Dad’s short answer: More than people think, and unlike robot vacuums the number here still means something. Suction is what lifts the dirty water back off your floor. A machine that can't do that leaves a wet, streaky floor no matter how good the roller is.

Why suction matters more here than on a robot

On a robot vacuum I'll tell you the pascal race stopped meaning much years ago. On a floor washer it's different, because suction is doing a job you can see immediately. It has to pull water — not dust, water — up off the floor and into a tank against gravity, through a roller that's actively wet.

If it can't, two things happen. The floor stays wet behind the machine, and the dirty water never makes it to the tank so the roller just keeps redistributing it. That's the streaking complaint, and it's a hardware problem you can't fix with technique.

The budget machine failure

I tested one budget unit that had no real active floor suction at all. It wet the floor and dragged a roller across it, and that's essentially a powered string mop with extra maintenance. It scored a five and got a Skip It, and the lack of suction was the reason.

This is the category where I'd caution hardest against buying at the very bottom. The gap between a budget floor washer and a mid-range one is much bigger than the gap between a budget and mid-range robot, because the cheap ones are missing the fundamental mechanism rather than just the convenience features.

What to look for: a stated suction figure and a dirty water tank that actually fills up. If reviews mention the dirty tank staying nearly empty after a run, that machine isn't lifting anything.

Dad’s Take: If the dirty water tank isn't filling, the machine isn't cleaning. That tank is the honest scoreboard.