Get a heavy duty extension cord for anything real.

You want 12-gauge (or lower) wire with thick, cold-weather insulation for anything outside a desk lamp.

The difference is wire gauge and insulation. Light duty cords (16-gauge or 18-gauge) are fine for phone chargers, a fan, or a lamp. Heavy duty cords (14-gauge, 12-gauge, or 10-gauge) handle power tools, space heaters, window A/C units, or anything that pulls more than a few amps. Lower gauge number = thicker wire = less resistance = less heat.

Insulation matters just as much. Heavy duty cords have thicker rubber or vinyl that stays flexible in cold weather and resists cracking. Light duty cords get stiff, brittle, and can expose live wires. That’s a fire and shock risk.

Safety rule: never use a light duty cord for something that draws power you can feel — a saw, a heater, a compressor. The cord will overheat, melt, or start a fire long before the breaker trips.

This is not where you save five bucks.