I would think Portugal is perfectly suited for solar heating. The challenge would be getting a solar system for heating water then connecting it in place of the heater. Another concern is if you are using it at night, you would see some amount of heat loss as the cover is off without any solar heating. You could try placing the solar hot water return pipes in front of a standard electric heater inlet though there would be some energy usage that could be rather expensive. The motor that runs the jets also uses a lot of energy. You'd need a pump to circulate the solar system (air or water) too. My inlaws have an air based system that circulates the hot air around a 6'x6x4' box of rocks (no idea what size rocks). They soak up the heat from the air then that heat is used to heat water. Supposedly, it is sufficient for heating the water in the Summer (without supplemental natural gas) which should have a similar climate (or they are cooler) than Portugal.
There is a product called a K-VAR that reduces the amount of energy used by motors: http://theofficialkvarstore.com/ which we installed. It only works with inductive loads and not resistive so it doesn't help with electric heaters. It was very reasonably priced and will work with any inductive load in the house.
