane on and a solar system to run it. I have measured a fridge that I own that is about 15 years old so I don't know if technology has improved, but looking at some discussions tells me it has not. The fridge I own draws a 2.4 second surge that is about 8 times the running current (1.2A RMS).

The reason they want electric is that they want a big fridge. I picked a Maytag 36" at random on their web site and it said it draws 20A. I am not sure what, exactly, this means. Seems high for running amps so is this actually the surge and the running amps are much less? Or what does that "20 A" mean?

I need to know so I can set up their requirements for panel wattage, battery bank and inverter size. I don't see inverters common for more than 2,000W, and it appears as though that unit may barely run the random pick I made.

Of course, they will pick a different fridge but the principles of sizing will be the same.

Thanks for any help,

Charles

Estragon · June 2017

20a would have to be the startup draw. I don't think I've ever seen a domestic fridge that doesn't run fine on a standard 15a ac circuit.

A "typical" modern energy star fridge will generally take ~1.5-2kwh/day, depending on ambient (and whether people like to stand in front of it with the door open all day) etc. A bigger unit may take a bit more, but the way the math works (surface area vs volume) the bigger unit can actually be a better deal in terms of watts/cubic foot.

Some features like water/ice through the door can be a fair bit higher. The energy star ratings are categorized, so a fridge with these features just has to be efficient relative to others with such features. They can be quite inefficient compared to a plainer model energy star unit.

A 2000 watt inverter should run a regular fridge just fine.

Lumisol · June 2017

I picked a 2000 watt Cotek to run a very small rv fridge based on the wattage it draws when on (80 watts). I would imagine a bigger fridge with a higher draw will need a larger inverter.
How many watts does the fridge you looked at draw? The start surge should be covered by the surge rating on the inverter.

Estragon · June 2017

I'm pretty sure some posters here have reported running full sized fridges off quality 1500w PSW inverters. I think one might even have used a 1000w. Obviously the closer you cut it in terms of inverter size, the less room for other loads though.

One solution might be to have an inverter dedicated to, and sized for the fridge alone.

chstoyer · June 2017

Thanks for your help. I agree the 20A must be the startup amps, I can't imagine the running amps would be that big. The fridge was rated 508 kWhr/yr in Canada (It's colder up there!) and 685 kWhr/yr in USA. The well water is not all that suitable for drinking so they will not want a water feed into the fridge and the lady of the house wants the solar to run the fridge only (they run a Honda generator a lot) but I am planning on expanding that initially to all kitchen lights and eventually beyond that to the TVs, etc. They will need the generator to run the well pump. Their cabin is at 11,000 feet near South Park in Colorado and it never gets really hot there. They go to AZ in winter as access to this place is not that easy from Nov-May.

I was planning on a 2,000 W PSW inverter. My daughter has one and it runs the teenagers CPU equipment, they have a small fridge and run that off a 1,500 W MSW inverter.

I am posting pictures of the startup current on the test fridge (I forget what make and model it is, it is an apartment-sized unit) and it is not handy to look at. I was incorrect in my initial post that the current surge is only about 1 second.

Estragon · June 2017

One other aspect of fridges and inverters to mention. It's possible that the inverter may go into sleep mode between fridge on cycles. This is a good thing, as the inverter can use a fair bit of power just being on but not in sleep mode - could be 20-40w. That alone could increase the daily wattage by a third or so.

This might be an issue if coming out of sleep mode triggers a defrost cycle though. It stands to reason that the defrost timer should reset to zero after a power failure, but it might be worth checking how this works on a particular fridge being considered. Shouldn't be a big issue with mechanical timers, but might be with an electronic one?

mcgivor · June 2017

At 11000 feet you may want to consider a larger inverter, unless rated for that altitude, than at sea level to ~3500 feet, due to derating factor of 0.8 at 11000 feet. This will apply to other components which are not specified to operate at such altitude

chstoyer · June 2017

I guess I am missing the point on why an inverter is derated for altitude? I know generators and other engines are if not turbocharged, because there is a 2/3 as much air at 11,000 ft as at sea level, but an inverter does not need air except for cooling. And where did you get the 0.8 derating factor?

While I am at it, is there any reason not to use a MSW inverter for a modern fridge. I know PSW is really best for TV, stereo, computers, etc, but does a modern large fridge need PSW?

mcgivor · June 2017

Rather than trying to explain the altitude issue I've provided some links
http://outbackpower.force.com/articles/Electricity_101/High-altitude-derating-factors-for-electrical-systems
http://power-topics.blogspot.com/2012/10/how-does-altitude-affect-ac-dc-power.html

mcgivor · June 2017

chstoyer said:

While I am at it, is there any reason not to use a MSW inverter for a modern fridge. I know PSW is really best for TV, stereo, computers, etc, but does a modern large fridge need PSW?

Modified sine wave inverters are generally hard on motors, the abrupt changes in the simulated sine wave create eddy currents which manifest as heat in the laminated core of the motor, depending on the profile of the simulated waveform, more small steps would be better than fewer large but never as good as a pure sine wave.

mike95490 · June 2017

The inverter/altitude will mostly be the less dense air does not carry away as much heat.
Nothing de-rating can do about the increase of possibility of internal arcing from less insulating air .

mcgivor · June 2017

mike95490 said:

The inverter/altitude will mostly be the less dense air does not carry away as much heat.
Nothing de-rating can do about the increase of possibility of internal arcing from less insulating air .

Guess you are right about derating not helping the increased possibly of internal arcing, the best solution to that would be to choose an inverter built specifically for that purpose like SMA for example. Derating output may be a consideration without additional cooling or larger heatsink, not many manufacturers include this in their specifications, except the higher end units.

lzhome · October 2017

Interesting thread here but got me a little confused. I'm looking to size my inverter to run a side by side refer on a backup basis. The GE label states 11.6 amps. I put a put a meter on and saw it go from 96w and .86a down to 3w and .06a. I'm guessing this shows consumption with compressor on then off?
Would appreciate comments on how I can put these numbers in context to size an inverter. Also I like the idea of a sleeping intverter with no load, who makes this in the 2000w range?

mcgivor · October 2017

There is also a defrost cycle, which tends to use higher current, but 11A seems high, are you sure about that figure?

lzhome · October 2017

mcgivor said:

There is also a defrost cycle, which tends to use higher current, but 11A seems high, are you sure about that figure?

Here is the sticker; it is a pretty big unit.

mcgivor · October 2017

That's not something I'd want to use in an off grid, curious to know what te 11.6A is, it must be the defrost cycle, the compressor could never use that, despite it's size, gargantuan would be a better description.

softdown · October 2017

I doubt that any current Energy Star rated refrigerator is going to consume more than ~200 watts when the compressor is on during normal usage. Not familiar with defrost cycles...I am thinking the high amp figures relate to start up surges....which refrigerators incur many times/day.

An active ice maker is going to destroy the math of course. It takes a lot of energy to make ice. While we on the subject of "bad well water"....have they tried using carbon to enhance the flavor of their water? Minerals do not necessarily effect flavor in a significantly adverse way. In fact, bottled water has some minerals added for flavor. Most bad flavors can be removed with carbon. Many minerals are, in fact, good for us. Calcium is very common in well water for example.

What does a Fridge amp rating mean?

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