Science fair lightbulb energy (help!) ?

Question

I'm looking for a way to test the amount of energy used by one light bulb (besides looking at your energy bill). Is there any way to do this? It can't be too difficult....no more then high school level. Thanks!

Answer 1

Do you have bulbs that aren't marked?   A 100 watt bulb uses 100 watts when operated at the rated voltage of 120 volts (assuming U.S. or other countries that use 120VAC).

Or do you want to isolate how much power goes into visible light and how much goes into heat?    That would be a bit more complicated - you'd need both a light meter and a thermometer (either conventional or non-contact).    Then you could test and compare incandescents versus fluorescents versus LED, etc.  

But if you just want a power measurement, there's an inexpensive watt-meter called the Kill-A-Watt that you might be able to find in a cheap tool store like Harbor Freight, which sells it for about $25-29.

Or you could instead use a clamp-on ammeter on one side of the line cord (use zip cord and split about a foot of it so you can isolate one side of the line).     Then use the power forumula P = I * E (where P is power in watts, I is current in amperes and E is the AC RMS line voltage) to compute the approximate power.

Answer 2

You need a ampmeter connected in series with a simple circuit. It will indicate the current the lamp is using. A more efficient bulb will use less current and therefore less energy.

Answer 3

I think Mythbusters did something similar.  You might be able to check their website to see what they did.