12-gauge vs 14-gauge extension cord for a 1500W space heater: 12-gauge is the only safe choice for continuous use.

A 1500W heater draws 12.5 amps, and 14-gauge cord is rated for 15 amps but derates to only 12 amps for continuous loads (over three hours). That puts you over the limit and creates a fire risk. A 12-gauge cord handles 20 amps (16 amps derated), giving you plenty of headroom. Don’t save ten bucks on the cord.