I learned something incredibly useful this week: For every 3 watts of energy consumption you save, you save an additional watt of power in cooling costs during the cooling season.

I bought my first CFL bulb a little over 9 years ago. I knew this was helping my AC costs too, but now I know how much.

Here’s the math:

number of bulbs x hours running per day x wattage x cost per KWH = cost per day.

Factor that with two scenarios, then take the difference. Divide that difference by 3, and you get the savings in cooling costs. Multiply that by the number of days you run your air conditioner per year–probably a minimum of 90, but it will depend on your personal definition of “hot,” and when it gets hot and how long temperatures stay that way where you live. I can see how some people might run their air conditioners 150 days a year. But once you figure out how many days per year you run your air conditioner, it’s an easy multiplication problem to figure out your annual savings.

So let’s look at what happens when you replace a 100W incandescent with a Philips L-Prize bulb, if you run both bulbs 10 hours per day.

Incandescent: 1 bulb * 10 hours * 100 watts / 1000 * .10 = 10 cents per day, $36.50 per year.

L-Prize LED : 1 bulb * 10 hours * 10 watts / 1000 * .10 = 1 cents per day, $3.65 per year.

Cooling savings = 3 cents per day, $4.50 per year. Total savings = $37.35 per year.

So even at its $50 introductory price, the L-Prize bulb paid for itself a little over a year when you factor in cooling costs. I was surprised.

Let’s look at what I saved by putting a motion sensing light in my kitchen.

Traditional: 4 bulbs * 14 hours * 10 watts / 1000 * .10 = 5.6 cents per day, $20.44 per year.

Occupancy : 4 bulbs * 4 hours * 10 watts / 1000 * .10 = 1.6 cents per day, $ 5.84 per year.

Cooling savings = 1.333 cents per day, $2.00 per year. Total savings = $16.60 per year.

So, at around $20, my motion-sensing switches are a good investment. In the bathrooms, where there are more lights, they’re an even better investment.

I think LED bulbs are worth it anyway. But reduced cooling costs help even more.

Since I have occupancy sensing switches in most rooms now, my LED bulbs stand a chance of lasting 20 years. If you move a lot, you may not care about that. Since I fully intend to stay put another 40 years at least, I do.

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