0
\$\begingroup\$

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.

\$\endgroup\$
4
  • 3
    \$\begingroup\$ 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. \$\endgroup\$ Mar 30, 2017 at 14:01
  • 1
    \$\begingroup\$ 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? \$\endgroup\$ Mar 30, 2017 at 14:52
  • \$\begingroup\$ @DmitryGrigoryev Power acceleration! \$\endgroup\$
    – winny
    Mar 30, 2017 at 14:57
  • 5
    \$\begingroup\$ Possible duplicate of Are watts usually measured in watt-hours? \$\endgroup\$ Mar 30, 2017 at 15:03

1 Answer 1

2
\$\begingroup\$

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.

\$\endgroup\$
8
  • \$\begingroup\$ 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 \$\endgroup\$ Mar 30, 2017 at 14:15
  • \$\begingroup\$ @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. \$\endgroup\$
    – winny
    Mar 30, 2017 at 14:18
  • \$\begingroup\$ 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? \$\endgroup\$ Mar 30, 2017 at 14:47
  • 1
    \$\begingroup\$ 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. \$\endgroup\$ Mar 30, 2017 at 15:43
  • 1
    \$\begingroup\$ @JoshWesling Yes, that's exactly it! You are welcome. \$\endgroup\$
    – winny
    Mar 30, 2017 at 17:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.