Comment 17666

By calsolar (anonymous) | Posted January 25, 2008 at 20:02:49

The 60000 hr comparison may not be compelling for people because it's hard to project that far. At 3 hours per day, that's about 55 years. Anyone lived anywhere that long, let alone have a light fixture that old? And as pointed out previously it ignores the time value of money.

It is probably better to look at how long it takes for the LED to pay for itself with savings.

Assume the ILB is free (it's already in the fixture). Assume all the replacements are free (you get the ones people are replacing with CFLs). At $0.10 per kwh, and assuming a 40 watt bulb, it takes 25 hours of operation to use a kwh. So the bulb costs you $0.004 per hour.

If the LED cost $40, and uses 4 watts, it costs $0.0004 per hour.

So what's the payoff time? H hours

IFL cost = H x $0.004
LED cost = $40 + H x $0.0004


$0.004 H = $40 + $0.0004 H

($0.004 - $0.0004) H = $40

H = 40 / 0.0036

H = 11,111 hours

So if you use the light 3 hours a day, it will take 3703 days or roughly 10 years for simple payback, so even longer when considering NPV.

If you use it 24 hours a day, it will take 462 days or a little over a year.

This gives you a feel for the dynamics. Lights used a lot make a lot more sense to replace. Your closet light just doesn't matter that much (CFL or ILB or LED) unless you leave it on all the time. It doesn't use that much electricity and it doesn't produce that much carbon.

If you live in Hawaii, your electricity costs 3x as much, so the time is cut to a third. 24 hour a day lights (traffic lights, emergency exit lights) are going to pay for themselves in a few months and keep paying.

If the LEDs cost 1/2 what I assumed, the payoff time is 1/2. If 2x as much, then payoff time is twice.

If it costs you to replace those bulbs every 1000 hours (office building or parking garage) you can include that cost and LEDs get better.

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