(This is all rambly and hard to format. There was a lot here I needed to address, and I don't feel as though I've done the best job laying it out. I think I did manage to explain myself somewhere in here, though.)
ENERGY IS PRICED ALL WRONG
Let's look at two different powers.
Burning Attack 5d (Costs 1 Energy, -5%) [24]
Burning Attack 500d (Costs 1 Energy, -5%) [2375]
In both cases, a single point of energy is required to use the power. That single point of energy cost three character points to purchase. But in the first example, the character saves one total character point from the limitation, and then each use costs a three-point-cost energy. In the second example, the character saves 125 points. They can spend half of those points on twenty energy, and get at least twenty uses of their power per day from those--and they've still saved sixty points on the price of the ability. 20/day or 20/combat are not worth very many points at all. It should never cost more points to get and use your limitation than it would cost to just buy the ability without the limitation, as in the first example.
Something is clearly very wrong here.
That's because none of these is tied to the actual cost of the trait in the first place. It's always a three-point Energy per use, and you save a variable amount of points from the limitation.
To make this super obvious:
Burning Attack 5d (Costs 16 Energy, -80%) [5]
Burning Attack 500d (Costs 16 Energy, -80%) [500]
One of these is a good deal. The other is a giant ripoff. In the first example, you save twenty points, but require forty-eight points' worth of energy for a single use of the ability. In the second example, you save two thousand points... and still require that same forty-eight points' worth of energy per use.
Having the use cost be the same (1 energy) but the amount of points returned per each energy used (a variable 5% based on the trait's price) just doesn't work. It leads to nonsense like this. It's especially bad for cheaper abilities, where you'd be better off just not taking the limitation in the first place.
My solution is to base energy pricing on the actual trait cost of the ability, where a character point's worth of energy always get your the same amount of use of your abilities.
That is: to power a 10% return on an expensive power should cost proportionally more than powering a 10% return on a cheap power. That is the only way to prevent these weird point crocks, where you're basically given free points on expensive abilities, and CHARGED points for the price of admission to the limitation (which severely weakens your ability!) on cheap abilities.
This must be very unlike what we see in GURPS, where the 10% return always costs you 2 energy, regardless of how large that return is. It could be one point or it could be one thousand points. Either way, you have to pay the same amount of energy.
ENERGY IS SHARED LIMITED USE
The key insight I've had is that Energy is a version of Limited Use where you can spread your uses between abilities.
Imagine you have an ability that costs three Energy to use, and that you have three Energy (that only recovers once per day; we'll get to other types of Energy later). This is the same as Limited Use 3/day.
If, instead, you have two different abilities that each require one Energy, while still having only three Energy, now you can see my point: this is Limited Use, but where the uses are shared between abilities.
FIRST LET'S LOOK AT LIMITED USE
Now that we know this is a form of Limited Use, we need to look at Limited Use. Is its pricing good? Can it be better? I think it can.
I will point out here that I think GURPS generally underpays for limitations (because it's trying to be universal in their application--but I have my own solution to that problem, which we'll see). So I'm giving perhaps higher limitation values than we're used to seeing while playing GURPS. Also, this is all only for demonstration purposes, so I may have messed up on the rounding in parts for the sake of providing cleaner numbers.
My new pricing for Limited Use is: U/(1.5+U), where U is uses per day. The result is a multiplier which gives you the new final price (the price after the limitation is applied). So .9 means you still pay 90% of the price, so it's equivalent to -10%. That gives me:
Limited Use Pricing Table
1/day 1/(1.5+1) (.4) -60%
2/day 2/(1.5+2) (.57) -43%
3/day 3/(1.5+3) (.66) -33%
4/day 4/(1.5+4) (.72) -28%
5/day 5/(1.5+5) (.769) -23%
6/day 6/(1.5+6) (.8) -20%
7/day 7/(1.5+7) (.82) -18%
8/day 8/(1.5+8) (.84) -16%
9/day 9/(1.5+9) (.85) -15%
And, of course, some abilities aren't very impacted by a limited number of uses per day, so for those we'll give less points back for such a restriction. I call this Use Frequency. For abilities that are used more often, we give a larger limitation value, and for those used less often, we give lower values. Here the GM must be careful not to give points back for limiting abilities that don't need to be used very often.
Use Frequency
High Use (Combat abilities, common adventuring abilities, and so on). x1
Medium Use (Abilities that might be used in combat but not often; adventuring stuff that occasionally comes up multiple times in a day) x.5
Low Use (Things where it's rare to use the ability more than once in a day; the GM should be very careful with allowing this at all) x.1
Limited Use final value after Use Frequency adjustment
High (x1) Medium (x.5) Low (x.1)
1/day -60% -30% -6%
2/day -43% -21% -4%
3/day -33% -16% -3%
4/day -28% -14% -3%
5/day -23% -11% -2%
6/day -20% -10% -2%
7/day -18% -9% -2%
8/day -16% -8% -2%
9/day -15% -7% -1%
But, wait, you might say, in GURPS an Energy Reserve recharges one point every ten minutes. How do we do something like that? Well, that's... weird. For simplicity's sake, I'll think of that as being equivalent to a limited number of uses per combat. Because the amount of time between uses is entirely arbitrary with the "one every ten minutes" of the RAW advantage. The GM can just decide how long it's been. But they can do the same thing when deciding the length of in-game time between combats. So I'm going to look at it that way.
Uses Per Combat
I start off by assuming that one use per combat has the same utility as two-and-a-half uses per day. That's just the number that makes sense to me. If it were two uses per day, I would much prefer once per combat. And if it were three per day, I would prefer that to once per combat. Having the ability to unload all in a single combat is very valuable. After all, with three uses per day, you can still just use it once per combat. The tradeoff between these two is being able to have multiple uses per combat if you need them; you give that up with a once per combat limitation.
Using the formula for Limited Use, we see that 2.5 uses per day are worth 2.5/(1.5+2.5), which is .625, which is -37.5%
So, let's try saying that for Uses Per Combat we'll use the same formula as before, but we'll first multiply U by 2.5, which will give us: (U*2.5)/(1.5+(U*2.5))
That gives us:
1/combat 2.5/(1.5+2.5) (.625) -37.5%
2/combat 5/(1.5+5) (.769) -33%
3/combat 7.5/(1.5+7.5) (.83) -17%
4/combat 10/(1.5+10) (.869) -13%
5/combat 12.5/(1.5+12.5) (.89) -11%
6/combat 15/(1.5+15) (.9) -10%
7/combat 17.5/(1.5+17.5) (.92) -8%
8/combat 20/(1.5+20) (.93) -7%
9/combat 22.5/(1.5+22.5) (.937) -6%
(Which I will admit seems a little generous; maybe we should make it where 3/day is equal to 1/combat--though even that would still give -5% for 9/combat. But we'll just go with this for now, since we're talking about energy and not focusing on Limited Use.)
HOW THIS ALL RELATES TO ENERGY
So we now have a way to know the limitation value for a given value of Limited Use. We can get decimal numbers, half-uses, and so on. If we want to know what half a use per day would cost, we can do that. We can figure out what one-and-a-half uses per day cost, and so on. How does this all relate to energy? We need to know all this later on, so we can choose arbitrary energy costs for our abilities, since there won't be linear costs.
CONSIDERATIONS
When you can share your uses between abilities, that gives you added utility. It's more powerful to be able to share uses than to have each ability limited to a certain number of non-transferable uses per day. The way I account for that is by slightly adjusting the limitation value when we use it for abilities powered by energy. So rather than U/(1.5+U), I'm going with U/(1+U). Which reduces the limitation value. The actual numbers can be adjusted. What's important is that this all works as a proof of concept.
FINALLY THE ACTUAL ENERGY RULES
An energy costs a single character point. 1 energy = 1 character point
(U/1+U)/2 = limitation value
(((U/(1+U))/2)S)/U = Energy cost per use
S is starting points (the price of your trait before the limitation is applied)
U is uses per day
Let's go through what all this means and why it is how it is. What is the method to all this madness?
(U/(1+U))
This is our base limitation value. It's the modified version of the limitation we explored earlier for Limited Use. For the Uses Per Combat variant, just adjust U, as described above in that section.
Then we have /2. Due to the way this is spitting out limitation values (by giving us a multiplier applied to the base price), this division actually doubles the limitation value. We go from paying .8 the original price to .4 the original price, because .8/2 is .4
The reason we're doubling the limitation value here is to account for the fact that we have to actually buy the energy points themselves to power the ability. We get points back to represent the Limited Use portion. For instance, 1/day is (1/(1+1)), which is .5, or -50%. But then we still have to buy the energy to pay for the one use it costs each day. That has to be accounted for somewhere. So what we're doing is halving that .5 to .25, and from that additional twenty-five percent that we save we can purchase enough energy to use the ability once each day and still on top of that receive the -50% that we're owed because we now have to pay a lot of energy to use our ability (which is equivalent to one use per day).
The key thing here is that energy cost is determined by the price of the ability. The more expensive the ability, the more energy it costs to power it for the equivalent to a certain number of uses. 1/day on a 50-points advantage costs a lot less energy to use than 1/day on a 1000-point advantage (twenty times less).
That's why we next multiply by S, which is the starting point total of the advantage. For a 1/day advantage, we would now be at .25*100. That gives us 25 points. That gives us the total amount of energy the ability is going to cost for the total specified number of uses. We have to spread that across that many uses, though.
So then we just divide by U, to account for the number of uses this much energy will be spread between. For 1/day, this is simple.
Let's look at a more complex ability.
Let's say we want four uses per day on an ability that costs 250 points.
(4/1+4)/2 for our limitation value of .4
(((4/(1+4))/2)250)/4 = 25 energy per use
So we buy this ability for (250*.4) 100 points. Then we want enough energy to use it four times per day, so we pay the 25 points it costs for each use four times, which costs another 100 points. So we can see that we've saved a total of 50 points total. Which is equivalent to the .8 our formula gave us before we divided it in half to account for points we would have to spend on energy to actually have a functional 4/day for an effective -20% limitation value. And now that we have this energy we can use it to power any of our other abilities that require energy.
All of this effort so there isn't any point crock or pricing weirdness in how energy is used. But I think it's well worth the effort!
The key thing is that an energy is always worth an energy. It doesn't do more sometimes and less others. It always creates a pricing equivalent to Limited Use, because it follows an adjusted version of the Limited Use pricing formula.
Basically, to reiterate how this works, we have so many uses spread over so much limitatio value. If 3/day is -33%, then we can see that each use is worth -11% (whatever that number of actual points is when applied to a given trait; for a 100-point advantage it's 11 points; for a 50-point advantage it's 5.5 points, and so on). But since for each use you have to buy energy, we double the limitation value. So that when all is said and done, you get your points saved from your limitation, with an additional amount saved to account for the need to purchase energy.
IN CLOSING
So we can see what we're really doing is multiplying the Limitation Value (L) by the starting price of the trait, and then dividing by number of uses per day.
L is limitation value
S is starting price.
U is uses per day (or combat; see above)
E is energy cost per use. And 1 energy costs 1 character point.
L = U/(1+U)/2
Giving us a final formula of: LS/U = E
