want to compute saving cost in installing motion detector to control lighting on and off, what is the formula to calculate in watt-hours, since given only is the total wattage of the light which 600 watts, what the standard says?
0.6kWh is the answer in the above example,power units are often confused for energy units the difference being used over time hence the prefix 'h' on the end, also there's the usual use of capitals were lower case should be used.
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.