Spell Points

True spell casters (Mages and Clerics, not Rangers, Paladins or Bards) are given a pool of spell points based on their level with bonus points for high statistics (Intelligence or Wisdom). Casting a spell costs its level in points. So a 1st level Magic Missile is 1 point, a 6th level Disintegrate is 6 points. This means you can cast any spell you have memorized over and over as long as you have points left. It takes 12 hours of sleep to regain 100% of your total points if you go all the way to zero (this assumes that using all your spell points is equal to total exhaustion; eight hours of sleep just isn't enough). Half your spell points would take 6 hours. You can regain points at slower rates over time while walking, riding, meditating, resting, etc. Spell point recovery tables were inspired by the psionic point recovery rules.

Spell Point Recovery in percentages of total points
Activity Per Hour 6 hours 12 Hours
Sleep 8% 50% 100%
Rest, meditation 4% 25% 50%
Sitting, talking, reading 2% 12.5% 25%
Walking, riding, mild activity 1% 6% 12%
Hard exertion, combat 0% 0% 0%

Spell Point totals
Using the charts in the Players Handbook, calculate a character's base spell pointS by multiplying the number of spells they have by the spell's level. So a 5th level magic-user has 4 1st level spells, 2 2nd level spells and 1 3rd level spell. (4x1=4) + (2x2=4) + (1x3=3), 4+4+3= 11 spell points for the base number. But, characters with high stats (Intelligence for magic-users, Wisdom for clerics) gain bonus points, 1 point per point of stat over 14 (15=+1, 16=+2, etc).

Spell points for non-primary spell casters- You could allow Rangers, Paladins and Bards to use spell points as well, but I have always considered them to be limited casters, they learn to cast spells by rote, they don't really understand how spell casting works, they're just repeating what they were taught. If you do give them spell points, I would only award half the bonus points for high stats. Also, I would have Bards and Rangers keep their spell point pools separate, unless you want to combine them for simplicity's sake.

More Memorizable Spells- The number of spells you start with a low levels is ridiculous, so I set up an alternate progression where when a character learns a new level of spells, they get four spells for magic-users and five for clerics. I also made the table the same for all magic-users and all clerics. Since characters get more memorizable spells, this system gives them more spell points as well. You could keep the number of spell points the same as above and give them more memorizable spells, giving them more flexibility, but not more power. Those tables and the new spell point totals are here- Spells per Level.

Alternate Spell Point Costs- I have seen other DMs use alternate spell point costs. One had reversed spells cost double (for example, if you have Cure Light Wounds memorized, but you want to cast Cause Light Wounds, it costs double) or another that really clamped down on things had spells cost the SQUARE of their level in points (a 3rd level spell cost 9 spell points, a 9th level spell cost 81!!). 5E has a spell point system which has a scale built into it, but bear in mind in 5E, priest spells have been reworked into nine levels just like wizard spells:

Spell Level Point Cost Class level Spell Points
1 2 1 4
2 3 2 6
3 5 3 14
4 6 4 17
5 7 5 27
6 9 6 32
7 10 7 38
8 11 8 44
9 13 9 57
10 64
11 73
12 73
13 83