## Monday, November 4, 2013

### You Do The Math - LED and CFL efficiency

Last year I bought an LED light bulb that was actually like a traditional incandescent bulb: a milky glass bulb that produces off-white, diffused light.  And since it put out "40 watts" of light, but only used about 7.5 watts, I went ahead and bought one.

But, it created a lot of excess heat, which LEDs don't do.  The heat came from below, in a metal housing full of electronics that convert 120 volts of alternating current to just a few volts of direct current.  It's this heat that robs an LED of it's great efficiency.

The energy efficiency is described as lumens per watt.  Lumens are a measure of light output, while watts are a measure of power input.  At 450 lumens, this bulb's efficiency was 60: not so good, even compared to a CFL.

Now, that same company has the best bulb I can find.  It produces 800 lumens of light, about equivalent to a 60 watt bulb.  Yet, it uses under 10 watts.   It's also dimmable, and on sale, and there are hundreds of them all over the store.  The efficiency is 81.6 lumens per watt.

However, my relatively expensive meter measures it at 11 watts (92 milliamps / 1000 x 120 volts), so that drops the efficinecy to 72 lumens per watt.  This puts it in the realm of all the other LED lights out there, and one CFL.  Thankfully, it is very bright, and probably does put out "60 watts" of light.

Another calculation of efficiency is dollars per lumen-hour.  Afterall, if a bulb costs you \$10, but doesn't last any longer than an incandesscent, you've lost \$9.50 (and you've wasted \$9.50 in energy, which was used to build and transport and sell this bulb) .  We must divide this \$10 by the number of lumens you will get in its lifetime.  This lifetime of lumens is it's expected hours of operation times the lumens it produces.  And, energy cost must also be added.

So, let's use the 9.8 watt bulb I'm so excited about.  If it really does last the 25,000 hours that it's rated, and your electricity is the same as the US average, 11 cents per kilowatt-hour, then the total cost is:
\$11.98 + (25,000 * 0.11 * 9.8 / 1000) = \$11.98 + \$27.50 = \$39.48

Total lumen-hours is:
25,000 * 800 = 20,000,000

Total cost is:
20,000,000 / \$39.48 = 506,585 lumen-hours per dollar

Let's see what a 60 watt incandescent costs, if it lasts the typical 1,000 hours:

\$0.50 + (1,000 * 0.11 * 60 / 1,000) = \$0.50 + \$6.60 = \$7.10
1,000 * 800 = 800,000 lumen-hours
800,000 / \$7.10 = 112,676 lumen-hours per dollar

The best CFL I could find produces 1640 lumens, similar to a 120 watt incandescent, yet this CFL uses only 23 watts.  It makes 71.3 lumens per watt, better than many LEDs.  If it lasts the advertised 12,000 hours, it's efficiency is:

\$4.49 + (12,000 * 0.11 * 23 / 1000) = \$4.49 + \$30.36 = \$34.85
12,000 * 1640 = 19,680,000 lumen-hours
19,680,000 / \$34.85 = 564,705 lumen-hours per dollar

So the best CFL and best LED give you half a million lumen-hours per dollar, while the incandescent gives less than an eighth million.  Amazingly, the CFL cost less in the long run.

Keep in mind that CFLs have been criticized for only lasting 3,000 hours, despite a 10,000 or 12,000 hour rating.  Even so, 3,000 hours gives over 400 million lumen-hours per dollar.

\$4.49 + (3,000 * 0.11 * 23 / 1000) = \$4.49 + \$7.59 = \$12.08
3,000 * 1640 = 4,920,000 lumen-hours
4,920,000 / \$12.08  = 407,285 lumen-hours per dollar

Here are many of the bulbs I found in September and October of 2013.

LED lights:

Utili Tech Pro  9.8w 800 lum 25,000 hrs round fins smoke dimmable \$11.98  81.6 lum/watt

Sylvania Ultra 14 w  1100 lum  \$29.98  25,000 hrs  78.6 lum/watt

GE Energy Smart 11 watts 800 lum \$15 15,000 hrs  72.7 lum/watt

Sylvania Ultra 12w 850 lum 25,000 hrs  flood fins softwhite  dimmable \$27.98  70.8 lum/watt

GE Energy Smart 10 w  700 lum \$20 15,000 hrs  70 lum/watt

GE Energy Smart 7 watts 470 lum \$10 25,000 hrs  67.1 lum/watt

Utili Tech Pro  7w 450 lum 25,000 hrs round fins smoke dimmable \$8.98  64.3 lum/watt

Utili Tech Pro  22w 1400 lum 25,000 hrs  flood vents softwhite \$37.98  63.6 lum/watt

Utili Tech Pro  8w 500 lum 25,000 hrs round fins clear \$17.98  62.5 lum/watt

Utili Tech Pro  4.8w 300 lum smoke/clear 30,000 hrs  \$14.98  62.5 lum/watt

Utili Tech Pro  4.8w 300 lum  candleflame smoke/clear 25,000 hrs  \$14.98  62.5  lum/watt

Utili Tech Pro  7.5w 450 lum 25,000 hrs round fins smoke \$16.98  60 lum/watt

GE Energy Smart 12 w 700 lum \$35 25,000 hrs  58.3 lum/watt

Utili Tech Pro  13w 750 lum 25,000 hrs  flood fins softwhite  \$24.98  57.7 lum/watt

Utili Tech Pro  20w 1150 lum 25,000 hrs  flood vents softwhite   \$29.98  57.5 lum/watt

Utili Tech Pro  4.5w 240 lum candleflame 50,000 hrs  \$9.98  53.3  lum/watt

Utili Tech Pro 4.5 w  240 lum 50,000 hrs  \$9.98  53.3 lum/watt

Utili Tech Pro  4w 200 lum candleflame clear 50,000 hrs  \$9.98  50 lum/watt

GE Energy Smart 1.8 w  75 lum \$20 2 pack 15,000 hrs  41.7 lum/watt

GE Energy Smart 2.3 w  94 lum \$20 2 pack 15,000 hrs  40.9 lum/watt

GE Energy Smart 2.5 w  80 lum \$20 2 pack 12,000 hrs  33 lum/watt

CFL

Utili Tech Pro  23w 1640 lum cfl  12,000 hrs 2 pack instant on \$9.98  71.3 lum/watt

GE Energy Smart 26 w 1750 lum \$6.58 6 pack 10,000 hrs  67.3 lum/watt

GE Energy Smart 16/25/32 w 600/1600/2150 lum \$7.98 2 pack 8,000 hrs  37.5/64/67.2  lum/watt

Utili Tech Pro  13w 825 lum cfl  12,000 hrs 4 pack soft white \$6.49  63.5 lum/watt

Utili Tech Pro  13w 825 lum cfl  12,000 hrs 2 pack instant on \$6.29  63.5 lum/watt

GE Energy Smart 13 w 825 lum \$2.82 8 pack 10,000 hrs  63.5 lum/watt

Utili Tech Pro  13w 800 lum cfl  12,000 hrs 4 pack bright white \$14.98  61.5 lum/watt

GE Energy Smart 20 w 1200 lum \$12.88 4 pack 10,000 hrs  60 lum/watt

Utili Tech Pro  5w 300 lum cfl  12,000 hrs 1 pack instant on softwhite \$4.78  60 lum/watt

Utili Tech Pro  5w 270 lum cfl  12,000 hrs 1 pack instant on daylight  \$4.98  54 lum/watt

GE Energy Smart 10 w 520 lum \$5 6 pack 10,000 hrs  52 lum/watt

Utili Tech Pro  7w 300 lum round smoke 8,000 hrs 2 pack \$9.98  42.9 lum/watt

Utili Tech Pro  7w 260 lum candleflame smoke 8,000 hrs \$ 5.98  37.1 lum/watt

Utili Tech Pro  7w 250 lum candleflame smoke 8,000 hrs 2 pack \$9.98 35.7 lum/watt