@tryinghere Sorry you have had a bit of a hard time on here OP, but I agree with the posters saying 15 minutes is a long time for a shower to be on.
I think because you still live at home you are a bit sheltered from how horrifically expensive everything is outside. You pay £400 a MONTH for your 'keep.' (Rent and bills etc...) £400 wouldn't pay for a week's rent on some houses in my village. (Some 3-bed, private-let, house rents are £1500 to £1750 a month.) JUST FOR THE RENT!
You are blessed and privileged to be paying £400 a month for everything. I am not having a go at you, (honestly!) because it's not your fault you haven't experienced it yet.
However, try and have some understanding towards your mum. She is clearly struggling.
Remember - most things in the house (that are electric powered,) and have a heating element, will use a lot of electric. Kettle, electric bar fire, hair straighteners, electric heating, blow dryer for your hair, microwave, toaster, fan heater, snack sandwich toaster, and yep, the SHOWER! Fortunately, most people will only use all of these items for short bursts of time. So shouldn't rack up the electric bill too much!
The shower on for 15 minutes will be using lots of electric. I take about 4-5 minutes for my morning shower, and often have a shower mid to late afternoon, as I go for a 3 mile walk every day. But that second shower is about 2 minutes. It's just to freshen up/get the film of sweat off me (Yep, so attractive I know! 😁) When I wash and condition my hair, I do take 10 minutes in the shower, but I only wash my hair every 3 days.