Extension cord rating should equal or exceed the appliance electrical rating on the unit with power requirements of wattage or amperage.
Do not let wattage or amperage scare or confuse you. Both state electrical consumption. The more power an appliance requires, the larger the electrical feed line should be. Undersized cords heat up and can cause a fire. There is no excuse for using an extension cord less than the size it should be. Saving a buck or three is not a good idea when hooking up an appliance that will be operating when you are asleep.
To determine wattage, multiply voltage by amperage. 115 volts x 10 amps = 1150 watts. For amperage, divide watts by voltage. 1150 watts = 115 volts = 10 amps.