Power would be the amount of energy an engine or battery has to enable something to work, I would presume more voltage/wattage/hp etc.. (It's been awhile since I've used those terms..so bare with me) A simple example: Car A has a 150Hp engine versus Car B with 200hp engine. The 200 has more power because of there is more energy which means there is more power that is being provided to the engine. So Energy would be actual forces that are needed to power something. The sun provides thermal energy, an alternator provides enough energy through current, to power the lights/start a car. (sorry if my technical things are bad but the principle is the same). Capacity would be the size of a battery/containters/etc.. (really anything, it is just how big something can hold/pull/fill another thing, towing capacity of 7000lbs, battery, etc..) needed for something to operate. An iphone 6s plus has about 2400mhz battery, versus a leading android with about 3600mhz battery. smaller versus larger so theoretically the smaller one wouldn't last as long. Hope that helps.

Power - мощность Capacity - мощность, емкость, производительность Energy - энергия Чтобы хорошо посоветовать лучше посмотреть на контекст. P.S. я не электрик, но инженер, потому можем разобраться.