Refrigerator and Heat Pump

fiziks09

Homework Statement

A freezer has a coefficient of performance of 6.30. It is advertised as using electricity at a rate of 457kWh/yr.
a)On average how much energy does it use in a single day?
b) What maximum mass of water at 20.0 degree celsius could the freezer freeze in a single day.

NOTE: One kilowatt hour is an amount of energy equal to running a 1-kW appliance for one hour

COP = Q / W

The Attempt at a Solution

I tried to use comparison like so;

if 457kWh is the rate for 365 days
then xkWh is the rate for 1 day

This gives me 1.25 J which is very wrong.

Please can i get clues on how to start or what to do on the "A" part of the question. It's really twisting my head..thanks a lot for your help..

Homework Helper
MHB
Hi fiziks09!

What you have is 1.25 kWh, which is 1250 Watt times 1 hour.
This is not 1.25 J, which is 1.25 Watt times 1 second.

You'll need to convert the unit...

fiziks09
okay..
but converting the unit still gives a wrong answer...

Homework Helper
MHB
How did you convert it?

fiziks09
okay i might have misunderstood. I'm so sorry..what conversion is being made?
is it from kWh to Joules or what? if so how so??

fiziks09
Allright i got it now..
since 1kWh = 3.6MJ (which is 3600000J)

1.25kWh X 36000000 = 4.5 X 10^6 Joules..

thanks for the help...

Homework Helper
MHB
Good!