As winters are very mild in the Algarve you rarely need heating during daytime.
But in the night, temperatures can drop with the average minimum temperature being 14C degrees. Quite regularly though, we have temperatures of only 7C or 8C degrees in the middle of the night.
That freshens up the area quite a bit and therefore we advise to always have some sort of heating. It just more comfortable in the evenings and on the occasional grey day.