What does Pa mean on a robot vacuum?
By Lachlan Ford
Every robot vacuum is sold with a big number followed by "Pa". It stands for pascals, a unit of pressure, and it describes how hard the robot's fan can pull at full power. The numbers get bigger with every new model, and it's natural to assume the biggest one wins. On carpet it often does, while on a kitchen full of tiles you'd be hard pressed to tell the difference.
The short answer
Pa (pascals) is a pressure figure: the strongest pull the robot's fan can make on its top setting. A higher number helps most on carpet and rugs, where grit sits down in the pile. On hard floors nearly any robot has enough, and the brush and the mop matter more.
How suction works
Inside the robot a fan pushes air out of the robot, and that leaves the air inside the robot thinner than the air in your room. The room’s air then shoves its way in through the slot underneath to even things out, and any dust lying near the slot gets carried in with it. Drinking through a straw works the same way. You lower the pressure in your mouth, and the ordinary air pressing down on the drink pushes it up the straw for you.
A pascal is the unit that pressure is measured in, and it’s a small one. The air around you presses on everything with a very large number of pascals. You’ve never noticed, because it presses evenly from all sides. The Pa figure on a robot is the size of the gap its fan can open up between the air inside and the air outside. A robot marked 10,000 Pa can open a wider gap than one marked 5,000 Pa, so the air comes in with more force behind it.
What the number on the box does and doesn’t tell you
The figure is a maximum. Robots have several power levels, and the number printed on the box belongs to the top one. The eufy Auto-Empty C10 shown here has four, running from Quiet up to Max. The default is usually a middle setting, because full power makes more noise and empties the battery sooner, so the headline figure describes the robot at its loudest.
Pressure is also only half of what a vacuum does. Go back to the straw, put a finger over the far end and suck as hard as you can. The pull in the straw is now as strong as you can make it, and nothing at all is moving. Cleaning needs a steady stream of air as well as a strong pull, since it’s the moving air that carries dust to the bin, and the box gives you a number for the pull alone.
The box is usually quiet about how the figure was arrived at. If the box does say where the figure was measured, it’s usually at the fan. That isn’t the same as the pull down at the floor. You can treat a much bigger number as a sign of a stronger fan, but two robots from different brands whose figures sit close together may well swap places on your floor.
A pressure figure has nothing to say about the brush or the route, either. The brush under the robot does a good share of the cleaning, because it flicks dirt up into the airflow in the first place. The route matters too, since a robot can only clean the floor it drives over, and one that maps the house covers it more evenly than one that wanders.
How it plays out in a real house
On tiles, timber and vinyl, dust and crumbs sit on top of the floor with nothing holding them down. The brush sweeps them towards the slot and a modest stream of air is enough to finish the job. This is why a robot with a small Pa figure can still leave a hard floor looking clean.
On carpet the number starts to matter. Grit works its way down between the fibres, and the air has to be dragged through the pile to bring it back up. More pull does help here, and it helps more as the pile gets deeper. A flat office-style carpet asks a lot less of the fan than a shaggy rug in front of the couch.
Many robots can tell when they’ve driven onto carpet and raise the suction by themselves, then drop it again when they’re back on the hard floor. That’s a more useful thing to have than a huge figure you need to switch on by hand. The strong setting is used where it counts and the battery is spared everywhere else. If your house is a mix of floorboards and rugs, it’s worth checking for.
Pet hair behaves differently depending on where it lands. On a hard floor it drifts about and the brush gathers it up easily. Once it’s trodden into a rug it wraps around the fibres, and getting it out depends on the brush as much as on the fan.
The runtime on the box is usually measured on the gentlest setting, so a robot working at full power through a house full of carpet won’t run for as long as the box suggests.
What to do with the number
Start with your floors. If they’re mostly hard, you can stop worrying about Pa. Nearly every robot has enough for dust on tiles, and your attention is better spent on the kind of mop it has and what its dock does.
If you have a lot of carpet, a few thick rugs or a dog that sheds, a higher figure is worth having, along with a robot that raises its suction on carpet by itself. Compare the numbers the way you’d compare any maker’s figures, which is to say loosely. A robot with double the Pa of another is very likely the stronger of the two. When two robots have similar suction figures, choose between them on their brushes, their mops and their docks.
For a sense of scale, look at the “In our list” box on this page. It gives the lowest, middle and highest suction among the robots we compare. And if you’d rather describe your house than read spec sheets, the Matcher asks about your floors and does the sorting for you.
