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Equating Prius Hybrids and Fluorescent Light Bulbs
4/6/06

Hi folks, Ben Larson here the new Clean Energy Program field coordinator here at UCS.  Scott’s letting me “barge in” on HybridBlog because there’s was a great question one of our activists on EnergyNet, a project of the Clean Energy Program to engage activists around the country, brought to our attention that is a bit of a “crossover.”  He heard that replacing 15 incandescent (the regular kind) bulbs with fluorescents (those cool, corkscrew ones) removes as much carbon dioxide as driving a Prius instead of a regular car.

Here’s where we at UCS get lucky actually having scientists and engineers in house to get some answers to separate fact from environmental urban myth. The final analysis might surprise you as much as it surprised us.

The following exchange is between Don MacKenzie, an engineer in UCS' Clean Vehicles Program (known to you HybridBloggers just as “Don”), and Jeff Deyette, a research associate in UCS' Clean Energy Program.

Don began by answering the question this way:

We can estimate the annual greenhouse gas savings from a Prius vs a Toyota Matrix, FWD, automatic.  The Prius gets an EPA-rated 55 mpg, vs 31 mpg for the Matrix.   Assuming 12,000 miles annually, it will save 169 gallons of gasoline per year. Since each gallon produces 11.1 kg of CO2-equivalent GHG emissions, the Prius saves 1,875 kg of CO2-equivalent annually. 

I would guess each bulb can save you about 85W (100W-15W), so 15 will save you 1275W.   If the lights are on 2400 h/yr (6.6h/day), they will save 3,060 kWh.   Using a national average of 1.35 pounds of CO2 per kWh, this will save 4131 lb or 1,875 kg of CO2.  So it doesn't seem like an unreasonable statement that the two are comparable.

Here’s Jeff’s response:

I agree that it does seem comparable. However, I would assume that a 23W fluorescent is equivalent to 100W incandescent. And 6.6 hours per day might be a little high for average use, so let's assume 4 hours per day instead.  So, if each bulb saves 77W (100W-23W), and operates 4h/day, 365 days/year, it will save 112 kWh of electricity.

At the emission rate of 1.35 lb/kWh, this is equivalent to 152 lb of CO2 (69kg CO2). Therefore 1875/69 = 27 light bulbs would need to be replaced.  Using our assumptions, the actual number of light bulbs that would need to be replaced is a bit higher than 15, but I think the comparison is still in the ballpark. It's definitely an interesting way to think about it.

Good ole’ ScottN then just had to stick his two cents in with two notes:

One is the fact that Don used the Toyota Matrix as a comparison point, which is a high-end mpg comparison for the Prius.  As you might know from the HybridCenter comparison chart, the typical midsize sedan in the Prius’ class gets 23mpg, so that’s a pretty huge difference there in what your assumption is for a “regular car.”  At 12,000 miles a year, the comparison chart shows the Prius producing 2.6 tons of global warming pollution vs. 6.4 tons for the average sedan (the Matrix would come in at about 4.5 tons, comparatively).

Second is the fact, that, of course, at the end of the day, the best way to think about it is that you should and buy yourself a bunch of fluorescent lightbulbs and a hybrid, not one or the other.

An interesting discussion, no doubt.  For more on the Clean Energy side of the picture, please come join us over at EnergyNet.  While you're at it, why not also signup for the new HybridCenter Driving Change Network.

Posted by: Ben 4/6/06

Original post and comments can be found on Hybridblog.org.