It's not a matter of power, but energy. It may take it twice as long as a 220v circuit, but when it's an unattended appliance that doesn't really matter. Per cycle they use about a gallon of water (3-4 per load total). Going from 70F to 140F requires 170Wh for a gallon of water. So if you run it at 1000W it takes 10 minutes.