I like many others have lost missions to forgetting to attach a solar panel or got a probe alllll the way to it's destination only to discover that it runs out of charge 1/3 of the way round the dark side. So I set about writing a plugin which will stop me being so daft - Fusebox was born.
It's active in the SPH/VAB and in flight. In the editor when you load a ship or select a root part it will open up. It shows you the amount of battery you have (in build maximum = current always of course