It is important for licenced amateurs to know how much power they are allowed to use and how much power they are actually using. In the UK the maximum amount of power permitted when transmitting is 400w, but not on all bands. On some bands and parts of others the maximum amount of power permitted is very much less. If you do not know how much power you are using, you might be in breach of the terms and conditions of your licence, in which case you will lose your licence. You have been warned.
Power is measured in Watts .
1 Watt is equal to 1 Joule per second. 1HP is the same as 750 watts. I suppose this is how much power a horse is using when it pulls a cart load of beer from the brewery to the pub. If you know two of the values you can easily work out the third one. If you know the voltage and current and want to calculate the power, you use the first equation.