According to :

The thing is, that for my 16 kw/h of electricity, the unit managed to
suck up just over 7 liters of water, which works out to be less that a
half a liter of water for every kw/h it uses. I was reading on the
energy star website, that modern dehumidifiers should be able to suck
up about 1.5 liters of water per kw/h used.

It doesn't sound that old, so, it should be doing better than that.

It may be that it just needs a thorough cleaning of the coils and fins.
If there's lots of dust on it, it will act as an insulator, and greatly
reduce efficiency. May need to resort to an air compressor and/or
fine & soft brushes.

In a basement, it may have picked up rather a lot of dust in a few
years if they did any renovation (eg: drywall sanding ;-).

Note also that if you're not getting the water right out of the
basement (eg: run the outflow directly into a drain), you'll be
losing some effectiveness through re-evaporation. Sorta like
having both the cold and hot side of an A/C in the same room - the
net result is the room doesn't get cooler...
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.