Rated power of a lamp is in watts -> Careful – if it's halogen / fluorescent / LED / metal Halide then there will also be additional losses in the transformer / ballast / driver / ballast respectively.
Electrical Supply Utilities charge in KiloWattHours – if you are a large utility, you may also be charged for apparent power, which is power drawn out of phase with the voltage. This happens extensively with old magnetic ballasts and magnetic transformers.
In addition, most electrical utilities will also charge more for on-peak power, and less for off-peak power.
Simply though, without taking into account control gears & out of phase power …..
A 600 watt load (incandescent lamp or mains voltage halogen only) turned on for 1 hour is equal to 600 / 1000 (kW) x 1 (h) = 0.6kWh.
A quick look at the Building code of Australia section J6 says upto 2 lights on a motion detector saves you about 45%, upto 6 lights will save you 30% and upto 200square metres controlled by a single detector will save a measly 10% – the figures are probably pretty conservative though.
, and more than 6 lights