Narwal Freo Z Ultra

Suction in pascal, as the maker measures it. More pascal helps on carpet, but it says little about how well the robot picks grit off a hard floor.

A brush that guides hair to the side instead of letting it wind around the shaft. It saves the cutting work in homes with long hair or pets.

The side brush swings out in a corner or along a baseboard, so the dust there is swept in too.

How the mop is built: flat pads, two spinning discs or a roller. Discs press harder on dried-on dirt, a roller rinses itself while it works.

How far the robot lifts the mop when it drives onto a carpet. From about 0.4 in (10 mm) a medium pile stays dry.

The pad slides out along a wall or a baseboard, so the last few inches are mopped as well.

The robot leaves the mop at the dock before it vacuums a carpet, so the carpet never gets wet.

The dock empties the robot’s dust bin, usually after every run. You only empty the dock’s bag or bin.

The dock collects dust in a bin instead of a disposable bag. No bags to buy, but you empty it yourself.

How much the dust bag in the dock holds.

How long the maker says a dust bag lasts with daily use. With pets or a lot of carpet it is shorter.

The dock washes the mop for you, so you do not rinse it by hand.

How hot the dock washes the mop. Above 140 °F (60 °C) grease comes off, which is what keeps a mop from smelling.

The dock dries the mop afterwards, so it does not sit there wet and start to smell.

The dock mixes detergent into the water itself, from its own bottle.

The dock can be connected to the tap and the drain, so you never fill or empty a tank. It does need a connection nearby.

The width of the dock.

How the robot maps your home and follows that map. Lidar measures with a laser and works in the dark, a camera looks along and recognizes more.

How the robot sees loose things on the floor, such as cables, socks or a pet accident. A camera with depth sensing sees more than a bumper that reacts on contact.

The highest threshold the robot takes, as the maker states it. Measure your own: the step into a bathroom or onto a balcony is often higher.

An extending leg or wheel that carries the robot over a high threshold. Without one it usually stops at about 0.8 in (2 cm).

How tall the robot is. It decides whether it fits under your sofa, bed or cabinet.

How wide the robot is. Narrower gets between chair legs more easily, wider usually means a bigger dust bin.

What the robot weighs. It counts most if you carry it up a staircase every day.

How much dust the robot holds before you empty it. It matters less with a self-emptying dock, which empties the bin after every run.

The noise in decibels, in the setting its source names: makers mostly give the standard setting, stores often the highest. 65 dB is like a vacuum at half power, 55 dB like a conversation.

The capacity of the battery. It hints at the runtime, but how much the robot draws counts just as much.

How long the robot cleans on one charge, at its quietest setting. After that it recharges and carries on where it stopped.

Which voice assistants the robot works with, such as Google Assistant or Alexa.