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Using a Timer with a Residential Refrigerator when Boondocking


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#1 Rif

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Posted 30 January 2013 - 11:06 AM

A couple days ago I posted in a thread about using a residential refrigerator when boondocking. I mentioned that I use a timer to turn off my refrigerator from the hours of 11 PM to 7 AM except for a half hour at 3 AM. I mentioned that this saved a lot of battery usage when we are boondocking. Some folks questioned that this could be the case and asked for a test with real numbers. This post is to provide the results of that test.

We are fulltimers who are currently boondocking in the La Posa South LTVA in Quartzsite, AZ. The unit is a 21.7 cu ft Whirlpool side by side refrigerator/freezer. Although it has an ice maker, I made no ice during the test period. The outside and inside temperatures were identical on the two days of the test. We used the refrigerator in exactly the same way both days, and although we did not count the number of times the door was opened, the usage between the hours of 9 PM and 9 AM was nearly identical both days.

Although I did not measure the temperature rise during the time the unit was turned off during this test, I have done so in the past. Prior to obtaining the timer, I manually turned the inverter off overnight to see how much power I could save. I have found the temperature rise to be in the area of 4-8 degrees in both the refrigertor and freezer sections when leaving the power off for 8 hours. It was because I wanted to minimize the temperature rise (and auatomate the process) that I decided to use a timer that would allow the refrigerator to be turned on for a short period during the night. With the timer in place I see a temperature rise of no more than 5 degrees, and usually only 2-3.

Electricity is provided by a Magnum MS2800 Pure Sine Wave inverter, an Onan 5.5 KW gasoline generator, and a Honda 2000i generator. The vast majority of the power comes from the inverter, which seamlessly handles the transition to and from generator power when it is available. Power to the refrigerator was never lost during the test except for when switched off or on by the timer.

The timer used was a simple 15A appliance timer purchased at Home Depot. The electricity consumed was recorded by using a Kill-A-Watt meter that was plugged into the timer. The refrigerator was plugged into the meter. Naturally, this does not then account for the amount of electricity used by the timer itself, but it is miniscule in comparison to that used by the refrigerator.

The test procedure employed was to record the cumulative KWH used by the refrigerator at 9 AM and 9 PM each day. The number of KWH used during each 12 hour period was then calculated and converted to 12V amp hours so as to be more meaningfull to battery and inverter users.

THE RESULTS

Daytime usage 66.7 AH
Night time with timer 22.5 AH
Night time without timer 46.7 AH

From those numbers we get:

Daily consumption without timer 113.4 AH
Daily consumption with timer 89.2 AH

Obviously the timer saves a significant amount of electricity, and at no appreciable loss in cooling performance.
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#2 Jack Mayer

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Posted 30 January 2013 - 11:58 AM

Thanks, Rif. Those are interesting numbers. Do you know the energy star numbers for your refrigerator?

Our 22.8 cf Fridgidaire uses approximately 114 DC Ah in a 24 hour period with doing nothing to optimize it - ice maker on, defrost on, water in use. I was surprised you are running about the same. Ours is a 2012 model.

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#3 mcbockalds

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Posted 30 January 2013 - 01:24 PM

THE RESULTS

Daytime usage 66.7 AH
Night time with timer 22.5 AH
Night time without timer 46.7 AH

From those numbers we get:

Daily consumption without timer 113.4 AH
Daily consumption with timer 89.2 AH

Obviously the timer saves a significant amount of electricity, and at no appreciable loss in cooling performance.


I am not surprised by the results, but let me point out what I see as a problem.

Dan's post did not question the idea that you would use less energy during the night with the timer cutting off the elec to the frig. (22.5 AH) compared to a night without the timer (46.7 AH). He thought, however, that during the next day you would use even more energy getting the frig back down to its daily temp, than you had saved during the night (saved = 46.7 - 22.5 = 24.2 AH). In other words, he thought that you would use more than the saved amount of 24.2 AH getting the frig back down to the daily temp.

But your numbers do not confirm an answer one way or the other to his question. We need to see two numbers for daily (24 hr) consumption. One 24 hr period following a night when the timer had been on and another 24 hr period following a night when the timer had not been on. Does that make sense? (Actually the two periods don't have to be 24 hrs long. They just need to be equal in hours and long enough for the frig to get back up to its daily temp setting after the night when the timer was on. Wouldn't 6 hours be plenty??)

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#4 Rif

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Posted 30 January 2013 - 01:43 PM

Jack,

The only number I can find is the annual estimated kWh. That's 537.
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#5 Jack Mayer

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Posted 30 January 2013 - 01:50 PM

Jack,

The only number I can find is the annual estimated kWh. That's 537.


Thanks. That is all I need.I will add you to the spreadsheet.

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#6 Bill Joyce

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Posted 30 January 2013 - 04:54 PM

John: His numbers were 9 to 9 which meant the fridge should already be back to temperature from being turned back on at 7AM. That is 2 hours and he already said a half hour was enough from his prior experience. So the other 12 hours should be the same for each day.

At least someone went and measured something! I keep seeing posts where people speculate and argue without any hard data, so I am really glad Rif made these measurements.
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#7 docj

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Posted 30 January 2013 - 05:16 PM

I agree that Rif's experimental approach to the problem was excellent, but mcbockalds is correct that the data is inconclusive until we see how much energy the fridge uses during the day following a night when the timer has turned it off. I hate to always be the physics teacher, but there is no free lunch in physics. Rif has verified that after a night of being turned off the fridge is a few degrees warmer in the morning. That energy has to be removed in order to get it back down to normal operating temperature and that energy will be reflected in an increase in the next day's energy usage. The correct approach would be to measure the energy usage during the day immediately following a night of timer usage.

At this point we don't know if the energy added to the next day will be more or less than how much was "saved" the night before. We can speculate but we don't know if the fridge is more or less efficient operating continuously when turned back on than it would be if it ran a few minutes at a time as it normally would. Adding to the complexity are secondary effects such as the fact that the fridge will be a bit more efficient when the room air is cooler, so are we talking about hot days and cold nights? if so, how hot and how cold?

We aren't striving for perfection, but before we declare timers a worthwhile investment we should find out how much energy is used the next day. However, we may not, in fact, care if we use more or less energy with the timer since its use permits us to use a generator during daytime hours to provide the needed energy. That alone may make it worthwhile, regardless of anything else.

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#8 Bill Joyce

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Posted 30 January 2013 - 05:30 PM

I am sure Rif had no idea he was having to write a research report for a peer reviewed scientific journal. He needs grad students and a bigger budget. I started out as a physicist, got a degree in math and ended up a mechanical engineer before I ultimately had a career as a software engineer. I guess the engineering mindset became dominant over the scientist and especially the mathematician.

See if I ever post real world measurements here. It is not worth it.

Edited by Bill Joyce, 30 January 2013 - 05:32 PM.

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#9 Marshall & Nancy

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Posted 30 January 2013 - 05:55 PM

Rif. "turn off my refrigerator from the hours of 11 PM to 7 AM except for a half hour at 3 AM."
Did you set your timer to turn it on for the half hour and if yes - why?
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#10 Dan Zemke

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Posted 30 January 2013 - 06:51 PM

Rif,

I had already concluded that I was mistaken about your timer not saving any energy. But I genuinely expected the savings to be tiny. I estimated earlier today that the savings would be about 5 AHs a day based strictly on the thermals and a 1.5 kwH daily power usage. I wouldn't have flinched if your measurements said 10 AHs, but 24 AH of power savings breaks my model. So it appears my calculations are grossly in error, or there is something else going on here. I won't be able to recalculate stuff and document it until Friday - but I will post it.

Thank you very much for providing all of us with the first data I've ever seen on the subject.

Dan

#11 Rif

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Posted 30 January 2013 - 09:52 PM

I've been out all afternoon so I haven't had a chance to respond to everyone until now.

Since I did the test between 9 and 9, the refrigerator had plenty of time to recover during the 2 hours after the timer turned it back on at 7 AM. It only runs 15 minutes or so before shuting off after the timer turns it back on, and it had only lost about 3 degrees by that time anyway. It loses that much and more just by leaving the door open for a minute while I'm trying to figure out which tupperware container my wife stashed the scalloped potatoes in. Doing the comparison between 9 and 9 is going to provide data that is as accurate and repeatable as anything I'm going to be able to do.

As for doing a 24 hour recovery test as some suggest, there is no way I am going to try to capture that data. The number of times the refrigerator door is opened, how long it is left open each time, and the temperature and mass of any items placed in the refrigerator during the day will all impact the daytime power requirement. Plus, all the time the door is open the 60 watt interior light is on. That alone would skew any data. Maybe you could convince your wife that you needed to leave the refrigerator door closed for the next 48 hours while you conducted a test for some doubting Thomas's on the Escapees Forum, but I'm not even going to try.

It is what it is folks, and I think the numbers speak for themselves. The results of this test confirm the "unscientific" observations I mentioned in the other thread that started this discussion and led to this test. Turning the refrigerator off overnight will save a significant amount of battery capacity with a minimal effect on temperature. That's why I tried this in the first place. I have a limited battery bank but a rather large solar charging setup. I really don't care how much power I use during the day because the solar system will normally generate more than I can use. However, I do care about how deeply I discharge my batteries overnight. Turning the refrigerator off overnight helps meet that objective. But it's also clear to me that the amount of electricity needed by the refrigerator to recover to its set point is nowhere near as much as what I have saved overnight by turning it off most of the night.

As for why I have the timer programmed to turn the refrigerator on for half an hour in the middle of the night, it is to limit the amount of temperature gain. As I mentioned, I found that the temperature in the refrigerator and freezer could rise as much as 8 degrees if I left the inverter off all night. Turning the refrigerator on for half an hour every 4 hours limits the drop to no more than 5 degrees, and usually no more than 2-3.

Edited by Rif, 30 January 2013 - 09:53 PM.

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#12 bobsea43

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Posted 31 January 2013 - 07:16 AM

Hey Rif,
Thanx for the test and the info. While it may not pass some peer review I am not a peer, but I do have a RR and recently went into a dry camping situation for several weeks so I am particularly interested in your results. I will turn my refrig off after we are thru with it at end of the day and turn it on in the AM first thing. Based on an instance several months ago where the inverter was accidentally off overnight I don't expect to see much of temp drop.

I appreciate you posting your test results. The empirical data has relevance to me.

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#13 BrianT

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Posted 31 January 2013 - 08:07 AM

Thanks, Rif!!

There are always possible variations in a scenario like this but I appreciate your taking the time to actually measure something.

I have wondered whether something like this could be helpful in reducing the size of battery bank needed in order to run a refrigerator / freezer on a solar setup.

My own thoughts are geared more towards running a deep freeze in a remote "home base" location but they are similar enough that this thread is of particular interest! I've had Amish neighbors that would run their deep freezes off of a generator on their milking equipment, which would run for a couple of hours morning and evening and that's all. And they worked well. I guess I'm toying with the idea that a 5 or 6 hour shot of daily solar power might be enough to keep a deep freeze going without a whole lot of battery input at all. Upright refrigerators that are opened and closed multiple times a day may be more challenging but your experiments are definitely worthwhile, "just to know".

Thanks,


Brian

#14 Smitty

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Posted 31 January 2013 - 10:03 AM

Appreciated the info sharing, and the time it took to obtain it...

(I wonder if I put a time on the DW's iron curler, shutting it off and on every few minutes - if I could have got by with only two 8D's house batteries:)! (I will not do that test....))

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#15 Lou Schneider

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Posted 31 January 2013 - 05:00 PM

If you have a frostfree refrigerator, turning off the incoming power for 7 hours (about 1/3 of a day) also decreases the number of defrost cycles by a like amount. The defrost cycles are controlled by a timer inside the fridge and the timer doesn't advance when the fridge power is turned off.

The refrigerator defrosts by applying heat to the freezer coils - usually from a resistance heater - to melt any accumulated ice.

This may be where a good part of the power savings are coming from - not to mention the benefit of not having the defrost heater energize while all of the power is coming out of the batteries.

Edited by Lou Schneider, 31 January 2013 - 05:21 PM.

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#16 PondPutz

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Posted 31 January 2013 - 09:53 PM

Rif,
Thank you for the information and your time to do the measurements,

That is what I was looking for, as I have a smaller battery bank, and up to 1000 watts of solar available.

Putz

Edited by PondPutz, 31 January 2013 - 09:53 PM.


#17 Dan Zemke

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Posted 01 February 2013 - 04:12 PM

The electricity consumed was recorded by using a Kill-A-Watt meter that was plugged into the timer. The refrigerator was plugged into the meter. Naturally, this does not then account for the amount of electricity used by the timer itself, but it is miniscule in comparison to that used by the refrigerator.

Rif,

My Kill-A-Watt meter would reset when the timer turned off the power to it. Does yours too?

I'm not trying to battle you, I just want to get to the best approximation we can.

Dan

Edited by Dan Zemke, 01 February 2013 - 06:20 PM.


#18 Lou Schneider

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Posted 01 February 2013 - 07:03 PM

Plug the timer into the Kill-A-Watt, not the other way around.
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#19 Rif

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Posted 01 February 2013 - 07:49 PM

Thanks Lou. That's what I did. The Kill-A-Watt never turned off. I read cumulative figures and did the math.
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#20 David & Lorna Schinske

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Posted 01 February 2013 - 07:52 PM

My Kill A Watt has a memory on it (not sure for how long but it has a battery backup). For those of you who are questioning it so much.... timers are fairly cheap (I just bought a two pack) plus they generally last for several years and a Kill A Watt can be bought for under $30. If you are interested in boondocking and/or solar, then getting a Kill A Watt for your own use could be a very good idea. And timers are very handy, especially the ones that you can set up to have multiple on/off times in a single day.