How much will it cost to operate 75 lights for one year if they come on at dusk?

Question

A typical sodium vapor security lights uses 500 watts of electricity. How much will this cost to operate 75 of these lights for one year if they come on at dusk and go off at dawn. Assume electricity costs $0.083 per Kilowatt-hour.

Answer 1

On an annual basis, midwestern US utilities use an average of 432 hours per month for flat-rate dusk-to-dawn operation.

432 hours x 12 months = 5184 hours / year

500 watts = 0.5 kW

75 lights x 0.5 kW / light x 5184 hours / year x $0.083 / kWh = $16,135.20 per year.

In reality the dusk-to-dawn load is very desireable to the energy companies.  You should be able to shop around and purchase energy at about half that cost if you sign an annual contract.  Even if your utility does not have open competition, they may offer "off-peak" or "time-of-use" rates that will be far lower.

Answer 2

too many variables.  dusk and dawn come off at different times seasonally.  stick with a fixed hour amount like 8 hours.

Realistically most electric companies have peak times.  but it's easier to have a constant like you provided.

Answer 3

Well, first of all, the average sodium vapor light doesn't use anything like 500 watts.  The lights used on street lights or barn style yard lights are 35 and 70 watts.  

  And it would really be nice if the KW per hour cost were that low, more like $0.135 plus fixed account costs.

  Under the conditions you give, the average night throughout the year is 12 hours, the 75 - 500 watt bulbs use 32.5 KWH per hour, so 365 * 12 * 32.5 * 0.083  = $11815.05

Answer 4

Well, it depends on where you are in the world. Because there are some countries that have 12hours of daylight and 12hours of darkness and there are some other countries that have like 3hours of sun and 21 hours of darkness during winter.

Let's suppose you are in a country that has 12 hours of daylight and 12 hours of darkness.

since you are using 500 watts per light

75lights*500watts/light = 37500wats

since you are going to use them 12 hours you get

37.5 kW* 12hours/day = 450kWh/day

a year has 365 days

450kWh/day*365days/year = 164.25MWh/year

and the kWh will be 0.083 dollars

164.25MWh/year*$0.083/kWh =$13632.75/year

hope this helps