If as with most apartment blocks the roof is flat are you not able to apply to your communidad to install a solar hot water system?
We have this on the costa de la luz coupled to an immersion heater [very rarely needed] but are going solar but still on the grid next year. A tank on the roof even with insulated piping going down an outside back wall would also give you a good head of water and a more powerful shower. Initial cost may be more but greater client satisfaction and less reliance on ever increasing electric bills should make it worthwhile. If you are thinking of short term rentals [and Valencia will probably go that route] it might be worth checking the requirements listed in the tourism permits already issued for say Barcelona and more recently Andalucia which includes air con for summer rental and heat for winter rentals. Not only that but you will need to provide a complaints book, declare all your taxes, issue invoices and more paperwork etc . . . so all has to be 100% perfect.