Don’t be a dim bulb

Photo of bulb by Jan-Erik Finnberg from, used under a creative commons license

Photo of bulb by Jan-Erik Finnberg from, used under a creative commons license


Light bulbs come up all the time when talking about energy efficiency. One comment I hear all the time, “I’ll switch to the more energy efficient bulbs as I use up the old ones.”

I’ve got a better idea, toss all the old ones you have and buy the most energy efficient bulb for your fixtures.

People who know me are aghast. I’m cheap and don’t like to waste anything. Throw out a perfectly good light bulb? I must have hit my head.

Don’t worry, frugality reigns. Even if it’s still in the package you’re better off disposing of it properly and starting fresh. Let’s go to the spreadsheet.

Haha! Scared ya! No spreadsheet, just simple math. I happen to need a light that provides about 1100 lumens so I’m using that as the basis of my search.

Lowe’s came up first, so I’m shopping there. They have a 2 pack of 20 watt CFL’s for $6.49. These are rated at 1100 lumens at 20 watts, with an 8,000 hour rated life. Just what I’m looking for. Let’s say I’ve bought them and they’re on the shelf. I’m going to round the cost to 3.25 each and run these things for their 8,000-hour life span. According the the NH Office of Energy and Planning, as of this writing the average cost of a kWh of electricity is $0.1629.

Now some math. At 20 watts it will take 50 hours to burn 1 kWh (1,000 watts/hour) so over it’s lifetime this bulb will use 160 Kwh. So the operating cost is $26.54. The total lifetime cost is $29.79.

Wait. There’s an LED option that only uses 9.5 watts for an 1100 lumen output. It costs $12.98 so it’s a lot more up front, but it’s rated for a 25,000 hour life. This bulb will take 105.25 hours to use 1 Kwh. To do a fair comparison let’s look at the cost at 8,000 hours. It will use 76 kWh in that time. Operating cost will be $12.38. Add the purchase price and you come up with $25.36. You’re already $1.18 ahead if you ignore the purchase price of the CFL (you threw it away).

The LED is going to keep going though, to 25,000 hours. Your lifetime cost will be $38.69 for electricity and 12.98 purchase price for a $51.67 total. Your hourly cost is $0.002067, not bad.

The CFL hourly cost is $0.0037. Even with all the decimal points that’s a lot more, and my Yankee thriftiness remains intact.

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