"Energy"

You probably heard about "energy" and "energy fighting" in War Thunder. The latter means a style in air combat with which the pilot strives to keep his energy high at all times, avoiding any large expense of energy. Good "energy fighters" are planes which by virtue of a low drag lose little energy at high speed (or are at least the least weak in energy fighting compared to turnfighting and boom and zoom tactics).

Energy is not something spiritual here; it's about physics.

Potential energy is the energy of altitude:

mass * local gravitational field * altitude difference = potential energy

Higher is better.

Kinetic energy is the energy of speed:

0.5 * mass * velocity * velocity = kinetic energy

Again, usually more is better. 

You see velocity twice (or squared) in the kinetic energy formula, but only once in the potential energy formula. This is important for the comparison between fast and slow fighters.

The kinetic energy difference between 800 kph and 900 kph is much greater than between 200 kph and 300 kph even though the speed difference is the same.

900^2 - 800^2 = 170,000

300^2 - 200^2 = 50,000

Meanwhile, the potential energy difference betwen 8,000 m and 9,000 m is the very same as between 2,000 m and 3,000 m. The faster you are, the more potential energy gain (altitude gain) can you get by sacrificing some (such as 100 kph) of your speed.

At battle rating 2.0-5.0 you can assume to be safe if all reds are either far away or a least 2.5 km below yourself. At battle rating 6.0-9.0 you can assume such only if all reds are either *really* far away or at least 4 km below yourself. 

The increased speed of the red fighters flying below you enables them to climb up to you within seconds.

Now about energy retention and building up energy:

The drag caused by movign a plane through a fluid (the air) approximately grows squared with its speed: The drag is quadrupled if you double your speed.

You need engine power to make up for this drag, and only what little engine power is left after maintaining your speed this way can be used to gather altitude (potential energy). To maintain a high kinetic energy thus requries a lot of constant effort by your engine.

Meanwhile, maintaining a high altitude is about as easy as maintaining a low altitude. Your engine power drops at higher altitudes and the drag drops in thinner air as well, but as a rule of thumb to maintain a certain high potential energy is as easy as to maintain a certain low potential energy.

The path towards energy superiority over your adversary at the beginning of air combat is thus not to accelerate to a high speed (built up kinetic energy), but to climb (build up potential energy).

Be aware: You're losing energy whenever you are flying faster than the top speed your plane can maintain at that altitude. The very high drag at that high speed bleeds your energy.

For reasons of manoeuvrability you should stop the climbing well before engaging a red fighter at about equal or lower altitude, though; build up speed again (even by dropping by 200-300 m) in time, for your plane is less manoueuvrable at its optimum climb speed (measured as IAS) than at a moderately higher speed.

One last note: Kinetic energy depends on your indicated air speed (the IAS). The other method of measuring speed measures the speed over ground and is useful only for navigational purposes ("how far did I fly in the last ten minutes?").