Pressure washer specs explained: bar, flow and area performance

A pressure washer spec sheet has three main numbers: pressure, flow and — on Kärcher machines — area performance. Here is what each means and how to use them, with the makers’ own figures.

Pressure (bar)

Pressure is how hard the water hits the surface. Most makers quote the maximum. Kärcher’s range page shows how pressure rises through its K classes:

• K 2: 110 bar · K 3: 120 bar · K 4: 130 bar · K 5: 145 bar · K 6: 160 bar · K 7: 180 bar.

Many machines let you turn pressure down for delicate surfaces. Kärcher lists the K 4 Power Control Flex, K 5 Classic and K 7 Comfort Premium as starting from 20 bar, and says on the K 4 that the pressure can be adjusted by turning the lance and checked on the trigger gun.

Flow (litres per hour or per minute)

Flow is how much water comes through, which affects how quickly dirt is rinsed away. Kärcher quotes litres per hour; Bosch quotes litres per minute. To compare, multiply litres per minute by 60: Bosch’s 7.5 l/min for the UniversalAquatak 135 and AdvancedAquatak 140 works out at 450 l/h.

Area performance (m² per hour)

Kärcher also gives an area performance — roughly how much surface you can clean in an hour: 20 m²/h for the K 2 class rising to 60 m²/h for the K 7 class. It is a useful way to match a machine to the size of your patio or drive.

Class figures versus the actual model

Kärcher’s range page gives figures for each class, but individual models can differ, so always check the model’s own page. For example:

• The class table gives the K 4 class 500 l/h, but the K 4 Power Control Flex is listed at max 420 l/h.
• The K 5 class shows 510 l/h; the K 5 Classic is listed at max 500 l/h.
• The K 7 Comfort Premium is listed at max 550 l/h.

Motor type

Kärcher says the water-cooled motor in its K 7 Comfort Premium has a lifespan at least twice as long as comparable machines with a universal motor; the K 4 Power Control Flex also has a water-cooled motor, while the K 5 Classic has a universal motor. Bosch lists an induction motor for the AdvancedAquatak 140. Weights differ a lot too: the K 5 Classic is 5.6 kg without accessories, while the K 4 Power Control Flex is 12.1 kg and the K 7 Comfort Premium 18.8 kg.

Hoses, reels and handles

Hose length decides how far you can reach without moving the machine: 3 m on the K 2 Classic, 8 m on the K 4 and K 5, and 10 m on a reel on the K 7. Bosch lists 7 m for the UniversalAquatak 135 and 8 m on an Easy-Roll reel for the AdvancedAquatak 140.

Water supply

Most pressure washers run from a garden tap. Kärcher gives a maximum inlet water temperature of 40 °C for its K 2, K 4 and K 5 models and 60 °C for the K 7 Comfort Premium. Bosch lists a self-priming capability of 0.5 m for its UniversalAquatak models, meaning they can draw water from a container such as a water butt.

Cordless pressure washers

The Bosch UniversalAquatak 36V-100 runs on a 36 V battery. Bosch gives up to 100 bar, 1.7–3.1 l/min and up to 45 minutes’ run time, and recommends a battery of at least 4.0 Ah. It is sold with or without a battery and charger, so check which version you are buying.

Detergents and nozzles

Machines differ in how they apply detergent — Kärcher’s Plug ’n’ Clean system on the K 4, suction on the K 5, and a high-pressure detergent nozzle on the Bosch models. Most come with a rotating “dirt blaster” or rotary nozzle for stubborn dirt and a fan jet for general cleaning.

Choosing in three steps

1. Size of job: use area performance (Kärcher) or the maker’s tier (Bosch calls its levels Easy, Universal and Advanced) to match the machine to your patio, drive or car.
2. How often: Kärcher describes the K 2 Classic as for occasional use on light dirt and the K 5 Classic for regular use.
3. Where: if there’s no socket or tap nearby, a cordless self-priming model may be the answer.

Read the manual before first use and follow its safety instructions. Search Amazon for electric pressure washers.