One solution might be to hire a pressure washer before buying and ask a company expert in the field what they recommend. Don’t forget to factor in to your costings any servicing that may be required to make sure the washer will work at optimum efficiency.
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.
On industrial machines it is normal to use a slower revving motor with a larger pump which means it can be used for longer periods without overheating. Pressure washers can be powered in various ways and each has its advantages. Electric power is the most common source with smaller motors using a 240 volt single phase supply from a conventional plug socket and larger commercial machines using a 3 phase 420 volt supply.