If we break it down to the basics, life really only needs two things: a stable and moderate range of temperatures that will allow biological reactions to take place and a source of energyThe capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow.. On the surface of the Earth, we use the Sun as our energy store.