Energy calculation example 1

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Example 1

Batteries Energy indication: eg 1200 mAh means for one hour (3600 seconds) 1200 mA = 1.2A To know the energy you have to multiply by the Voltage, say this is a 9V battery: P = 9 * 1.2 Joule/second, total energy is E = 9 * 1.2 * 3600 = 38880 J

(You have to calculate in standard units, like A=Ampere, V=Volt, C=Coulomb, then you get J=Joule.)

So the same battery, but running at 1.4V has a total energy of: E = 1.4 * 1.2 * 3600 = 6048 J. Now...these are the calculations, but reality is chemistry inside a battery, so depending on what is inside and the quality, the battery might be able to deliver this energy or less.

If you know the energy in Joule, you can compare:

eg eating a banana means your body gains (on the average banana) 202 kJ.

eg charging my cellphone is 4 hours at 5V with a current of 700 mA, calculating: Energy = 4*3600*5*.7 = 50400 J = 50kJ so with the energy out of a banana I could charge my cell phone 4 times...at the same time delivering the same energy using a dynamo and my muscles for 4 hours (hand crank USB device) is not really comfortable for a human being!

eg a human being of 60 kg running up a flight of stairs, 3 meter high is 1900 J = 2kJ [1] Can you keep running up flights of stairs during 25 * 3.5 seconds = 90 seconds for charging your cell phone? Yes you can, but the dynamo would have an efficiency of 50%, this becomes 180 seconds, 3 minutes...which is still perfectly possible, but slowly you'll get a bit tired of charging your cell phone this way?