Wattage Equivalents Are Marketing, Not Physics

The wattage equivalence label was invented to help shoppers replace an incandescent bulb without thinking about it. It worked. It also left an entire generation of buyers specifying luminaires by a number that describes power consumption rather than light output.

What the equivalence figure really means

An LED lamp marked as a 60-watt replacement typically draws 8 to 10 watts and produces 750 to 850 lumens. The equivalence is a comparison with an old incandescent bulb that produced the same light for 60 watts of input. It is a useful shorthand at a retail shelf. It is a poor specification.

The two numbers that matter

Lumens describe how much light leaves the lamp. Watts describe what it costs to run. The ratio between them, lumens per watt, is efficiency and it now varies by more than a factor of two between a cheap LED and a good one.

Where this goes wrong in a specification

A buyer writes 60W max on the purchase order, the factory fits a socket rated to 60W, and the lamp ships with a 9W LED. The fixture now looks under-specified on paper and exactly right in the room. The fix is to specify lumens and a maximum wattage, and to say which one the socket rating refers to.

Heat is the practical limit

In a small drum shade or an enclosed glass diffuser, heat has nowhere to go. Halogen and incandescent lamps in that geometry raise the temperature of the socket, the wiring and the shade itself. LED lamps run far cooler, which is why a modern lamp can be smaller and higher output than a 1990s equivalent.

Dimmability is the third number nobody writes down

A bulb can be efficient, bright and completely incompatible with the dimmer in the lamp. Compatibility is a three-way negotiation between driver, dimmer and load, and it belongs in the specification for any lamp that ships with a dimmer or a three-way socket.