Usually On

B63 lists for Insubstantiality
B63 lists for Invisibility
B108 specifies it is mutually exclusive with Reduced Fatigue Cost enhancement.
B263 uses it twice in the Spirit Templates
==Powers==
P99-100 discusses as an alternative to Always On for things which are off by default. In this context it is a limitation
P109 discusses as an alternative to Switchable for things which are on by default. In this context it is a +5% enhancement which is cheaper than Switchable. It is still worth points compared to while conscious which is a limitation to always-on abilities. It has the benefit of not costing FP to turn off, but 
P112 lists it under limitations too, just as P109 implies it is a limitation for things which are usually off by default. However the directions here are confusing, it says to see P109 but P109 actually says to see the AlwayS On note on P100 continued from P99. This pricing is also a bit confusing:
*if Always On would be -20% or less, it is HALF
*if Always On would be -21% or more, it is 10% less than that (ie -11% at AO -21%, -20% at AO-30% and so on)
**B110 defines it ranging from -10 (so -5%) to -40 (-30%)
==alternative approaches==
Using Costs Fatigue for Limited Enhancements on Switchable +10% produces interesting results.
 
Usually On is a limitation for things which are inconvenient to be left on (P100 like a damaging aura) or an enhancenet for things which are convenient to leave on (P109 like Claws/DR) but either way the effect is the same: 1 FP/second to turn it off.
*Costs Fatigue, by contrast, can be thought of as "Usually Off" where 1 FP/second can be paid to turn it on for -10%, with special -5% "per attack" options for transient abilities or "per minute" (half 2+) for non-transient ongoing ones.
Reducing +10 to +5 is like -50% as a limited enhancement, which is either Costs Fatigue 5 (5 x -10%) or Costs Fatigue 10 (10 x -5%) depending on what approach is used.
Effectively you are paying 5 FP per second or 10 FP for 1st minute (5 FP per additional minute) to gain the effect of an enhancement.
This is obviously nowhere near the first price (1 FP per second is 1/5 as much) so it may be derived from the second.
10 FP per minute is 1 FP per 6 seconds, while 5 FP per minute is 1 FP per 12 seconds. In either case this is more expensive. So it seems to fall between both values.
The normal parameters of switchable ("whether it switches on or off – or remains in its current state, whatever that is – when
you’re knocked out.") if selectable for UO probably favor switching ON when someone is knocked out: otherwise the FP cost would probably kill them as it drained 1/second while unconscious.
*Like with Costs Fatigue, spending that fatigue probably requires a voluntary effort.
While conscious is worth considering here too, the -5% value could cancel out the +5% value as a zero point feature. The question is whether or not the modifiers are compatible.
This normally modifies things Always On by default. It does give an out: it goes out while unconscious, so net -45% might mean "a fire aura I must use when conscious but which falls apart when I lose consciousness". This might reduce the value of AO though, since there is less danger (ie: your house won't burn down in your sleep if falling asleep turns the ability off)
If applying WC to UO, this could mean "I use a concentrate to turn on the ability, but then must spend FP to turn it off, unless I lose consciousness". This gives a way to turn it off aside from FP (just knock yourself out) which isn't entirely useful.
If you lose the ability to turn it off ALREADY by being knocked out (can't spend the 1 FP per second) then it perhaps shouldn't be worth -5% since you're not losing anything aside from needing a concentrate to start using it again.
There seems to be some element of Requires Concentrate within While Conscious: not an ongoing Concentrate but rather the attention span
*it probably makes sense for Magic as Sorcery : the requirement to stay conscious to "maintain" a spell (refresh it's duration) at certain intervals. There seems to be no "pay up front" with FP unless you use Maintain Spell since Extended Duration is not an option for Trading FP for Enhancements with standard magic, as it would be for Sorcery since it's a power.
**this could be related to Simultaneous Uses penalties
Probably how magic could work is buying Independent as an enhancement, at which point you could buy Extended Duration but ONLY in respect to the independent duraiton, since you are not buying it on the base duration, which is what the prohibition is intended for. Independence is basically how Maintain Spell is intended to operate (can't control it) and the spell called Independent is even called upon as guidelines for "progamming" a maintained spell
==see also==
*Uncontrollable which could have a power using up your FP against your will
*Ectoplasmic Projection where you can't just turn off the power to stop FP loss since you need to ge tback to your body first, or evict a spirit 
==Power-Ups==
GURPS Power-Ups 8 pg 19 lists generally.
==see also==
*Always On
Category:Limitations
Category:Enhancements