When you attack an invader you are given 2 options. Normal Attack, and Enhanced Attack. Using normal attacks, takes longer but uses a lot less energy.
I will use my account to attack a level 2 invader
As you can see in the following pictures right now a single hit will take 335 units of energy. This based on invader knowledge and hero skills
This time it took 6 hits using normal attacks, as you are attacking you will notice that for a while at least, each attack will receive an additional 15% attack bonus as long as you hit the invader at least once every 60 minutes. When I used the Enhanced attack it used it took 9 attacks to kill another invader. So for me to kill a level 2 invader, it took 50% more energy. When you get strong enough to start attacking the Uber invaders that are found around the Place of Power(POP) these will take around 60 hits with the with somewhere between 950 and 1150 units of energy depending on what skills knowledge and armor you are using. Enhanced attacks against Ubers take a huge amount of energy. On of the players I have pulled information from said it took him 150 hits at 900 energy each to kill an Uber using the enhanced attack while it only took him 67 hits to kill an Uber with the normal hits. So it it took more that twice as much energy to kill one Uber. There are a few times when it it appropriate to use the enhanced attacks but they are rare. Usually most players will let you finish an invader if you start attacking it. In particular during competitions especially if there are only a few Uber invaders other players will start attacking the same invader and if you want to be the one who finishes it off you might need to use the enhanced attack then. Know that some of the other players will be doing the same thing.
Right now my maximum sustained hit is 600% so If I take 40 hits, each hit has an increased attack of 16% this is why the normal hits are so much more effective.
As you work against invaders try to make it so that you have taken the invader down to 30% health by the time you hit your maximum sustained hits