I got one before toddling off to Portugal a few years ago. Putting it on for an hour before bed didn't use much power from the battery - and anyway once I got to Spain the sun was beating down on the solar for at least 10 hours a day so it was inconsequential.
However, I gave up with it after a couple of nights because it's not like a mains blanket that you use under the bottom sheet at home - it has to go on top of you. I really don't like having anything between me and the duvet, it was uncomfortable, and putting it on top of the duvet meant I didn't really feel any benefit.
I still have it somewhere if you'd like it.