Teho = energia/aika
P = E/t
Tehon yksikkö on watti, W ( 1J/1s = 1W).

Energia = 3000 J
aika = 20 s
P = W

E = 1200 J
t = 60 s
P = W

E = 2 kJ
t = 40 s
P = W

E = 3 MJ
t = 4 min
P = W = kW

(Huom. Nämä ovat vähän erilaisia):
P = 180 kW
t = 20 min
E = kWh

P = 10 W
t = 20 vuorokautta
E = kWh