Power fueled by rare energy

[somecallmetim]

I am trying figure out how to price an ability fueled by an energy reserve. Powers lists only being able to power an ability from ER as a -5% limitation. In the setting the magical energies they would be using to power these abilities are very rare. The only way to refill the reserve involves spending the night in one of a very small number of places of power and making a successful roll against the Religious Ritual skill. The locations should be rare enough to be worth fighting over. I think that should qualify for the Special Recharge limitation, which is -70%. But that applies only to the cost of the ER itself. -5% seems like a very small amount for something that will substantially limit the characters use of certain abilities. Any suggestions on how I should work out a fair price?

[Nereidalbel]

-5% for the ability, and buy a fairly large ER with -70% for Special Recharge. If needed, make the PM for these abilities and the ER worth more than (-10%). Don't forget that you should also add Costs FP to determine how much ER is drained per use.

[Anaraxes]

The -5% is for not being able to use your normal FP in addition to the ER to power the ability.

If you had a 10-point ER and a 10-point FP pool, powering from one or the other would really be a feature; powering from either gives you 20 points to work with, and is relatively advantageous. For that matter, if the point level is such that you have a large ER, say 100 points, then not being able to use those last 10 is barely a nuisance most of the time. -5% might well be generous in many circumstances.

And of course, if a power doesn't Cost FP, it doesn't need the ER at all. I'd allow the -5% only in conjunction with Costs FP. Likely these two are added into the Power Modifier for the setting so that all abilities must have those disadvantages to make sure the magic has the desired feel.