Energy Efficiency

Published on February 23rd, 2011 | by Susan Kraemer


LEDs v. Incandescent Bulbs – the Math

February 23rd, 2011 by  

I have a mental block about the upfront cost of LEDs, that is like the block most people have about solar.

(To me it seems so obvious that solar electricity is cheaper than utility electricity in my state that I get frustrated that every roof doesn’t sport solar. It’s as if people have some kind of a mental block about solar. They think it’s a good thing – for some time in the future, and of course, they mean to do something about their energy choices – some time in the future, sure, but in the meantime…)

But I have to confess, I am just like that – when it comes to LEDs. Sure I know they are cheaper in the long run, and are more efficient by a very long shot than incandescent lights (about a tenth the energy needed to make the same light), but somehow I can’t get over the upfront cost. And there’s no PPAs or leases for LEDs, obviously,  like there are for solar, to eliminate that “oh, but the upfront cost!” argument.

But some math from Marc Gunther at Greenbiz has provided the evidence needed to change my mind. Let’s see if it does.

First take a conventional 60 watt bulb. Take the equivalent in an LED – which uses only 12.5 watts, while providing the same amount of light. Now compare the electricity costs between the two, run for the duration of a 25,000 hour period, which is nearly three years if run 24/7, or about 12 years if run 6 hours nightly.

If you pay 12 cents a kilowatt hour, electricity will cost $37.50 to run the 12.5 watt LED for the 25,000 hours. But you will pay $180 to run the 60 watt incandescent the same amount of time.

That is one saving right there, the cost of electricity.

But another saving is this. Only the LED will actually last the 25,000 hours which is about 12 years if you are turning it on at 6PM every night and off at midnight. However, the incandescent bulb will only last 1,000 hours, so one bulb actually cannot even do this test.

Instead you’d need to buy 25 incandescent bulbs to stand up to the duration of one LED bulb to run this 25,000 hour test.

We long ago replaced all our incandescent bulbs in our house with CFLs, and that math is not so extreme – but we are moving this year. So the choice will come up again for us. Next time I look at an LED and think, “but it’s so expensive”, I think I will consider this math.

It really doesn’t seem quite so expensive any more. Especially if you pay a higher rate than the average for electricity.

Susan Kraemer@Twitter

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About the Author

writes at CleanTechnica, CSP-Today and Renewable Energy World.  She has also been published at Wind Energy Update, Solar Plaza, Earthtechling PV-Insider , and GreenProphet, Ecoseed, NRDC OnEarth, MatterNetwork, Celsius, EnergyNow, and Scientific American. As a former serial entrepreneur in product design, Susan brings an innovator's perspective on inventing a carbon-constrained civilization: If necessity is the mother of invention, solving climate change is the mother of all necessities! As a lover of history and sci-fi, she enjoys chronicling the strange future we are creating in these interesting times.    Follow Susan on Twitter @dotcommodity.

  • hammerman

    For $13 at Home Depot the 9.5 (60W) replacement LED at 2700K are perfect replacements. You save about $1-$1.20 per 60 watt bulb replacement off your bill.

    • Bob_Wallace

      I’ve got a couple of the Cree LEDs from Home Depot and I’m impressed.

      51 watts per hour saved. Four hours a night, 30 days a month would be ~6 kWh. At $0.12/kWh, a $0.72 per month savings. You must run your lights longer and/or pay more for electricity.

      Whatever, in about a year these LEDs pay for themselves. And that’s without adding in the cost of new incandescent bulbs. A one year payback is a 72% return on investment.

      I would like an 100 watt replacement. Hope they come out with that soon.

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  • Richard Faulkner

    “The quality of light” is also questionable. Colour rendition on cheaper models is generally terrible so you do have to look at the high end products and consider it a long term investment for an acceptable change-over. Also, as already has been mentioned, the quality of electronics should match the lifetime expectancy of the LED for it to be worthwhile.

    As a bit of a hippie I am pro-LED, Almost daily there are press releases claiming new levels of comfort and efficacy. Couple that with no mercury and less rare earth phosphors (which are soaring in price, so CFL won’t be much cheaper forever) used in their construction, then environmentally its a no brainer.

    From a fiscal point of veiw though its not still all roses in terms of energy use.

    There is another factor creeping into the cost of ownership equation if you live in a northern region such as I in the UK. Filament bulbs produced a great amount of heat as well as being a useful and comfortable light source. As well as being a source of heat that contributed to warming a room, the siting of them means they produced a nice thermal blanket at ceiling level which kept other heat sources circulating in the room instead of rising straight through. The EU has banned, or is in the process of banning, the sale of filament lamps, so unless we invest more heavily in flooring insulation on each story at the same time as replacing our lighting we will have to reckon on our heating energy units increasing to compensate, thus reducing the apparent savings and increasing pay back time.

    If you live in a hot climate, you should be on the LED bandwagon as the opposite will occur and you could potentially reduce aircon bills without having to sit in the dark. Hmmmm, I’ve always fancied life on a ranch. Does Texas have room for one more nerdy limey?

    • Anonymous

      If you want to heat your room then use the electricity on an efficient room heater.

      That ‘light bulbs as a heat source’ just doesn’t make sense. It’s an old, worn-our right wing anti-efficient lighting argument.

  • john

    My problem is one of quality. The greedy bastards do not tell you their CFL is going to die from cheapo electronics making it an extremely expensive investment for a 6 month light bulb. I will not touch LED given my experience with CFLs performing horribly and I’m not sure which china-made brand out there. I jumped on early; but the tech was never new so the early bulbs shouldn’t have been crap.

    We need warranties and have them provide easy replacements OR the government goes after them for false advertizing! The run around for a replacement is a nightmare.

    LED costs even more so the scam becomes even worse. We also could use a standard for DC lighting which would help a little as it would remove most the electronics used in the bulbs. (I don’t see why we couldn’t have LED/CFL units separate of the converter– then new houses can have a new socket and old ones get converters.)

    • Hmm, off, I’ve had CFLs for years and have no such problems. In fact, don’t think I’ve ever replaced one.

    • Anonymous

      I’ve been using nothing but CFLs for almost 20 years. I’ve never had one burn out.

      My oldest has dimmed over the years so it’s now used for ‘ambiance’, not reading. But it’s still working….

      • RagingTyrant

        Where on earth did you get them? We have a few dozen in our home, but I havt to replace them all the friggin time.

        • Bob_Wallace

          The oldest? I probably ordered it from some place like Real Goods “back then”. They were fairly new and close to $20 each. I just got one for my most used lamp due to the price.

          More recently I get them at the local home/building supply place. They often go on sale for $0.50 or $1. Looking at a couple they aren’t brands I recognize – MaxLight and FEIT.

          Mine are in table/floor lamps that don’t trap heat. Seems like most of the failures I’ve heard about are when they are used in a ceiling can or some other place where they get too hot.

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  • LED longer life span, low maintenance cost but too expensive.

  • Former electical reliability engineer

    I love LEDs BUT they wear out very fast if they are run at “full power”. Often times the brightness is spec’ed at a certain power rating. The lifetime at those ratings are often quite short (100 hours or so). So do the math but look at power vs brightness vs lifetime.

  • Nice article – the math gets much less convincing if you compare CFL to LED. I’ve done a number of such tests myself on my site, and given improvements in CFLs in recent years, it seems to me LEDs in general applications are still a few years away from taking over. That said, I’ve managed to find a handful of niche applications for LEDs that save me about $20 a year.

  • RemiPG

    I find that equivalence claims for LEDS (and CFLs for that matter) are often overstated on the products I buy.

    A 60 watts incandescent would output around 700 lumens while the 12.5 watt leds bulbs available at my grocery store, would output around 450 lumens… (hardly “equivalent”)

    Leds are still more efficient than incandescents (35 lumens per watt instead of 12 lumens per watt for incandescents) However, CFLs I purchased were giving out 70 lumens per watt…

    I think overstated output explains why a lot of people a disapointed by CFLs and LEDs. But if they did their own math they would be able to know what they should purchase to replace Incandescent lightbulbs accuratly…

    • My own experience is from one I have that a company in California is pioneering – a 100 watt equivalent spotlight LED (flat front with little LEDs in it – not like the image) that looks as brilliant and warm as a 200 watt halogen light – not like a typical 100 watt incandescent – yet it uses only 10 watts. (I don’t have the packaging so don’t know the lumens.)

  • There generally is not a payoff switching from CFLs to LEDs. The main differences are better looking light and no warm up time. In addition, CFLs contain mercury, and LEDs do not.

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