Richard Garrison of the Sustainability Subcommittee of the Buildings Committee at Collington has developed this useful analysis of the benefits of LED bulbs. Hopefully they will be of use way beyond our campus.
LED bulbs generally look and provide light like standard incandescent bulbs, but they use only a fraction of the electricity, last far longer, and therefor save a lot of money (they run a lot cooler, too.) . In fact, to match the lifespan costs of one 60-watt equivalent LED bulb using traditional incandescent bulbs you would have to spend about $154 more for electricity and buying about 20 incandescent bulbs.
Bulb Types
Incandescent bulbs (the common ones we all grew up with) emit light because electricity running through a filament inside the oxygen-free glass envelope causes the filament to become white hot and emit light. Incandescent bulbs are simply electric heaters which emit light as an incidental side effect. Incandescents contain no mercury, but run hot.
Compact Fluorescent Lamps [CFLs] cause a coating inside the bulb to fluoresce, emitting light. CFLs contain mercury and run almost as cool as LEDs.
Light Emitting Diodes [LEDs] are electronic devices that cause electrons to bounce between low and high energy orbits releasing photons (light) in the process. Many LED bulbs are similar in size and shape to standard incandescent bulbs, so they can be used for lampshades employing spring clips which attach to the bulb itself instead of a harp. LEDs contain no mercury and run relatively cool.
Electricity used by a bulb, regardless of type, is converted essentially100% into heat @ 3.4 BTU/watt-hour. Lower wattage bulbs emit less heat for lower air conditioning loads (thus saving even more).
Lumens
We all grew up using a bulb’s wattage as an indicator of its brightness or light output, but the wattage is merely a measure of the electrical energy consumed by the bulb. Light output is measured in “lumens” regardless of bulb type or wattage. Here is a rough conversion between traditional incandescent wattages and lumen output:
- 100 watts – about 1600 lumens
- 75 watts – about 1100 lumens
- 60 watts – about 800 lumens
- 40 watts – about 450 lumens
Human perception of light is far from linear. A 1600 lumen bulb does not appear to be twice as bright or to throw twice as much light as an 800 lumen bulb, and the difference between 700 lumens and 900 lumens might not even be noticed.
Color Temperature
Color temperature is a way of describing the color of light emitted, commonly expressed in degrees Kelvin. The light emitted by common incandescent bulbs is usually yellowish, or “warm,” usually around 2700 degrees K. Depending on technical design common color temperatures for all three types of bulbs range from “warm” (2700 K) to “bright white” or “daylight” (5000 K – 6000 K).
Bulb Lifespan
Bulb lifespan is not a precise thing, but here are some common averages for the bulb types in question (To give some perspective on bulb life, a year is 24 x 365 = 8,760 hours):
- Incandescent 750 – 2000 hours
- CFL (Plug-in) 10,000 – 20,000 hours
- CFL (Screw base) 8,000 – 10,000 hours
- LED 25,000 – 50,000 hours
The three kinds of bulbs in question are widely available with standard screw bases in a broad range of lumen and color temperature outputs, and light distribution profiles (e.g., spot, flood, diffuse) at many retail outlets (e.g., Home Depot, Lowes, Amazon, etc.).
The calculations were performed as follows:
- (Watts x 25,000 hrs) / 1000watts per kilowatt-hour = kilowatt-hours used over 25,000 hour operation
- kilowatt-hours used x price per kilowatt-hour = electricity cost over 25,000 hours
- 25,000 hrs / hrs average bulb lifespan = number of bulbs needed to operate for 25,000 hours
- # bulbs needed for 25,000 hr lifespan x price/bulb = total bulb cost for 25,000 hrs operation
- Electricity + bulb cost for 25,000 hrs of operation = total overall operating cost per fixture