calculating the efficiency of a microwave oven

If I want to calculate the efficiency of a microwave oven, I have the input Energy (= power (watts) x time (s)) and the output which I measured using temperature difference (q=mct). So do I divide output/input x 100 or do I find the difference between them (=work) then divide it by the input energy?

• Make sure your units balance on both sides... – Solar Mike Mar 24 '19 at 17:25
• yes they do input energy: microwave is 650W and I'm heating for 20 seconds, so 650W x 20s = 13000J output energy: experimental value of 7698.56J – Shahad Mar 24 '19 at 17:28
• so to calc effieicny do I just divide the 2 then x 100? or do I need to find the difference between them then divide by the input energy? – Shahad Mar 24 '19 at 17:30

The equation for efficiency in thermodynamics is as follows:

$$\eta = W_{out}/W_{in}$$

and to get this as a percentage you just multiply $$\eta \;\mathrm{x}100=\%$$