LED against incandescent

A 100-light incandescent mini set draws around 40 watts. The LED equivalent draws around 5. That is not a marginal efficiency gain, it is a different order of magnitude, and it is why the two are worth treating separately in every calculation.

The practical consequence is that chain limits stop mattering. Five incandescent mini sets fill a 210 watt run; you would need forty LED sets to do the same. Most people never approach the limit with LEDs, which is why so much of the old advice about counting sets feels irrelevant now.

Take a display of twenty 100-light sets running six hours a night for thirty nights. Incandescent: 800 watts, 144 kWh, which is a genuinely noticeable amount on a bill. LED: 100 watts, 18 kWh.

At typical residential rates that is the difference between a meaningful line item and a rounding error. It is also the difference between needing a second circuit and not.

Every watt a set draws leaves it as heat. That is not an efficiency claim, it is the only place the energy can go. Even the fraction that leaves as visible light is absorbed by the tree, the wall and the room within a few feet and becomes heat there. So the eight-to-one power difference on this page is also, near enough, an eight-to-one heat difference.

The number that matters is not the set total, though. It is the watts per bulb, because that is the heat concentrated at the one point that touches a branch, a curtain or a tight wrap of tinsel.

The spread is the useful part. A C9 bulb puts more than seventeen times the heat of an incandescent mini into a single glass envelope, and about nine times what an LED C9 does. The old advice about keeping bulbs clear of dry foliage was written when C7 and C9 were what people hung, and it still holds for them. It was never really about mini strings, and it does not transfer to LEDs at all.

In practice: an incandescent C7 or C9 set wants air around it and no contact with paper, fabric, dry needles, or a tight coil of its own cord. LED sets of any format can be bundled, wrapped close or buried in a wreath without that concern.

LEDs are not cold, though, and sets sold as “cool to the touch” oversell it. A 20 W LED C9 set still puts 20 W into the room. What changes is where it comes off, mostly the base and the driver rather than the envelope, so the bulb you touch is cool while the set as a whole is not a zero-heat object.

We publish no bulb surface temperature. You will find confident figures for one, and they disagree with each other by enough to be useless, because the answer depends on the bulb format, the glass, whether it sits in open air or inside a lens, and whether it is buried in foliage. We have no measurement we can stand behind, so there is no column for it above. The watts per bulb are what we can source, and they are enough to rank the formats.

Heat is also not what causes most holiday lighting fires. The failures that do are on the what goes wrong page, and the first of them is the overloaded chain, which is a cord heating up, not a bulb.

Color. Incandescent light has a continuous spectrum and a warmth that cheap LEDs do not reproduce. Early LED sets in particular had a blue-white cast that people disliked for good reason. Good warm-white LEDs have largely closed this gap, but the cheapest ones have not.

There is also the flicker question: inexpensive LED sets often run direct from rectified line voltage with minimal smoothing, which produces visible flicker in peripheral vision and in photographs. If your phone shows banding when filming a set, that is what you are seeing.

Do not chain LED and incandescent sets together on the same run. The chain limit is set by the cord in the first set, and putting a heavy incandescent load behind a light LED set is exactly the case that overheats a cord.

The calculator handles mixed displays: add each type as its own row and it works out the chain limit for whichever type is tightest.