The Electric Charge Meter is a small, lightweight box that you can slap onto the outside of your Kerbal creations. It shows the ship's battery level with an easy to see green, yellow, or flashing red light. The yellow and red warning levels are tweakable in the hangar and in flight.
The part takes into account ALL of the power sources on the ship that are actively providing energy. If you add or remove power sources then the meter will automatically adjust itself. Enjoy!