Your utility company charges by the kilowatt/hr used. Let’s base this example on 12 cents per kilowatt hour with a lighting system that has 800 watts of lights, and operates 5 hours per night.
1 kilowatt = .001 watt

800 (watts) x 5 (hrs) = 4000 watt/hrs

4000 watt/hrs = 4 kilowatt/hrs

4 x $0.12 = $0. 48 per night $0.48 x 30 = $14.40 per month