This is one of the questions we are asked all of the time. Good residential units have boiler steam temperature between 240 and 298 degrees farenheit. For serious cleaning avoid any steam cleaner that has a lower average steam temperature.
Hotter steam means a quantum increase in cleaning effectiveness.
What's more it is necessary to start with a much higher temperature than you would think you might need, as all vapor steam cleaners lose some temperature on the way to the tip. The very best machines have boilers made out of stainless steel which heat up faster, and do not rust. Without a safety cap there is danger of steam burns if the water reservoir is opened too quickly after usage. On the continuous feed machines there are two reservoirs for water. One is for filling and the water does not get hot.