On a 230v. 50 amp system with a 5.5 kW heater it will take about 6 to 8 hours to get the water from about 60 degrees Fahrenheit from the garden hose to about 100 degrees Fahrenheit. It will also depend on the spa you have purchased. If you purchased an 8-foot spa, it may take 6 to 8 hours. However, if you purchased a 7-foot or smaller spa then it may take 5 to 7 hours to heat the water.