The Pressure Washer Spec That Most People Ignore
Marketing puts pressure figures front and centre. "250 bar!" sounds impressive. But for most cleaning tasks — removing mud, algae, road grime, food residue — flow rate (litres per minute) has as much impact on cleaning speed as pressure. Understanding both metrics helps you choose more wisely.
What Pressure Does
Pressure (bar/PSI) determines the kinetic energy of the water impact. High pressure is needed to:
- Break the adhesion of strongly bonded deposits (cured concrete, dried paint, hardened scale)
- Cut through thick mud or clay
- Remove material from porous surfaces (concrete block, brick)
For these tasks, higher pressure genuinely helps. But beyond 150–180 bar, many surfaces start to be damaged by the pressure rather than cleaned by it.
What Flow Rate Does
Flow rate (L/min) determines how quickly you rinse loosened debris away and how much surface area you can clean per unit time. A high-pressure, low-flow machine cuts dirt loose but leaves the debris swirling on the surface. A moderate-pressure, high-flow machine rinses more thoroughly and works faster on large areas.
The Cleaning Units Formula
Cleaning Units (CU) = Pressure (bar) × Flow Rate (L/min)
Example comparison:
| Machine | Pressure | Flow Rate | Cleaning Units |
|---|---|---|---|
| Machine A | 160 bar | 6 L/min | 960 CU |
| Machine B | 130 bar | 9 L/min | 1,170 CU |
Machine B has lower pressure but 22% more cleaning power. It will clean a driveway faster and use no more water per metre squared — despite "lower" pressure.
Practical Implications
For home patios, driveways and vehicles: look for machines with at least 7–9 L/min flow rate rather than chasing maximum bar figures. For professional construction or industrial cleaning where you genuinely need high force: both high pressure and high flow rate (the expensive professional units) matter. For agricultural spraying: flow rate dominates — you need volume, not pressure.
FAQ
Why doesn't my high-pressure machine seem to clean as fast as my neighbour's cheaper one?
Likely a flow rate difference. Check the L/min specification on both machines. Higher-bar machines are often paired with lower-volume pumps to keep the motor within size constraints. A machine rated 180 bar / 5 L/min may genuinely clean more slowly than one rated 120 bar / 9 L/min on typical suburban tasks.