Ardenas Barehand:
Okay, see, the first I recall even hearing about Energy Reserve was within the past year or so, right here. It's not in Basic or Magic, as far as I recall, and those are the only books I have (well, and Low Tech, and 3-4 DF PDFs).
Energy Reserve came out in
GURPS 4e Thaumatology. It was riffing off the idea presented in
GURPS 4e Magic pg 15 "FP for spellcasting only" by making the "Spellcasting Only FP" its own pool separate from FP.
ER is in
DF 3 The Next Level on pg 39 where it gets a write up (same write up from Thaumotology), and it's mentioned in several
DF books after that.
DF 5 Allies adds the "Drains Familiar" wrinkle to a Wizard's ER (they can at this point have 2 types of ER, Personal and Granted by Familiar). The Familiar's ER is their FP, so it only recovers when the Familiar rests... which brings it full circle back to the Advantage's roots as FP.
So, to sum up, the many different Energy Pools a Wizard or Psi can have are:
FP, ER (Magic/Psi), ER (Familiar). Then they can have up to 2 Power Items; 1 Power Item (Magic/Psi) and 1 Power Item (Endurance).
There is an optional advantage Extra Power Items (Type) [15 points] which adds one extra Power Item of a specific type. However each Power Item is still treated as a separate Power item, even if they are of the same source.
Other Heroes have a similar energy pool maximums, just one less as they cannot have Familiars (depending on Gwythaint, frex Druids might have Familiars, Clerics might be able to get Divine Servitors that grant ER).
Power Items (Heroic) are the same as Power Items (Magic/Divine/Psi) except they require a 'Heroic' power source such as Heroic Archer, Weapon Master, or Trained By a Master. The energy cannot used for all FP expenditures (so not for missed sleep, FP lose to forced marches, or non-combat Extra Effort), only FP for special Skills and Abilities (frex Flying Leap, Power Blow, Botte Segrete) and Extra Effort
in combat. The item must be a weapon, shield, or armor.
Endurance Items require a $1,000 ritual to attune the item to be an Endurance Item, the only requirement is that it be a worn or carried item. The energy from an Endurance Item can only be used for exertion, Extra Effort, and mundane loss (frex missed sleep, starvation, forced march).
Multi "Source" Characters can have a Power item for each source. So frex Ardenas who has Magery and Trained By a Master, could a "Wizard's" Power Item (staff, wand, fancy pointy hat, etc) and a Martial Artist's Power Item (hand wraps, 'kicking' boots, belt, monk's prayer beads, etc). They cannot be the same item, so he couldn't have a swank staff that he uses for both. They are charged and used separately and can only be used for their respective Professions.
Now this does mean some characters are kinda screwed. For instance Stenet does not have Weapon Master, so he cannot have a Power Item (Heroic), which he needs, but could have a Power Item (Divine), which is useless to him (he has no Divine abilities that cost energy). He'll definitely be getting a Power Item (Endurance) as soon as his swanky new armor is out of the shop.
Yeah... sorry this started out as a quick "ER is based on FP" post and morphed into "Power Items in case someone doesn't know about them" post.