How can I heat up my apartment more efficiently?

Question

I live in a studio apartment that I rent from my parents, in France. Each winter, it gets very cold. The region is very sunny, but the specific area around my building is significantly colder (due to being in a small valley, a water stream nearby, tall trees, etc). The building is very old (1800s) and made out of stone. My studio has two floors: the second floor (main area) and a bedroom upstairs, under the roof.

I did not heat my apartment from spring to November, but this past few days, it has been very cold and I need to act. The temperature in the studio dropped from 20ish degrees celsius to 11.5 in a matter of days. The problem is that I track my power consumption using an app provided by the electricity company, and as soon as I turned on the heaters, my daily cost went from 1.90€ to 7.50€. I unfortunately cannot afford to pay this daily for the whole winter.

Here is how the apartment is heated.

Second floor (main floor, kitchen, office, bathroom):

• There is a new generation "ceramic inertia heater" underneath the kitchen window (facing south).
• There are two old school electric radiators under the windows facing North
• There is a heating towel rack in the bathroom, that I never really use
• The kitchen window is well located but really tall trees prevent a lot of sunlight from entering the room. Each day, I have to turn on the lights, except between 11 AM to 2 PM approximately.

Third floor (bedroom, under the roof)

• There is a ceiling window (Velux) on the roof, facing South

• There is an old oil radiator that I can move around and that I sometimes turn on at night.

Additional details:

Last week, I did not heat my apartment and it went down to 11.5 degrees Celsius when I came back from work at 6 PM. The apartment below mine is not inhabitated, neither is the one across the hall from my entrance door. I cannot install a wood furnace, a petrol heater or the like. All windows, Velux included, are double-paned glass.

My current heating strategy: This is what I am currently doing. I turn on the kitchen window radiator to 13.5 degrees. I put the ones on the two other windows oon anti-freeze mode with the dial on 2 or 3. I do not use the bathroom's radiator because the heat from my shower is enough for comfort. At night, I sometimes turn on the oil heater in my bedroom, to the minimum setting. I wear a sweater, slippers, and put a drape over myself when I sit at the computer for a long time.

I'm being as frugal as I can be, but I am very cold and expected to pay 200+ euros at the end of the month for electricity only. I am willing to keep living like this, but if you have any advice or solution, it is more than welcome.

Here is a plan of my apartment (proportions are wrong):

Answer 1

Add insulation: start adding heavy curtains to the windows on the north side of the building, then insulate the ceiling of the top floor (under the roof), if you have enough height, consider a suspended ceiling otherweise just add insulation panels under the shingles from outside.

Consider double-glazing, at least the windows facing north.

Use heat-pump to heat instead of resistive heaters, it's 3 five more efficient than electric (your 8€/day will go to around 3€/day just for that). A 3-split system may be enough (also if you already have the AC, check if it can also do heat, as said is much more efficient than resistive).

Consider moving the PC in the bedroom, so you'll avoid to heat a room with no much use (or, better, move the bed at the second floor so you avoid heating the loft that is much more wasteful having 5 sides exposed instead of 4)