Dad’s short answer: Enough to pull the liquid back out, which is the entire job. A spot cleaner that sprays well but extracts poorly leaves the stain wet and driven deeper — which is worse than not treating it at all, because now it's wicking.

Suction is the whole machine

Anything can spray water on a stain. What makes a spot cleaner a spot cleaner is that it pulls the water back out, taking the stain with it. Weak extraction means you've just added moisture to a mess and pushed it deeper into the backing.

That's why suction is the spec I'd weigh most heavily here, ahead of tank size, ahead of heat, ahead of everything on the front of the box.

It's also why corded machines have an advantage in this category. Sustained suction is exactly what a battery struggles to deliver.

The honest test

Look at the dirty tank. After treating a spot, the dirty tank should have a meaningful amount of liquid in it, and the carpet should be damp rather than wet. If the tank is nearly empty and the carpet is soaked, the machine isn't extracting — it's just spraying.

That's the same principle I use on wet/dry floor washers, and it's the most reliable real-world test there is. The dirty tank is the honest scoreboard in every one of these categories.

If you're reading reviews before buying, that's the detail to look for: does the reviewer show the dirty tank? Anybody who does is telling you something real. Anybody quoting only a suction number is reading the box.

Dad’s Take: Watch the dirty tank. If it isn't filling, the machine isn't cleaning — it's just wetting your carpet.