Psykers draw the energy for their Power from the currents of the Immaterium, and as such walk a narrow path that can easily fall into damnation, or worse.
To represent their ability to harness the power of the Warp, Psykers posess the talent known as Psy Rating. This is a determiner of the individuals skill at manipulating the warp to perform Psychic Powers.

To manifest a Psychic Power, a Psyker must first perform a so-called Focus Power Test. This represents the caster concentrating on gathering power from the chaotic tides of the Warp to manifest an event in realspace, be it a Fireball or something as subtle as a telepathic call.

Psychic Powers are not without risk, and can cause Phenoma within the real world, unwanted effects as foul energy leacks from the warp. Worse still, a Psyker may attract the attention of creatures within that hellish domain, causing the dreaded Perils of the Warp.

Because of these risks, Psykers train from a young age to use their abilities with great caution, and to hide their presence from evil entities. In game, this is represented by the ability of Psykers to use their power to mitigate the dangers of the warp, both to themselves and those around them.
Whenever a Psyker attempts to manifest a Power, they choose how much Psy Rating they are investing into the Focus Power Test. This ranges from 1 to their current maximum Psy Rating. Every point of Psy Rating adds a +5 bonus to the Focus power Test.
However, any roll of a double while manifesting a power causes a Psychic Phenomena, coupled with a roll on that table. A Psyker can mitigate the danger by using less Psy Rating, as each point of unused Psy Rating imposes a -10 to the roll on the Psychic Phenomena table.

In desperate times, a Psyker may attempt to draw more Power from the Warp than he is capable of safely harnessing. This practice is called "Pushing" and it as dangerous as it is powerful. While pushing his power, a Psyker may add more Psy Rating to a Focus Power Test, and a subesequent Power, than his maximum. How much extra Psy Rating can be added is determined by what type of Psyker the individual is, up to +4 for Unsanctioned Psykers and those of Daemonic origin, and +2 for Bound or Sanctioned Psykers.
Pushing is immensely dangerous however, and automatically causes a roll on the Psyhic Phenomena table as the pskyer allows latent energy to pour into the real world. Any Psy Rating added also adds a +10 to the roll on the Phenomena Table. Sanctioned Psykers such as Astropaths have several failsafes and mental blockers to draw away excess power such as when Pushing, as such they only add +5 to the Psychic Phenomena table for every extra point of Psy Rating.

After a Psyker uses a Power, then they must spend a number of Half-Actions equal to the Psy Rating they used for that Power calming their minds before attempting to wield their mental power once more. For every Half-Action used in this way, they recover once point of spent Psy Rating.

If a Psyker attempts to use a Power before calming their mind, then they suffer a -5 penalty to their Focus Power Test for every point of Psy Rating that they have not regenerated. They do still gain the bonus from the Psy Rating they invest in a Focus Power Test however.
(Note that how much a Psyker's current used Psy Rating amounts to is determined by the last power they manifested.

Example: A Psy Rating 6 Eldar Warlock attempts to use a Telekinesis Power at Psy Rating 6 after having cast a Psy Rating 4 Power the turn before. He did not spend any Half-Actions calming his mind and as such suffers a -20 Penalty to his Focus Power test as a result, which coupled with his bonus of +30 from the Psy Rating he is investing brings his Focus Power Test to a +10.

The Invocation Skill can be used by a Psyker to aid in clearing their mind. Successfully using the skill immediately restores a Psyker's full Psy Rating, taking a Full Action to concentrate. A failure means the Psyker has not restored his Psy Rating, however he counts the Full Action used as two Half Actions for the purposes of restoring used Psy Rating.

So you have a problem in your campaign where a psyker-player is unbalancing the game by using powers too often? Instead of changing the system, and I'm not saying it doesn't need tweaking, is there perhaps something less drastic to handle your problem? Can you tell us more about the situation? I ask because I've not had this problem and I'm not picturing the scenario where tweaking becomes necessary. I played for awhile in a DW campaign and there was a Librarian in it that had no issue constantly calling on powers, but that didn't seem to unbalance the game.

Actually no, I have not had any problem with a psyker unbalancing the game. Instead the issue came from the psyker player himself, he did not like the use of Fettered and Unfettered as being the only uses of Psychic Powers (Pushing is almost anathema in his eyes, seeing as he plays an Astropath that would rather die than risk possession.)

I had a long discussion with him and other players from the group and we thought about the mental processes that would probably be happening within a Psyker's mind when he is manifesting. The conclusion we came to is that a Psyker most likely becomes a direct conduit to warp energies, the psyker gene allowing his mind to convey these energies into the real world through Psychic Powers.

Now we thought, if it is a conduit then he needs to prepare his mind, and the more power used the greater the subsequent effect on his mental state. And this is how we then got to the whole mechanic of Psy Rating being a ressource.

As for the Psychic Phenomena changes, this actually came straight from the psyker player. He didn't think a Psyker should ever be able to use a power willy-nilly, as with Fettered, but that the potential for dangerous fallout should be mitigated based on the Psyker's skill. And from that came the mechanic of having Psy Rating also mitigate the potential effect of Phenomena as the Psyker uses his remaining skill to stem the overflow of warp energy.

As for Invocation, I just found it as a neat way to make that skill less... well.... useless.


The tl/dr of it is this:
These housrules are supposed to better represent Psyker's as strange beings that can harness barely understood mystical powers.

As for Daemonic and/or Xenos Psykers, most alien minds would function similarly enough to not warrant changes, and I handle Daemons using Psychic Powers as them simply using a tool (i.e. they would never need to recuperate after using a power and never generate Phenomena)

It sort of is based on their skill. Are you strong enough to Fetter-pull the stunt you need? The player perspective might be "willy nilly, look at me, no consequences!", but the mechanic is that you are measuring exactly your mental might, making the least changes to the environment, and trying your DAMNEDEST not to attract the attentions of those things that also live where your power comes from. Being PR 4 is great, but can you get the task at hand done with PR 2? No, you don't believe so, then you better test your mutation, and your faith in Him on Terra.

The player perspective might be "willy nilly, look at me, no consequences!", but the mechanic is that you are measuring exactly your mental might, making the least changes to the environment, and trying your DAMNEDEST not to attract the attentions of those things that also live where your power comes from.

^

That is in fact exactly what I tried to emphasize with this Housrule, by giving Psyker's a wider option than simply Full, or Half PR (or all-out Push but that's for suicidal Psyker's that tempt fate). I wanted it to feel like a powerful Psyker being able to leverage their skill more effectively, and that the more skill a Psyker has the more their control over the inherent risks of Warp energies.

In the current system, it makes no difference if you are a veteran Astropath or a feral Unbound Psyker, in terms of the dangers you face when rolling Psychic Powers. If you Fetter, you will never suffer any discomfort, and if you Unfetter its 1 in 10 that something happens and you can't stop it. That feels limiting to me, and also a bit cheap.