Water heaters and thermos
Hot water comes either from an electric storage thermos or an instant gas heater (or the boiler itself). Sizing and choosing between them changes both comfort and the bill.
Sizing the thermos
A couple in a flat manages with 50–80 liters; a family of four wants 100 or more, especially without a night-rate plan to heat cheaply. Undersized means cold showers at the worst hour.
Instant gas
Endless hot water and no storage losses, but it needs a gas installation with its inspections and combustion venting. In old city flats, the change is sometimes simply not legal to do.
Limescale is the killer
Barcelona's water is hard: thermos elements and heater exchangers scale up and lose efficiency. Anodes and descaling every couple of years extend the appliance's life for the cost of one visit.