When you buy a compact vacuum cleaner it's important to think about every possible feature because when space is of the essence, the small details really do count. The best compact vacuum cleaners will come with most (if not all) of the following features.
The capacity of the vacuum cleaner's dust bin or bag will essentially determine how often you'll need to empty it. If you have a large house and you don't want to be going back and forth the bin, it's worth investing in a vacuum cleaner with a bigger capacity. That said, if you have trouble lifting or manoeuvring a vacuum cleaner, bear in mind that the bigger the vacuum cleaner's capacity, the heavier it'll be.
A high wattage (input power) doesn't always mean better suction because the amount of this power that's turned into air watts (suction or output power) depends to some extent on the vacuum's design and efficiency.