The fundamental physical quantity of energy is a measure of the ability to do work. Energy can occur in many various forms, for example as heat energy. It is measured in the same units as physical labor, namely Joules (J) – established by law as the basic unit of energy – or (among others) in Kilowatt-hours (kWh) or Watt-seconds (Ws). The conversion is 1 J = 1 Ws. Work is the energy per unit time, measured in Watts (W) or Joules per second (J/s).

Fig. 3/2: The sun – our most important energy source