Categories usually define the essential style of Energy being used. Magic uses Mana, Quasi-magic uses objects found in nature, and Racial uses one's own body.

Power Sources define specific Energy sources (except Mana, which is universal to all magic). They provide the most basic powers and traits connected to those sources. They are meant to be as self-balancing, so whether you take 10% or 100% of your levels in a given power source shouldn't throw off balance.

Disciplines are not always used. They are subdivisions of a Power Source. Sometimes, they add to a baseline, other times, they just change flavor. The total amount of power gained from a Power Source, including any disciplines, should be the same in any case.

Prime Skills are actual skills you take ranks in. They allow access to feats. They can sometimes be used as a basis for custom powers and/or crafting.

General model:

• Max: 4 + 1 per 2 levels
• Recharge: short rest (between encounters)
• Threshold: 100% of normal maximum
  • Using more than this amount of Energy (from any source) in a single power or maneuver causes 1 strain (magical) or fatigue (physical) per point over the threshold.

Power Sources should provide:

• Special energy on-demand with costs or long-term consequences; or, ways to reduce energy cost
• Explanation for what energy is, and what circumstances it recharges in

Should it be possible to "invest" in Energy and Recharge?

• Probably not. Energy is equally useful to everyone, so it makes no sense to bind Energy to anything other than their most useful stats. And either way, everyone will max it out, so it's ultimately the same as if you can't invest in it at all.
• Your power sources are an investment in Energy.

What is Energy? Do non-casters have it?

• Sure, and so do birds, bees, and even rocks.
• Energy is Qi, Mana, Prana, or what-have-you. It's the animating force of life, the momentum left over from the Big Bang, or the will of god(s). It creates something from nothing, makes order from chaos, triumphs over base nature.
• Everyone has it. High-level people (and monsters) have more. Said people (monsters) regenerate faster, since regeneration is based on total Energy.

Quick spitballing:

• True Flame
  • When you need more Energy, just Ignite: encounter, immediate; your body is wreathed in silver flames. You immediately gain 50% of your max Energy poolas True Flame Energy. At the beginning of each round, you gain the same amount, increasing by that amount each time (round 1: 50% max, round 2: 100%, round 3: 150%, etc). If you finish any round with more True Flame Energy than your energy threshold, you suffer 1d6 unavoidable holy fire damage per point of overage. This damage cannot be healed, it must heal naturally. If this damage reduces you to -100% hp, your body is disintegrated permanently.
• The Affinity
  • Need more energy? Just do a Vital Transfer from the environment. Of course, this incurs Vitality Debt. TBD.