To describe how energy is redistributed in a system, we calculate each change using the right equation, then compare them all in joules (J). Every store is measured in joules, so this is a common scale.
Choose the equation to fit the process. For thermal changes use change in thermal energy = mass × specific heat capacity × change in temperature. For mechanical changes use work done = force × distance moved, or the kinetic and gravitational potential energy equations. For electrical changes use energy transferred = power × time, or energy transferred = charge × potential difference. For example, a 100 W lamp on for 30 s transfers 100 × 30 = 3000 J.
At home, energy use is measured in kilowatt hours (kW h) using energy = power (in kW) × time (in h). A 2 kW heater on for 3 hours transfers 2 × 3 = 6 kW h.
The energy shown for a whole system must balance. If an appliance takes in 500 J of electrical energy and 400 J is useful, then 100 J is wasted, mostly as thermal energy.