Are portable air conditioners inefficient? They seem to cool down very well

[Universe93B]

I have this Haier model portable conditioner (1 exhaust hose) model number qpca08jamw, which is 9000 BTU and it's 3 years old. I last used it 3 years ago in my house when our upstairs central HVAC went out in the middle of July and 99 degrees outside in NC. Since then, I've let friends borrow it when their central a/c went out in the summers and they praised the unit for keeping things cold. We've all used it in 1 bedroom and leave the door open, so it eventually cooled the entire floor.

Why are these units known to be inefficient? It doesn't use that many more watts than a window a/c unit. Is it because the 1-hose exhaust doesn't get rid of all the hot air fast enough? Or because the excess water gets evaporated from the heat of the condenser?

There's a Physics part of this also: They say because the portable a/c spits hot exhaust air outside, it creates a negative pressure in that room, causing outside hot air to leak back in. what if this is a room where there is another door that leads to a part of the house that has a central HVAC that is working well? What are the implications if that door is open or closed? If that door is left open, I'm assuming that would help ventilate and air exchange with the room that has the single hose portable air conditioner?

These things seem pretty powerful to me:

 

[harshness]

The measure of efficiency is the EER.

This unit is rated at 6.8 EER. It is a stop-gap solution to a few hot days, not a summer-long cooling solution.

An efficient conventional window unit may have an EER of 11-13 and a good dual-split would be just short of that.

It all comes down to what kind of construction work you're willing to do to make something more efficient fit in.

Window coverings (inside and/or out) may get you to a point where the central air will be sufficient. This is probably going to get you the best bang for your buck.