Apple Mac power draw

You are welcome.

Well, the 80 watts usage would depend on what you were doing at the time. The actual power usage could vary greatly depending on screen brightness, CPU usage, GPU usage, etc. In most cases the actual usage would be less.

The unit they charge you for is energy, or kWH, not "per hour". The power spec in Watts is Joules per second, so you multiply by the time to get energy. i.e., 1 Joule per second for 1 second is 1 joule.

If the continuous power usage was indeed 80 watts, it would take 12.5 hours to use 1 KW/H.

Sure, but loads for equipment are specified in Watts, not kWh. You need to multiply the number of watts * the number of hours.