Amp draw for fridge - nothing is adding up

sarahps33sarahps33 Registered Users Posts: 5 ✭✭

Hey there, so I'm hoping someone can help me out. I'm throughly confused about my fridge amp draw that will be in my e350 camper that I'll be fulltime living in in just 2 months.

I have a magic chef 10.1 cubic foot 'normal AC' compressor fridge. On the back of it it says
1.5a
160w

From what I understand if I divide watts (160) by volts (115) I'd get amps (1.39a)

Which is already different than what is listed on the back of the fridge.

So then I'd want to multiply the amps by how many hours a day it would run, right? I guess I'll just say 50% of the time to be safe, so 12hrs.

1.39ax12hrs = 16ah

Then I'd have to convert that to DC right? Which would be the amps multiplied by 12v.
16x12 = 192 Amps to run the fridge for 12 hours a day.

Can someone tell me I'd I did this right?

Then there are questions, like I've read a little about duty cycle of a fridge, does that just mean when it runs? Would my 12 hr running time account for that?

And to make things even more confusing I have energy star rating for the fridge.

It says
297kwh per year

So I followed this calculation
297000 / 365 days a year =
843 wh per day.
843 / 24hrs a day = 35.15 watts per hour.
Watts (35.15) divided by volts (115) = .31 amps

Which is even farther off from the above calculations based off what's on the back of the fridge.

So if I take .31a x 12v = 3.72 amps.
3.72amps multipled by 12 hours of running = 44.64.

So the calculation using the back of the fridge info leads out to 192 ah a day while the energy star leads to 44.64 amps a day.

Hence why I'm terribly confused! Lol. Which one is closer to reality? I'm trying to figure out how many batts I'll need in my bank and panels - etc so that's why I'm trying to get all of this "locked in."

Thanks so much for reading,  looking forward to your alls responses!


Comments

  • PhotowhitPhotowhit Solar Expert Posts: 5,810 ✭✭✭✭✭
    Yes, The power used to run anything on thermostat will vary.

    The difference in the 1.5 at 160 watts vs the division culd be them using a minimal number for voltage and rounding, or simply the power factor of the motor.

    Guessing at the 'duty cycle' of the compressor, is just that a guess. I'd use and try to understand the Energy Star numbers. You will want to look up the conditions they are based on, Something like an ambiant temerature of 74 degrees and opening 4 or 6 times a day. So in colder rooms it will require less energy and hotter rooms more energy.

    Since it runs on AC, I'd get away from calculating using amps!!! Mostly because you are ignoring the added energy used by your inverter. Most small ones will peak at around 90%. If you aren't using a pure sine wave inverter the modified sine wave will also add to your energy use due to added heeat when running motors.

    So in that perfect world of Energy Star you'll use somewhat close to the 843 watthours per day at the fridge and may lose another 85 - 130 watts running the inverter. Another 50 watts if running a modified sine wave inverter....
    ....all of which varies with the ambiant room temperature and how often it's opened.

    Clear as mud?



    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    edited December 2021 #3
    Welcome to the forum Sarah,

    The rating labels on appliances are (mostly) worst case numbers for a running appliance. They are not very useful for understanding/planning out a solar power system.

    For example, I have a standard freezer that takes around 150 Watts when plugged in and cooling from a "warm" start (freezer is at 70F internally). And by the time it reaches around 0F, it takes around 110 Watts. The compressor takes less Watts (power) to run when the compartment is freezing vs "warm".

    Also, labels should list worst case running. Say it is designed to run at 105 VAC low line voltage, then the running amps would be closer to:
    • 160 Watts / 105 VAC (typical "low voltage" utility limit)= 1.52 Amps (assuming linear relation between different voltage levels)
    • 160 Watts / 132 VAC (typical "high voltage" utility limit) = 1.21 Amps (high line voltage)
    Also--AC (alternating current) power has other factors that need to be taken into account. One is something called "Power Factor". Induction motors tend to not be "ideal" electrical loads. They take their current slightly out of phase with the AC voltage sine wave. This causes the current (Amperes) to be higher than if you just calculated current draw from the Watts (power) number.

    Induction motors have a PF of (typically) around 0.80 to 0.95 -- So the current draw may be worse than the the "power equation" would indicate. For example (just made up numbers, you have to measure yours):
    • Say 120 Watt running at cold temperatures. And 120 VAC nominal voltage, with PF=0.80 
    • Power = Voltage * Current
    • Current = P/V = 120 Watts / 120 Volts = 1.0 amps (ignoring PF)
    • the "real" power equation for AC/non-DC power = Power = Voltage * Current * Power Factor
    • Current = P * 1/voltage * 1/PF = 120 Watts * 1/120 VAC * 1/0.80 PF = 1.25 Amps
    • VA = Voltage * Current (this is not power, it is Voltage * measured Current)
    • VA = 120 Volts * 1.25 Amps (measured with PF) = 150 VA (volt amp)
    What does this all mean...

    First, you should measure your actual loads using a Kill-a-Watt or similar meter. This relatively inexpensive meter is great for understanding your present loads, conservation, and planning for an off grid solar power system:

    https://www.amazon.com/s?k=kill-a-watt+meter

    This is "good enough" for understanding your loads (it gives the basics needed: Volts, Amperes, Watts, Watt*Hours, Power Factor).

    How does this help you...

    VA (volt amps) is used for sizing the wiring, AC inverter, transformers, etc. VA helps you design a system that is "strong enough" to withstand the physical effects of current (the heating effects of current on wiring, AC inverters, transformers) and for planning on solar harvest and battery sizing (Watts and Watt*Hours are Power and Energy used--And that is "what the battery bank "sees"--mostly).

    What is Power Factor? Math wise, it can get complicated (involving Calculus, vector math, etc.). But one way to think about it... When you peddle a bicycle, you can put your force synchronized with the peddles and efficiently move forwards. Or you can be "out of phase" with the peddles a bit and "waste" some of your force (standing on the peddle at the top or bottom of the cycle--Does not help you move the bike--But the peddle arms must be strong enough to take that force). PF is the electrical equivalent of how "efficiently" the current is being used with respect to the voltage sine wave (or your overall force with respect to peddling):

    https://en.wikipedia.org/wiki/Power_factor

    A simple starting point is to use the Kill-a-Watt meter plugged in for 24 hours to measure your Watt*Hours used.... And plan your solar usage that way.

    Watts is a "rate" like miles per hour (50 mph)
    VA is used to plan wiring and AC inverter sizing
    Watt*Hours is an "amount" like miles driven (50 mph * 24 hours = 1,200 miles driven)

    Do not get too much into "exact" accuracy. Any numbers within ~10% is pretty much "dead on" for solar planning.

    With variable energy usage devices like a refrigerator... You need to measure your actual usage (fridge plugged in, opening door, adding/removing hot/warm/cold food, making ice, etc.) as you normally would (yes, making ice takes more energy and you can measure it with a Watt*Hour meter). For a typical refrigerator running in a typical home, they cycle around 50% on/off (on for 20 minutes, off for 20 minutes, etc.)... Your usage may look like this:
    • 120 Watts * 0.50 duty cycle * 24 hours = 1,440 WH per day
    • 1,440 WH per day * 365 days a year = 525,600 WH per year = 526 kWH per year (yellow energy star tag rating)
    Now, the VA and starting surge design issues (examples--Generally we always do "worst case design" so PF is usually "lost in the noise"--Don't worry about 0.80 factor when we "double" the size of the system because of starting surge issues and PF does not play into battery sizing/array sizing, etc.):
    • 120 Watts * 1/0.80 PF = 150 VA
    • 150 VA / 115 VAC "your inverter voltage" = 1.30 AAC (amps AC)
    Sizing for 1.3 amp wiring/AC inverter is a "don't care" here... 14 AWG minimum AC wiring is rated for 15 Amps--1.x amps will not make a difference here (PF lost in noise).

    Then there is starting surge... A typical induction motor AC compressor can take upwards of 5x rated running power to start:
    • 150 Watts * 5x starting surge factor = 750 VA starting (technically we are in "VA land", not Watts for starting surge on induction motor)
    Because of the starting surge, we need to size the minimum AC inverter to manage this starting surge (of a few seconds). Typical residential AC inverter max Watts = max VA rating. And we recommend a minimum of 1,200 to 1,500 Watt (VA) rated AC inverter (and battery bank to support starting surge) for reliable staring/running of a refrigerator freezer (plus a few lights, cell charging, etc. loads).

    Regarding:

    It says
    297kwh per year

    So I followed this calculation
    297000 / 365 days a year =
    843 wh per day.
    843 / 24hrs a day = 35.15 watts per hour.
    Watts (35.15) divided by volts (115) = .31 amps

    Which is even farther off from the above calculations based off what's on the back of the fridge.

    So if I take .31a x 12v = 3.72 amps.
    3.72 amps multiplied by 12 hours of running = 44.64.

    So the calculation using the back of the fridge info leads out to 192 ah a day while the energy star leads to 44.64 amps a day.

    Your math is sort of correct. Using 297 kWH per year is "usually good enough" for solar planning (supposed to be average fridge usage in a 90F room--Average worst case for summer).
    t
    Note here: Watts is a "rate". "Watts per hour" does not make "math" sense. Same with Amps (rate)--Amps per hour is not correct.

    Watt*Hours is amount of "energy " or "work". Like MPH vs Miles driven. Or gallons per hour pumping vs 100 Gallon water tank (total amount of water pumped).

    To avoid confusion, we typically convert all power & energy usage to Watts. As you have seen, a mixed power system (120 VAC vs 12 VDC) when you throw Amps into the mix, it quickly gets confusing. For example, both are true:
    • 120 VAC * 1 amp = 120 Watts
    • 12 VDC * 10 amp = 120 Watts
    Watts (and WH) are a 'complete" unit. Tell us 120 Watts... That is all we need to know. With Amps--We always have to keep track of the "working voltage" to do the math. For a boat, car, etc. where there is only 12 VDC amps and amp*hours does work--Because we know that everything is 12 volts.

    The "way I do the math" for solar... Say only interested in solar power to fridge. The math looks like this:

    843 Watt*Hours per day load for fridge and 1,200 Watt minimum AC inverter (support starting surge). Using our rule of thumbs for system design (reliable, relatively cost effective, using flooded cell lead acid battery bank):
    • 843 WH per day * 1/0.85 AC inverter eff * 2 days storage (no sun) * 1/0.50 max battery discharge (longer life) * 1/12 volt battery bank = 331 AH @ 12 volt battery bank (20 Hour battery rating)
    Also, in this case because a refrigerator has a heavy starting load and a minimum of 1,200 Watt AC inverter--Rule of thumb:
    • 1,200 Watt inverter * 100 AH battery bank * 1/250 Watt inverter = 480 AH @ 12 volt battery bank to "support" 1,200 Watt AC inverter (refrigerators are "solar unfriendly" loads)
    In this case, need ~480 AH @ 12 volt battery to support 1,200 Watt inverter (up to 2,400 VA starting surge support).

    Two calculations for solar panels. First based on rate of charge for battery bank. Second based on your location (hours of sun per day) and your daily loads.

    Rate of charge: For lead acid batteries (and works well for others like Li Ion) 5% minimum rate of charge (weekend/summer system usage) 10%-13% typical for full time off grid system solar charging:
    • 480 AH battery bank * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 452 Watt array minimum
    • 480 AH battery bank * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 904 Watt array nominal
    • 480 AH battery bank * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,175 Watt array "typical cost effective" maximum
    And we need to size the array based on your location and daily energy usage. For a fixed array facing south in Frederick, Maryland:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Frederick
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 51° angle from vertical:
    (For best year-round performance)

    JanFebMarAprMayJun
    3.05
     
    3.62
     
    4.27
     
    4.72
     
    4.79
     
    4.94
     
    JulAugSepOctNovDec
    4.98
     
    4.76
     
    4.66
     
    4.52
     
    3.25
     
    2.73
     

    Toss the bottom three months (assuming not used in winter, or use a genset for bad weather to charge batteries), pick 3.62 hours of su per day (February) "break even":
    • 843 WH per day * 1/0.52 off grid AC system efficiency * 1/3.62 hours of sun per day = 448 Watt array for February "break even"
    For full time (baseline) loads (like a refrigerator) which needs power 24x7 -- Highly suggest only using 65% to 50% of predicted solar output for those loads:
    • 448 Watt array * 1/0.50 "base line fudge factor" = 896 Watt array for February baseline loads
    So using the above SWAG numbers, it looks like an 896 to 904 Watt array would be a good starting point for full time off grid power to support the small fridge.

    Lots of guesses above--But that will give you a good sizing estimate for a basic solar power system. If this was an RV, you probably would be using Li Ion / LiFePO4 type batteries and something like 1/2 the above AH capacity (Li Ion support "surge current" very well, and charge "more efficiently" from solar than Lead Acid).

    Anyway--Enough of my guesses. Any corrections or questions about the above?

    The estimates here are "relatively" conservative to give you a long term reliable system. Refrigerator/freezers are what usually separate a "small" cabin/RV system from a "medium sized" solar power system.

    The choice between a "relatively" inexpensive home refrigerator and a DC based compressor refrigerator can be huge... A typical (what is "typical"?) RV DC compressor fridge is probably closer to 500 WH per day and does not have a starting surge current that is much higher than the running Wattage. A relatively expensive DC camper fridge may be worth it--Smaller battery bank, smaller solar array, no "large" AC inverter "wasting power".

    Doing several (multiple) paper designs can really help you better optimize your system.

    For DC WH/AH meters, there are a lot available these days:

    https://www.amazon.com/s?k=dc+watthour+meter&crid=FF3B89UWXZJK&sprefix=dc+watthour+meter,aps,187&ref=nb_sb_noss

    And there are 12 volt 10 cuft refrigerators:

    https://www.dometic.com/en-us/outdoor/food-and-beverage/refrigerators/rv-refrigerators/dometic-dmc4101-242459

    Max power is 156 Watts (rated) @ 12 VDC (15 amps). I could not find any energy usage numbers--But for a starting estimate:
    • 156 Watts * 0.50 duty cycle (guess) * 24 hours = 1,872 WH per day (sounds too high--I would aim for 500-1,000 WH per day compressor fridge)
    Anyway, lots of brands and models for 12 VDC compressor fridges... You may already have your 120 VAC fridge--So there is that sunk cost to consider.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 706jim706jim Solar Expert Posts: 473 ✭✭✭✭
    edited December 2021 #4
    Ok here's a short answer:

    You will need a minimum of 1000 watts of panels to run that fridge using solar as the power supply.

    Otherwise a generator will be needed to make up the shortfall.

    I have pretty much the identical fridge and run it from a 2500 watt system and wouldn't want much less.
    Island cottage solar system with 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 29th year.
  • sarahps33sarahps33 Registered Users Posts: 5 ✭✭
    Thank you so much for the replies. I ended up getting a fridge that was about half that size. Hoping it will take down the load a little at least. 
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