Energy is a quantity/number that is always conserved in a complete system. It can quantify how much work has been done on an object, or the amount of work that it has the potential to do – therefore giving us an indication of what can happen in that system. Objects can store energy in different ways, and objects can have multiple energy stores at the same time (a flaming arrow possesses energy in a kinetic, gravitational, thermal, and even a chemical store). The SI unit for energy is joules.