PSI, Bar, Power of Pressure washers - What does it mean?

23 August 2018

What does PSI mean or stand for?
PSI stands for ‘pounds per square inch’. The PSI is the energy with which the water will hit the surface you are cleaning.

What does LPM flow rate mean or stand for?
LPM stands for ‘litres per minute’. Usually referred to as a rate of water flow in machinery with a fast output of water such as a pressure washer or pressure washing machine tool accessory.

What does ‘Bar’ in pressure mean?
It’s a different scale to PSI for measuring pressure. In every Bar there is 14.5 PSI. The Bar is the energy with which the water will hit the surface you are cleaning.

How do I know the power of my pressure washer engine or motor?
Petrol and diesel pressure washer engines are usually measured in horsepower (HP). Electric motors are usually measured in kilowatts (kW). 1 kW = 1.341 horse power

Most important things to consider when buying a new pressure washer!
The most important factors you need to understand when looking at a commercial pressure washer and deciding which one you want, are...

1) Engine Type & Size

2) The flow rate - LPM (litres per minute)

3) The pressure (PSI or Bar).

Basically the higher the two numbers on your pressure washer rating the more powerful the machine you have. If you want to use a multi-nozzle attachment, it is more important to opt for a pump with a higher flow rather than a greater pressure. For multi-nozzle use, insist on a pump specification with at least 13 LPM.

What else to consider when buying a new pressure washer once you’ve decided one the engine you want?
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 a petrol or diesel model is the LPM and the Bar. The LPM is the flow rate of water 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 rating the more powerful the machine you have. Higher flow tends to wash away dirt, higer pressure tends to cut through tough dirt, so it all depends on the task on hand.

Comparing pressure washer machines' output power
Looking just at the LPM and Bar figures of different machines is a difficult way of comparing them. One industry specialist has produced an equation that gives you a rough guide as to which overall is the most powerful machine. The sum is LPM x Pressure in Bar, then divide this by 600. This will give you a number to compare machines, the higher the number the more powerful the machine is. Just roughly...!

Comparing industrial pressure washers' output power
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 and the other has a direct drive. Machines with reduction gearboxes between the engine and the pump will likely be more expensive than direct drive machines. The reason for this is it enables the pump to run at lower speeds, thus increasing the life of the innards of the pump, especially the seals. If there appears to be little difference in specification you’d at least expect the higher-priced machine to be built from higher-quality components that are going to last longer. For instance, the most of the hose reels and accessories from GENERAL PUMP are tested to a burst pressure of at least 4 times the rated pressure. Well known competitors' accessories are only rated to 3 times the maximum pressure.

How much pressure do I need?
This is completely dependent on the type of work you’ll be doing. If it's for car valeting then 130 Bar at 10 LPM is sufficient. If it's driveway cleaning then 150-200 Bar at 14-15 LPM would suit. If it's professional or industrial pressure washing work then anything up to 276 Bar.
 

Can my pressure washer run off a mains water tap?
Not always....it depends on the pump type fitted. If its a direct drive pump with an output of 13 LPM maximum, it would be best to have a water supply with a positive displacement, eg. mains water tap. Direct drive pumps don't like having to suck the water up themselves and can overheat if they can't suck up sufficient water. If you have a geared pump fitted, the pump would prefer to draw water from a barrow or bowser. This way it can draw as much as it wants, and because the pump is running slower than a direct drive pump, it has a stronger suction. Generally speaking a geared pump will require more water in a minute than what a mains water tap can produce.