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.
Not all bagless vacuum cleaners are the same. The type of separation system makes a big difference to the performance of the machine and the amount of filter maintenance required.
Watts are a means of measuring the amount of power that's put into the motor. However, because the output power or suction you receive at the end of the hose depends on the vacuum's design and efficiency, air watts, which measure the output power and suction, are considered a better means of comparison for vacuum cleaners. Remember however, a clogged filter will quickly impair ait watt performance.