Let me state it this way:
If your power situation is so critical that a few watts either way will make or break your power 'budget' for the day, then either stop watching movies or watch them during daylight hours (with solar input) only.
Now having said that, if you have sufficient power budget, it really comes down to cycling the laptop battery, or not cycling it. Laptop batteries do not last forever.
Cycling them DOES mean that each charge-discharge cycle removes one cycle from however many cycles it will provide. If it can fully cycle 1000 times, and you fully cycle it 300 times a year, that's around 3 years of life, and then a new battery will be needed.
If however, you only fully cycle it 50 times a year, you could get many more years of battery usability than the hard drive and screen and keyboard and operating system will probably be usable for.
If at all possible, I run my laptops on external power whenever I have it available.