I would not advise the low wattage heater panels if you want to keep your apartment warm.... thay are 425watts and you will need lots of them to warm an apartment.
I bought some from Media Markt but took them back as they do not give out enough heat, better to buy the more expensive but still economical programmable electric radiators from places like Leroy Merlin... we bought their own brand and they are very good. Even better but still more expensive are the ceramic versions of the programmable radiator.
Heating properties in Spain is not cheap though and you should be prepared for a 2 month electricty bill of €200 plus for the coldest 2 months.
Barry