Psi stands for pounds per square inch

 Lpm is litres per minute.

In 1 bar there is 14.5 psi

Petrol and diesel engines are measured in horsepower (Hp)
Electric Motors are measured in kilowatts (kw)

1 kw = 1.341 horse power

The most important factors you need to understand when looking at a pressure washer and deciding which one you want, after you’ve decided whether you want an electric or petrol/diesel model is the Lpm and Bar.

The Lpm is the flow rate in litres per minute that your pressure washer produces.

The Bar is the energy with which the water will hit the surface you are cleaning.

Basically the higher the two numbers on your pressure washer the more powerful the machine you have.

But the two numbers interact together.

Take this example: If a bird flies into a building at 30mph it will hit the building and bounce off, only causing damage to itself.

If a jumbo jet was to fly into the same building at 30mph the building would most likely be destroyed.

So what this means is that you could have a pressure washer with a very high Bar, but if its got a low Lpm flow rate it won’t be as powerful as a machine with a slightly lower bar but a higher flow rate.

How do you compare machines then?

Just taking a look at the Lpm and Bar figures of different machines is a difficult way of comparing them.

Malcolm Smith of powerwashinguk however has produced an equation that gives you a rough guide as to which overall is the most powerful machine.

 
Lpm x Pressure in Bars, then ÷ by 600. This will give you a number to compare machines, the higher the number the more powerful the machine is. Again like I said this is a rough guide.

Power isn’t the only thing to consider

You can have two equally powerful machines but one is more expensive than the other. There may be an obvious reason for this, like one has a reduction gearbox, but on the other hand there may not be. You’d expect the higher-priced machine to be built from higher-quality components that are going to last longer.

How much Power do you need?

This is completely dependent on the type of work you’ll be doing.

Car Valeting, the occasional small jobs: 130 bar at 10 lpm

Professional Driveway Cleaning: 150-200 bar at 14-15 lpm

Boat yards: 500bar

Paint Stripping: 1000bar

Tip: You have to remember when using a hot pressure washer you don’t require as much brute force because the hot water aids the cleaning process.

Water Supply

It’s all good having a hugely powerful machine, but you’ve got to supply that machine with enough water to keep it running. You can of course run your pressure washer off a buffer tank, but if the water supply is too slow you’ll have to keep waiting for your buffer tank to fill up again.