That is definitley too high. 40m2, gas heating, engery saving bulbs, good about keeping the lights off, etc. I estimate about 300 - 400zl over Jan - May period.
But, if you do a good bit of cooking with an electric cooker you can add a couple hundred zloty to that.
AND, if you have a 3 KW electric heater that you use for 3-4 hours a day you can add 700 - 800zl to that. And if you don't have Tarif II night electricity, well add several hundred more. So in a situation like this you could see 2000zl.
I agree with Ziemowit's advice. Turn off (unplug) everything for 30 minutes and see if the meter keeps moving.
Have you see the actual invoice for the electricity? What was the meter reading at the beginning of January, and what is it now?
And read some of the points on this thread: https://polishforums.com/real-estate/poland-options-dealing-dishonest-landlord-42375/