Whether it is for safety or economic reasons, knowing how to calculate watts can come in handy. Since electrical power (watts) is the result of electrical force (volts) pushing current (amps) trough a circuit, there are three different ways to calculate watts.
The figures you will need--amps, volts and, possibly, ohms--may be stamped on the body of the device, on a label of it or in literature that came with it. In some cases you may need a meter capable of detecting these values. Jot these figures down.
If you know both the volts and amps, multiply them. For example, there may be an indicator on your 9-volt tape player that the device uses .3 amps. That means your tape player uses 2.7 watts.
When you do not know the amperage but know the ohms (resistance), first divide the volts by ohms. This will give you the amps. Take this amount and multiply it by the volts to get the wattage. Let's say you found an old 12-volt reel-to-reel recorder, and no amps are listed on the specifications, but it does list the ohms as 4.2. Divide the voltage (12) by the ohms (4.2). Rounding up the answer to the next hundredth will result in 2.86 amps. Multiply that amount by the volts and you will see the wattage is around 34.32.
You can also calculate watts if you know only the ohms and amps. Square the amps and multiply that result by the ohms. So, if you came across an old radio and the only specifications you can find are that it uses .4 amps and its circuit has a total resistance of 312.5 ohms, this means it will use 50 watts (0.4 x 0.4 x 312.5).
Check your answers by reversing the mathematical process. In the example given for ohms and amps, 50 divided by 312.5 is 0.16, the square root of which is 0.4.