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As much as I hate to use this phrase, there is an app for that. Philips has a free app called BatterySense that shows you how much of your charge remains and how long you can count on the battery lasting both for tasks you are engaged in, as well as letting you check to see how much you can expect for tasks that you are just thinking about starting up. So you can see how long you can watch Twin Peaks before the battery dies and you have to get back to work. BatterySense also suggest things you can do to extend battery life, both in real time and as a general set of suggestions.
Apple provides a list of suggestions to extend battery life that you might also find helpful at: