Although there are 3 basic considerations with regards to gas pressure washers, we will be elaborating on three other features that people usually tend to overlook, but that matters a lot if its professional use you plan for.
For gas powered pressure washers, the primary concern is the HP rating of the engine as it creates the volume and pressure. In many cheaper models, the pressure is higher without a proportional increase in the HP, this is something you must watch out for. If you have the required HP for the PSI and GPM rating then you basically have a powerful pressure washer. Domestic users should look in the range of 5HP or 110 Volt washers and if you plan on washing only your car then a lesser powered washer will suffice. Pros should opt for 6.5 HP or more washers. Some prefer the 13HP motor of commercial grade washers but this is usually required for garages and large manufacturing plants.
Volume In Terms Of GPM
Gallons Per Minute or GPM determines how much water the pressure washer can expel per minute. This lets you figure out the effective power of the washer along with the PSI rating.
Pressure In Terms Of PSI
Although PSI is the least important of all the three measurements, it still matters a lot. When you have enough GPM and HP having high PSI matters but without the other two, PSI hardly makes any difference.
A better way to measure cleaning power is to multiply GPM with PSI. Together, it shows just how effective a pressure washer is. Compare this with the HP rating of the washer and you can get a realistic idea of the power a pressure washer possesses.
Many low grade pressure washers rely on pumps made locally or imported from China. Make sure that the pump is from a reputed brand such as Legacy, Comet, General, AR, Karcher and others. They will give you above par performance, last longer and have readily available spare parts. Also look at the components used in the pump. If it uses ceramic plungers, thermal relief valves, brass heads and such high quality parts then its a high quality pump.
Vertical Vs Horizontal Shaft
The engine shaft direction also matters. Vertical shafts are cheaper and usually last a shorter duration. On the other hand Horizontal shafts are expensive but very reliable. If you plan on keeping the pressure washer for more than three years opt for a Horizontal shaft washer.
A pressure washer at the end of the day is designed for one specific task. So, pay only for what you plan on using. Some professional grade washers retail around $400 while consumer grade washers come for $99 to $200. Neither is a bad investment as it all depends on how much use you have for them. Longer jobs, frequent need will be taxing on the washer and having a commercial grade gas pressure washer obviously makes better sense. However, for odd household jobs go low key with an inexpensive pressure washer.