What is your opinion about lights with battery power for outdoor filming? 

They can be extremely useful especially when you need to go to places where electricity is unavailable such as forests 

I think some of it's subjective and depends on the lighting set up and needs for the scene. If you need those huge lights that will light up a city block then you will need a generator. 

It is all good until your battery runs out and you still have a lot of projects to film. Also as Kim said, it could work only if you don't need powerful equipment

What kind of battery could power a huge light? I mean it could work for little lights but when the work get serious I highly doubt they are even an option.....