Can I find run time of a refrigerator given the Watt Hours?

Question

I took the following measurements on my mini fridge with a power meter.

Voltage: 123 V
Current: 0.71 A
Power: 50 W

I ran for 4.75 hours and the meter read 52 Watts used. So I know that the watt hour rating is about 11Wh per hour (54W/4.75Hr).

Can I then conclude, since the wattage during on-cycle is 50 watts (and thus if ran for a complete hour would be 50 watt hours), that run time per hour would be 11Wh/50W = 22% of 50Wh

Then: 22% of an hour would be 13.2 minutes of actual run time.

The math seems to make sense to me but I can't find any examples on the internet.

and the meter read 52 Watts used Watts is Joules (unit of energy) per second. So you cannot say about 11 watts per hour. To get the amount of energy (in Joules) you need to multiply Watts x time (in seconds). Instead of looking for examples on the Internet, try to understand how energy calculations work. It is not difficult. Real engineers ALWAYS check if the units make sense, if they do not the calculation is WRONG. — Bimpelrekkie, Mar 30 '17 at 14:01
Out of curiosity, what stops you from dividing that 11 "Watts per hour" by 4.75 again to get 2.3 Watts per hour per hour? — Dmitry Grigoryev, Mar 30 '17 at 14:52
Possible duplicate of Are watts usually measured in watt-hours? — Dmitry Grigoryev, Mar 30 '17 at 15:03

score 2 · Answer 1 · answered Mar 30 '17 at 13:59

2

You have messed up the different physical quantities energy and power, especially when you get to "watts per hour". But in general, if your 50 W refrigirator consumes 52 Wh in 4.75 hours, your duty cycle is 22 % which sounds very resonable for a refrigirator.

answered Mar 30 '17 at 13:59

winny

14,707
6
46
66

The fridge itself is rated at 1.1 amp and 75 watts, what I posted were my actual readings, if my understanding is wrong, please explain – Josh Wesling Mar 30 '17 at 14:15
@JoshWesling That's normal. When you certify your device/refrigirator/whatever, the agency will only check that the consumption is less than the name plate for it. Hence, they always list higher current than actual consumption in real life. – winny Mar 30 '17 at 14:18
My power meter totals the total watts used while the fridge is running, after the 4.75 hours the total displayed was 52 watt hours, maybe I used the wrong units in my original explanation? – Josh Wesling Mar 30 '17 at 14:47
@JoshWesling No it does not. It totals the total WATT-HOURS used while the fridge is running. It's not the wrong unit, it's the wrong physical quantity. "How long did the experiment take? Five pascals." "How long is the bridge? Four PSI" – winny Mar 30 '17 at 14:55
If its the total watt hours while the fridge is running (which is what I thought), what is wrong with my calculation? – Josh Wesling Mar 30 '17 at 15:18
@JoshWesling Your calculation is fine when it comes to dividing the values to get the numerical answer. Your dimensional analysis of the same calculation though, is not. – winny Mar 30 '17 at 15:24
1

Okay so i think I have the wrong units its not 52 watts used, its 52 Wh, that's for helping me understand and confirming my numbers are correct. – Josh Wesling Mar 30 '17 at 15:43
1

@JoshWesling Yes, that's exactly it! You are welcome. – winny Mar 30 '17 at 17:01

Can I find run time of a refrigerator given the Watt Hours?

1 Answers1