An 18-gauge cord is fine for LED lights under 100W.

Yes, you can use an 18-gauge extension cord for your LED Christmas lights if the total wattage is under 100 watts. It’s totally safe and common.

The math is simple: at 120 volts, 100 watts pulls about 0.8 amps. A 50-foot 18-gauge cord is rated for around 10 amps. You have massive headroom. Even if you packed a whole 100-foot string of C9 LEDs (which is maybe 30–40 watts), you’re still well under the limit.

The real issue isn’t the gauge — it’s the cord’s construction. Make sure it’s outdoor-rated if it’s going outside, and don’t plug multiple cords into each other. Also don’t run the cord under rugs or through windows where the door can pinch it. That’s where fires actually start.

You have bigger things to worry about than this cord.