The biggest culprit in any old apartment is a draft, often via old windows and non-insulated exterior walls. Here are two inexpensive solutions that you can install yourself and can potentially save you a bundle. I use these in my apartment and they work.

Window kit

https://www.homedepot.com/p/Frost-King-Indoor-Window-Insulation-Kit-3-per-Pack-V73-3H/100135637

Outlet kit

https://www.homedepot.com/p/Frost-King-1-Gang-Socket-Switch-and-Deco-Wall-Plate-14-Pack-OS14H/100180324