Whatever the process, energy is measured in joules (J), so we can add up all the changes on a common scale and show how the energy has been redistributed. The total is conserved, so the energy supplied equals the energy transferred to the stores and the surroundings. A joulemeter can be used to measure the energy used by an appliance.
For a mechanical process, work done = force × distance moved in the direction of the force (W = F s). A 20 N force pushing a box 3 m does 20 × 3 = 60 J of work. If the box slides along a rough floor, this energy is transferred to the thermal store of the box and floor.
For an electrical process, energy transferred = power × time (E = P t), with power in watts and time in seconds. Energy can also be found from charge × potential difference (E = Q V), where charge Q = current × time. For example, 0.5 A for 40 s moves 20 C, and with a potential difference of 6 V this transfers 20 × 6 = 120 J. For heating, use ΔE = m c Δθ.
At home, energy use is measured in kilowatt-hours (kWh). One kWh is the energy transferred by a 1 kW appliance running for 1 hour. Use power in kW and time in hours: a 2 kW heater running for 3 hours transfers 2 × 3 = 6 kWh.