# Is Ohm's law required to calculate Watts?

A buddy and I were having a conversation. One of us is convinced that Ohm's Law and Watts are the same thing. After much research I can not find anything directly pointing Ohm's Law to defining Watts, what I have found it that one way of getting Watts is to leverage parts of Ohm's Law, but that you can calculate Watts without the usage of Ohm's Law. I feel that this has to be the case as Ohm didn't do his work on resistance until after James Watt had already died, however the naming of the "Watt" was not official until after the research the Ohm had done and had basically nothing to do with Watt himself. Can somebody please clarify on this?

• Yeah, you can definitely calculate Watts without Ohm's law. For example, by dividing Joules and Seconds. – Ladislav May 21 '15 at 6:35
• "... until after James Watt had already died" James Watt was a mechanical engineer who worked on steam engines (among other things). The fact that his name is used generally for power (of all types) is not relevant to this chronology. – Roger Rowland May 21 '15 at 6:40
• Corrected in question, should have been more clear that Watts is general and not really related to the engineer it was eventually named after. – Heath N May 21 '15 at 6:53
• @Arduinology It might be better to explain why you (or your buddy) believes that Ohm's Law is required to calculate power. – Roger Rowland May 21 '15 at 6:54

The Watt measures power, which is energy per unit time---more specifically, Joules per Second. In familiar units, multiplying Volts (Joules per Coulomb) by Amps (Coulombs per Second) yields Watts, i.e., $P=IV$. This applies to all lumped electrical components, be they resistors, batteries, transistors, etc.
Ohm's law really isn't much of a law, but rather a particular relationship between Volts and Amps that happens to hold for resistors. Reciting it, $V=IR$. Simply put, for resistors $V \propto I$ and we can then find a proportionality constant $R$ to help us relate them. This relationship doesn't hold for all components, and it's not needed for calculating power.