Power units such as Watt, horsepower, BTU/hour and other actually describe something very basic and simple:
how much work must be done, to do something
To simplify things lets use an example:
lets say we have a heavy cart and we need that cart to move 20 miles per hour.
we tie one horse to the cart and start moving, the maximum speed is 10 miles per hour.
we then tie another horse to the cart and start moving, the speed now is 20 miles per hour (mph) meaning, in order to move the heavy cart in a 20 miles per hour speed, we need the power of 2 horses also called 2 horsepower or 2 hp.
similarily other units such as Watts, KiloWatts, BTU/hour etc... describe the amout of work needed to do something