# Terms and definitions

The unit of energy I have chosen is the kilowatt-hour (kWh).

Power is the rate at which something uses energy. The main unit for power is the kilowatt-hour per day (kWh/d) (or Joules, but that is not practical as a Joule is too small a unit: 1 kWh is 3.6 million joules). The power of 1 J/sec equals 1 W. 40W = 1 kWh/d; 1kW = 24 kWh/d. A device could use one kilowatt; it doesn’t use “one kilowatt per second.”! The “per second” is already built in to the definition of the kilowatt; compare with ‘the knott’…
One 40W lightbulb, kept switched on all the time, uses one kilowatt-hour per day. One kilowatt-hour per day is roughly the power you could get from one human servant.
volume = flow x time. flow = volume/time. Energy is like water volume: power is like water flow. energy = power ×time (kWh = kWh/d x d)

When we “use up” one kilojoule of energy, what we’re really doing is taking one kilojoule of energy in a form that has low entropy (for example, electricity), and converting it into an exactly equal amount of energy in another form, usually one that has much higher entropy (for example, hot air or hot water). Only low entropy energy is “useful” to us.