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Ask Mr. Tight-Watt: light bulb standards  Smart Choices Archive

Ask Mr. Tight-Watt: light bulb standards

Mr. Tight-Watt sheds light on CFL questions.

Ask Mr. Tight-Watt

Mr. Tight-Watt welcomes readers’ questions. Mr. Tight-Watt checked out a variety of resources to answer this reader’s questions.

Question: I am looking for information on CFL dimensions and wattage ratings; is this manufacturer specific information or are there defined standards to the physical sizes available? Also, I understand there is a co-op CFL wholesaler; how do I find this source? – Alston, A Smart Choices newsletter reader

Answer: CFL sizes (bulb dimensions) are pretty much standard, although you may run into some variations on bulb height among different manufacturers. Likewise, wattage ratings vary slightly by manufacturer. All use about 25% of the energy of standard incandescent bulbs with similar lumens of light output.

Most bulbs are “medium” or “Edison” base; which means they will fit into most sockets in the U.S., although candelabra size (smaller) bases are also available.

For more information on bulb sizes and shapes, check out ENERGY STAR® for general bulb shapes and Service Concepts for types and uses.
Flood lamps
Flood (reflector) lamps are identified with the letter R in the description, e.g. R20. External Flood lamps are identified with PAR in the description, e.g. PAR38.

The number for flood lamp description is a unit of bulb measurement, denoting the maximum diameter of the lamp expressed as a number of 1/8's of an inch. For example, some common sizes are:

PAR-38 (4.75")
PAR-30 (3.75")
PAR-20 (2.5")

More information

Also remember your co-op's online store for discounted prices on a variety of energy-efficient bulbs and products.


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