Electric storage heaters are "dumb" devices that generally just store up energy in heavy bricks during the night (for which you will need to have a day/night or smart meter with day and night tarrif) & releases it slowly during the day. In some cases you might also have an additional wall thermostat that turns a "boost" option on & off and this is where you might be having issues - check to see if there is a thermostat unit anywhere on the wall.
Some units don't even have a boost option, and just blindly storage up heat & night & slowly release it on default.
The trouble with these in my experience is that they tend to release nearly all the heat during the morning and afternoon while I am out of the home, and then I need to turn on manual boost which of course is charged at peak rates. So you end up paying a lot to heat the home at a time you are not there. Then again, my experience is mainly from using them in shoddy pre63 flats that were probably underpowered for the size of the unit and probably had close to zero insulation in the home. There are 3x storage heaters in my house, all of which were broken when I bought the place (I assume the tenants had short circuited them by trying to dry clothes on them).
Another issue is that if you size them properly many of these are huge units using 2.5kw to even 3kw which will go through quite a lot of electricity, but it will heat the room properly.
You can get storage-like electric heaters now that include panels that store up heat and then switch off, thus reducing the amount of time the heater is on by 30-40% - its still an expensive way to heat a room. The main issues I've found is poor placement & underpowered heaters.
Newer electric heaters have to comply with new "Ecodesign" standards since 2018 to detect open windows, distance from users, etc.
https://www.dimplex.co.uk/news/ecodesign-everything-you-need-know
Then there are companies like Technotherm in Germany that produce more sophisticated heaters that store some energy, and can take power from PV panels. Solar power obviously might help in some cases if for example you have a battery and were storing some power during the day or using it to warm up a cold home.
With energy efficiency like you get from such devices it might be worth investigating mini splits or even air to air heat pumps, but these are relatively expensive - eg I got a quote of 2.4k plus electrical work for a highly efficient bloc and indoor unit for a 26m2 living room - these things are very close relatives to air 2 air heat pumps so similar performance but if efficiency 300-400% for heating it does mean you'd be heating such a space using a considerably lower amount of electricity per kw output - with storage or standard panels you get 1kw output for 1kw input. But at that cost probably better off pricing a proper air 2 air heat pump which I see some models from Daikin can feed up to 5 indoor air units.