if I have a power out put unit which have 20kW, & I want it keep to run 0.5hr, exclude power lose, how much J of energy I need?

Quote from scania :

if I have a power out put unit which have 20kW, & I want it keep to run 0.5hr, exclude power lose, how much J of energy I need?

1W = 1 j/S

1800 x 20000 = 36000000 joules

thx
if it is a electric car, how much battery I need? (mah?)