Fusion
Fusion is an active generator program which costs 5 PWR to use, but then generates +3 PWR per turn for the next 4 turns while on cooldown. Fusion can be chosen as a starting program, but cannot be acquired otherwise.
Strategy
It is almost always correct to use Fusion consistently every 4 turns. In this case, it generates a net of +7 PWR every 4 turns, for an average of +1.75 PWR per turn. So it generates significantly more PWR than Power Drip, although Fusion gives PWR later because of its upfront cost. Overall, Fusion is considered to be a relatively good generator because it gives a good amount of PWR and has only minor drawbacks.
Unlike Power Drip, Fusion must be used actively by clicking on it in the Incognita view. For new players this will take some getting used to, and it is easy to miss out on PWR by forgetting to use Fusion on the first turn or as soon as it comes off cooldown. It is also necessary to manage your PWR use carefully while Fusion is on cooldown, so that you will have 5 PWR left to re-activate it; if you make a mistake with this, then in the worst case you might become unable to generate PWR for the rest of the mission. Beware of ambushing with a Volt Disruptor when you need to reactivate Fusion soon.
The main drawback of Fusion is that it costs PWR right at the start of the mission, slowing down hacking of Security Cameras on the crucial early turns. Modulate can reduce Fusion's efficiency by raising its upfront PWR cost, and a badly-timed Siphon can sometimes prevent you from using Fusion at all, especially if you have already used up the PWR from Consoles and you have no other PWR source.
In the last few turns of a mission, you do not need to save 5 PWR to re-activate Fusion, so you can use more PWR if necessary. Be careful, though, because if you mis-judge it and aren't able to leave the mission, you can get stuck with no PWR and no way to generate more.
Fusion is even more efficient in combination with Charge, which makes Fusion cost only 3 PWR to use and extends its cooldown so it generates +3 PWR for an extra turn, for a net average of +2.2 PWR per turn. In contrast, Fusion should never be combined with Overdrive, which makes Fusion cost 6 PWR to use, and reduces its cooldown so it generates +3 PWR per turn for only 2 turns — that is, Fusion with Overdrive generates no net PWR at all.
When playing with 0 starting PWR as a custom difficulty setting, Fusion is a very poor choice of starting generator, because you will typically need to hijack half of the Consoles in the facility just to start using it.