If an independent power source is needed which is not constrained by the electrical supply then a diesel or petrol engine model is the answer. This type of pressure washer can be more powerful than electric machines and is ideal for off site work. These machines are often mounted in a tubular steel frame for robustness.
In a hot water pressure washer the design inlet temperature should not be exceeded as it may affect the seals in the internal machinery. The size of the motor that drives the pump to produce the water pressure. The motor size is normally expressed in Kw for an electric washer and HP for an engine driven version. The motor must be correctly sized to match the pump to ensure optimum performance. On cheaper machines the pump may be smaller than the ideal size meaning it has to work harder to create the required flow and pressure and as a consequence will not last as long.
To reduce uncertainty ascertain how long the washer would be used each week and how hard it would have to work. To have a washer working at full tilt all the time will not improve its longevity. It is also important to think about the kind of cleaning required is the dirt caked on? Do you want to clean large floor areas or smaller pieces of equipment. Do you need a hot or cold water version?