# Physics BMAT Question

Could someone explain how I could calculate efficiency only using this info? The answer is 66%

Hi!

I think i have the same study book as you, here’s the solution they gave:

we have to know the average household has 240V of electricity available to solve
(in my opinion the IMAT test would give us this info we don’t need to know it by heart)

energy efficiency = useful E / total E * 100%
wattage the bulb is receiving = 0,5 * 240 = 120W
however our bulb only draws 80W
so E efficiency = 80/120 * 100% = 66%

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Hi!
completely agree with the fact that they need to give us the avg. household value i was using 220V (this is the avg. household value in my country) and was getting efficiency of around 72% and was confused why it’s not 66%

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yes i think they would have to give us the figure otherwise it wouldn’t be fair to international students. In any case i googled it and the EU standardized it to 230V so i’ll just remember that number

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Ohhh I had no idea that was the average! Thank you for the solution! I was wondering how to solve it without knowing the initial voltage!