There are several ways to get FPs for use. This article explores several of them and how thy can affect magic use.
- Extra FP 3/level.
- Energy Reserve (Powers) 3/level
- Threshold Magic (Thaumatology)
- Raw Magic
- Powerstones and Manastones
- Power Objects
Can be used for anything normal FP can, is vulnerable to things that drain normal fatigue, and recovers at the rate of 1 per 10 minutes rest. Fit (p. B55) doubles recovery rate but this does not help FP used for magic or powers. Breath Control is a cinematic skill that allows recovery of FP at the rate of 1 per two minutes, not cumulative with spells like Recover Energy. Recover Energy is a spell that recovers FP at a faster rate depending on skill level and mana level.
Cant be used for normal FP purposes, is not vulnerable to typical FP draining activity, and recovers independently of rest and at the same time as recovering normal fatigue. A few levels of ER thus go a long way as it effectively doubles a casters available FP and that recovering without resting is a nice bonus. Recover Energy applies to both at the same time.
Modifiers: -20% for recharging 1/hour, -60% for recharging 1/day.
Introduced in GURPS Thaumatology this as an update the S. John Ross's UMana article introduced in Roleplayer many many years ago. It changes spell casting to a strategic exercise more than just a tactical one as you have a decent sized pool but limited daily recovery. However this is a setting switch and there are no point costs for the trait. There are traits to modify it however.
Raw Magic Store
This advantage from Thaumatology (pp. 227-229) lets you gather and store energy. Each level (costing 5 points) stores 1 bit of raw magic equaling 5 points of FP. See the referenced pages for more details.
Powerstones and Manastones
These cost no character points and are equipment that can be bought and sold. Powerstones recover 1 energy per day in normal mana so are more of a strategic resource. If you use them your unlikely to have access to that energy till after the adventure. Manastones are cheaper and do not recover so are disposable items. Another item is Paut which is a potion that recharges FP.
These are similar to powerstones in that they are a limited strategic resource. However they are simpler in their mechanics. You can recharge them in town for a small fee, but they do not recover on their own. Again they are equipment and cost money, not character points.
Default Magic vs. Powers as Magic
So users of the magic system have numerous options to increase their capability with cash as well as points. Those using powers as magic such as Sorcery only have options that cost points.
Most magic as powers abilities should have a 1 FP cost to keep them from overwhelming others in the party. Since normal magic can cost several FP for powerful spells this is an advantage to the powers type, though high skill offsets that.
Playing a Sorcerer one should invest in ER. But getting gear that helps your magic is a staple of dungeon fantasy so how can we do that?
Metatronic Generators, Pyramid 3/46 provides a decent answer.
Lets look at a powerstone type device. Assuming TL 3 and tiny size it would cost $175/point so $525 per level of ER. This is quite a bit more expensive than a powerstone but recovers much faster! A stone that only recovered 1/per day would cost less or about $210 per level of ER, more expensive than a low level powerstone but cheaper for more powerful ones. A power object like device would be about $157.50 per level (using Special Recharge, -70%). Double the $$ cost for TL 4.
Given that Sorcery uses less FP on average this gives a comparably useful item.
The Strategic Option
For some settings the strategic option of Threshold Magery has an appeal, but how to build it?
Suggested value is Threshold 30 and Recovery 8 and a suggested idea is changing the values as long as keeping the same multiple. Also there advantages that increase the Threshold or Recover rate.
So 30*8= 240 Increased Threshold +4% per point and Rapid Recovery 5% per point.
An ER like the above could be 29 ER (Recovers 1/day, -60%)  + ER 1 (Recovers 1/hour, -20%)  for 38 points, though that is not really allowed as they should not stack because the hourly recover would be munchkinny if it helped the bigger pool.
30/8 takes 3.75 days to fully recharge.
Using suggested defaults Threshold would be 6  or 1.2  and Recover 2  or 0.4 .
Those numbers change with a different starting default.
So a 6/2 for 10 points compared to ER 3 for 9 points isn't desirable for players as they will get much more mileage out of straight ER since it would fully recharge in a half hour. What if we scale that up though? 5 levels would be 30/10  (Incidentally the cost of Variable Energy Access) where a normal would be ER 16  half as useful in one shot and takes just over 2.6 hours to fully recharge,
Another look; on 50 points those 5 point advantages as enhancements would be +10% for either +6 (20%) Threshold or +2 (25%) Recovery rate.
So building it as a new advantage could look like...
You have an Energy Reserve that has a high capacity but recovers much slower than the standard Energy Reserve. The ER holds 30 energy and recovers 8 per day.
New Enhancements: Increased Cap +6, +10%; and Increased Recovery 2, +10%.
New Limitations: Reduced Cap -6, -10% (Max 5 levels); and Reduced Recovery -2, -10% (Max 5 levels).
Other optional modifiers: From GURPS Horror Corrupting, -20%; from GURPS Powers (p. 119) Abilities only, -10%; One Power, -50%; and Stunts only, -10%; and from GURPS Power-Ups 8: Limitations Aftermath, Aspected, Attracts Threats, and Backlash.
Note: The new limitation values mach up with the suggested version for Magery under Threshold Magic!
Strategic ER is not just for Magery, its a variant for of ER that can be used with any source or power theme where the desire is to make a more strategic option available.
With the Aftermath limitation it could for example represent physical stress.