When looking at combo dimmers most of them are rated for 600 when using incandescent, but only 150 when using leds. Surely the switch should be able to shed the same amount of heat regardless of what bulb is used -- why the difference?

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    Pretty well answered here, I think: diy.stackexchange.com/a/201548/18078 – Ecnerwal Oct 31 '20 at 16:14
  • @Ecnerwal: that suggests incandescent lamps are actually de-rated because of start-up current spikes, not the other way around. I believe the OP needs to look at power factor; some LEDs have miserable PF; as low as 25% for some naïve/cheap capacitive dropper based bulbs, compared to 100% for incandescent ones. That needs to be accounted for, even if not billed for (yet, wait for smart metering to remedy this and prompt the need for new labels on bulbs). – dandavis Oct 31 '20 at 19:50
  • It depends on the type of power fed to the LEDs to determine what kind of heat x watts will burden the switch with. Rather than break it down by tech (which is complex and confusing to consumers to say the least) they go for a worst-case scenario. It doesn't hurt marketing because 150W of LEDs is generally PLENTY for most rooms; way brighter than 600W of Edison bulbs. – dandavis Oct 31 '20 at 19:55

The main difference would be the harmonics and distortion that occur in the driver that powers the LED lights.

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