What kind of extension cord should I use for power tools and appliances?

Get a heavy duty extension cord with 12-gauge (or lower) wire and thick, cold-weather insulation for anything beyond a desk lamp. Light duty cords (16-18 gauge) are fine for phone chargers or a fan, but heavy duty cords (14-10 gauge) handle power tools, space heaters, and window A/C units. Lower gauge number means thicker wire, less resistance, and less heat. Never use a light duty cord for anything that draws real power — it can overheat, melt, or start a fire.