Anonymous asked in Science & MathematicsEngineering · 8 years ago

How much does it cost to run one 13W bulb 24/7 for 30 days at $0.090/KWH?

I did the math myself but I don't know if I was right or not.


Tell me if my math is correct:

There are 720 hours in one 30-day month.

1 Watt = .001 KWH

1 Watt x 720 = .720KWH

13 Watts x .720 KWH = 9.36 KWH

9.36 KWH x $0.090 = $0.84


So my math says that one 13 Watt light bulb costs $0.84 (84 cents) per month to run 24/7 at the rate of $0.90 per KWH.

Is this correct?

Update 2:

So if I leave two 13W porch lights on during the day AND night, then the extra cost is:

Gets dark: 8:45pm

Sunrise: 5:45am

Extra hours: 15 hours extra

15 hours x 30 = 450 hours extra each moth for each bulb

2 bulbs = 450 x 2 = 950 extra hours for two bulbs

.950 KWH/mo x 13W = 12.35 KWH/mo

12.35KWH x $0.090 = $1.1115/mo

So the extra cost for leaving the porch lights on all day is $1.11 per month.

In one year, it costs less to leave on the porch lights than it costs for the actual bulbs!!!

1 Answer

  • 8 years ago
    Favorite Answer

    13 W = 0.013 KW

    0.013 KW * ($0.090 / KW*h) = $0.00117/h

    $0.00117/h * 30 days * (24 h/1 day) = $0.84

    Your conclusion isn't quite right. The light bulb that you're referring to has a very low watt compared to the 40 and 60 watt light bulbs. The 13-watt light bulb is energy smart; therefore, it is designed to save energy and hence reduce the cost. The energy smart light bulbs actually last for years (i.e., each bulb can last for 5 years), so, if you take into account their life time and how much energy they save, you will find that it does not cost less to leave on the porch lights than it costs for the actual bulbs. Hope this helps :)

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