Off Grid Refrigerator – Big Unit, Small Power

Keeping a refrigerator in an off grid environment is a challenge and expense.  In this series, we cover our experience with several units.  In the previous Tips 2 & 3, we spoke of the new Frigidaire unit, here I’m seriously pleased to follow up on that last bonus tip with good power news

One of the more difficult parts of creating and living within your power budget is getting good power data to use.  Yes you can “worst case it” by using UL power listed on appliances.  The result, however, will be a moderate to largely exaggerated system budget.   Yes, you will have more than enough power, yet the price tag may scare you away from your off grid move!   So realistic power numbers are crucial.

What is the duty cycle of a refrigerator?  Its the percentage of time that the compressor is on versus off.  The worst case assumption is to assume 100%, or that it is on all the time.  You could assume 50%, but again, its just a guess.  The duty cycle will be affected by:

  1. Room temperature for the refrigerator
  2. How often, and how long either door is left open ( That is, how many kids and adults feeding from it)
  3. Quality and thickness of the refrigerator’s insulation
  4. How well the magnetic strips seal the doors closed

There are probably others, but those are the big ones that come to mind first.  The good news here is that I recently purchased a P4400 “Kill a Watt Monitor for measuring power.  Below we start with a real refrigerator model, and a real family of 4 and a 24 hour measurement period.  Without further delay, here are the initial facts:

  • Frigidaire  18 cubic foot freezer/refrigerator (ice maker not used)
    [ Label inside reads Electrolux model FRT18IL6JW1 ]
  • Measurement time of 24.67 hours.  Timer has unit off 7hrs at night.
  • Power measured was 640 Watt Hours total.  115-130W when compressor running, 44W light when door open.
  • Test conditions:  “Normal” family of 4, 2 adults, 2 teens.  Kitchen temperature mostly 65F since winter.

Ok, what does all the data mean?  Well if we did the worst case calculation, and ignore the light inside, we get 24hr x 130W or a total of 3120 WH.  So if you left the door open, or took it off, that’s the power required in your budget.

What about our 50% duty cycle?  Still you would have 1560 WH on your budget!   The actual measured value of 640 WH is less than half of that.  How much power do we save simply by using the timer to turn it off at night?   If you 640 WH adn divide it by the actual 17.67 hours of plugged in time, you get 869 WH.  You save and 229 WH every day, by simply using the timer at night!

Now this is not a perfect number, but much better.  I would guess that in the summer the value might be somewhat higher because the kitchen temperature can be 80F sometimes in August.  We’ll have to revisit this measurement this summer to provide you with the best possible information!

  • UPDATE note:  The freezer part of the unit was about 85% full with frozen blueberries.  This helps the freezer hold its temperature during the nightly power off time.  In a similar way the refrigerator was moderately full too.  I’m certain if we tried this with everything empty the results would be worse.

[Note:  The series continues with the updated refrigerator data]

Was this information helpful?  What do or would you use for your off grid fridge?  What’s your experience?  Please share your thoughts in the comment section below:

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Related posts:

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  2. Quick Refrigerator tip #2 and #3
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  4. Power Budgets [3] Measuring Real Power
  5. Power Budgets[5] Online Calculator Gets You Started
28 Responses to Off Grid Refrigerator – Big Unit, Small Power
  1. Dave Doolin | Website In A Weekend
    February 3, 2010 | 2:58 pm

    Given I’d go off-grid in the Cumberland Plateau region of Tennessee, I’d probably try and route at least some of my cooling through springs. The area is rife with springs and caverns. There’s a lot of spring houses in the region.
    .-= Dave Doolin | Website In A Weekend´s last blog ..Your First AWeber Autoresponder Followup Series =-.

    • marshall
      February 3, 2010 | 3:50 pm

      Its all about energy efficiency. Free and natural cooling sources are at the top of my list when available!

  2. Arild Jensen
    March 11, 2010 | 8:49 am

    My clients who are boaters reduced the number of fridge door opening by placing a supply of cold drinks in an ice chest. If a pack of ice or at least a lot of cold water from a spring so much the better. these boaters tend to be cruising in tropical climates and fridge load become a big item when anchored out. With a cooler on hand they can restrict fridge door opening to meal times only. Some also go as far as only using top loading fridges as well as freezers.

  3. Arild Jensen
    March 11, 2010 | 8:54 am

    Speaking of top loading fridges and freezers. Freezers have set point well below freezing but fridges are set some four degrees above freezing. I have seem DIY plans for modifying the temp controller so a freezer short cycles and maintains a temp just above freezing thus making it a fridge but with a top loading lid. This prevents spilling out the cold air every time you open the fridge.

  4. Arild Jensen
    March 28, 2010 | 11:24 am

    People Who are interested in the duty cycle of a freezer or fridge could wire up an elapsed hour meter to show how many hours the appliance run in any given 24 hour period.

  5. Rusty
    December 7, 2010 | 9:52 am

    My fridge has been on a timer for many months now (approaching a couple years soon) & I have it off from 10 p.m. till 7 a.m daily, 24/7
    The secret, I’ve found, is to freeze water jugs & place them in the fridge each evening. Then back into the freezer each morning. As long as no one spends minutes at a time with the fridge door open, my fridge never goes above 40 deg/F. During winter, my bottles are outside the backdoor right next to the fridge & they freeze using “free energy” while allowing me to keep the freezer full of goodies!
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    • marshall
      December 8, 2010 | 4:23 pm

      Good point Rusty. In all this refrigerator work I never mentioned that you should always have them full! full of food, or jugs of cold or frozen water. This give your system more “thermal inertia”.

  6. robert woodliff
    August 19, 2011 | 12:39 pm

    Dependant up how your life is arranged, one thing I noted some time ago was that a top loading freezer I left my winter jumper on developed a damp patch below the jumper…and concluded that as in the best capatalist tradition the manufacturer has stripped the insulation down to save cost, so the area of insulated between the jumper and lid had cooled down enough to condense out water. So if you have an old freezer and don’t want to throw it out( ‘cos its works ) add additional insulation glued / sealed in place ( to stop condensation / mold developing ) to reduce the energy requirment. And if possible, arrange it, so the back of the fridge is in a cooler space, ( like the window AC units ), if you can hang the back of the fridge into atmosphere on the cool side of the house / dwelling, the heat it is attempting to shed should be removed more quickly reducing the motor running time, and energy consumed.

  7. arild jensen
    March 8, 2012 | 2:36 pm

    This past year many of the energy saving fridges are using LED interior lights instead of the traditional filament bulb. this runs cooler and does not add heat to interior of fridge. It also consumers less energy. there are now LED bulbs with screw bases so they can be used as replacement for the old style appliance bulb while saving energy.

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  13. hugh conroy
    December 24, 2012 | 5:00 pm

    Energy Star refrigerators in North America are a sales scam!
    One of the most efficient refrigerators in the N.A. market uses around 300KWH per year. You can buy a Sunfrost for LOTS of money, they are rated at 150 KWH per year.
    We just got a Bosch refrigerator/freezer from Germany that uses 132 KWH per year, cost less than $1000, and has separate compressors for fridge and freezer sections. Refrigerator/freezers with thus kind of efficiency are common in Europe, for 2013 in E.U. regulations mandate even more efficiency. Units with the North American so called “Energy Star” ratings would be illegal to sell in Europe because they are so very inefficient.

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