Tenants unhappy about electric bill need advice please

[Mathuw Chandler]

I own a 2 family unit in st louis, this is upper level and lower level.  I used to occupy the upper level while the lower level was being gut rehabbed.  While living upstairs i had high electric bills winter only (all elec apts) like 300$ avg in winter time when running heat at about 65ish.

I have a feeling the furnace size is pushing it, not incredibly small and not oversized for the unit about 1100 square feet. 

I think when the furnace is on bills skyrocket, so im thinking the size (possibly) though it does not seem small, and the duct layout have a lot to do with it as it is a 1925 all brick home, with new windows.  

Recently (last week)  i added door sweeps, weather strips to all exterior doors and door sweeps to all interior doors which needed them, bought them plastic to install over (new) windows. and have scheduled to get the furnace cleaned professionally

The market rate of the apartment is 875 and i  am renting it to them for 800 should i be concerned that they are upset about high electric bills,  given i have done more than a slum lord to ensure they are happy and they are 75 dollars under market rat?  Only thing they par is their own electric and rent, i pay trash sewer water

I understand it sucks for a higher than average utility bill and i had to deal with it when i lived up there, i had the same high bills.  However i am doing more than avg for a LL and they got a good deal on the apt.

This is my first time renting only 3 months in i want to ensure that i am doing the best i can but also want to ensure that i am not over stressed for no reason.

Please let me know your thought, i can elaborate and give more details if needed.

Sorry for the unorganized post as this was stream of consciousness writing.