A refrigerator has a 1000W compressor but the compressor run

A refrigerator has a 1000W compressor, but the compressor runs only 20% of the time.
a. If electricity costs $0.10/kWh, what is the monthly (30 day) cost of running the refrigerator?
b. A more energy-efficient refrigerator with an 800W compressor costs $100 more. If you buy the more expensive refrigerator, how many months will it take to recover your additional cost?

Solution

Refrigertor power consumtion 1000W

power consumed per month = 1000*30*24 W

as the refrigerator runs for 20% of the time

actual power consumed = 1000*30*24*0.2 W

                                       = 144 kWh

monthly cost = 144*0.10 = 14.4 $

b) power consumed by efficient frize   144*800/1000 = 115.2 kWh

   monthly bill = 115.2*0.1 = 11.52 $

savings per month = 14.4 -11.52 = 2.88$

additional cost = 100$

months to recover = 100/2.88 = 34.72