Efficiency of a Microwave

I wanted to find out what percentage of the power supplied by my
microwave ends up in the food as heat. It was tricky because I did
not have a thermometer. I used the following procedure.

Measuring input power:

1. Unplugged the refrigerator and checked that no computers or the
   stove were running.
2. Timed how long it took for the wheel on the power meter to
   revolve once.
3. Put some chili in a bowl in the microwave, turned it on for three
   minutes.
4. Timed how long it took for the wheel on the power meter to
   revolve once

Measuring output power:

1. Emptied a tray of ice into a glass bowl and added
   room-temperature water.
2. Waited till the water felt as cold as the ice.
3. Poured the water through a strainer into a measuring cup. Measured
   the volume of water.
4. Poured the water back into the bowl. Covered the bowl with
   a lid.
5. Put the bowl in the microwave and set the timer for 1 or
   2 minutes. Waited for the bell to ring.
6. Took out the bowl, poured the water through a strainer into the
   measuring cup and measured the volume.

Data

Input power

Trial	Time with Mic. Off (s)	Time with Mic. On (s)
1	74	17
2	77	19

Output power

Trial	Time (s)	Init Vol of Water (mL)	Final Vol of Water (mL)
1	120	500	600
2	120	125	330
3	60	330	410

Calculations

Power meter formula from

Michael Bluejay.

Base Power = ( 3.6 * 7.2 ) / (Time with Mic. Off)

Total Power = ( 3.6 * 7.2 ) / (Time with Mic. On)

Input Power = Total Power - Base Power

This gives 1.17 kW, 1.03 kW. The average is 1.1 kW.


Output Power = ((Final Vol of Water) - (Init Vol of Water))
* (1.0 g/mL) * (333.55 J/g) / (Time)

This gives 0.28 kW, 0.60 kW, 0.445 kW. The average is 0.44 kW

This gives an efficiency of 40%. Unfortunately, the 90% t confidence
interval for the Output Power is (0.17, 0.77) so more trials are
needed.