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Is the warlock a third-caster like the Eldritch Knight (meaning that each of its levels count for 1/3 of a caster level in the multi classing rules), a half-caster like the Ranger (meaning that each of its levels count for 1/2 of a caster level in the multi classing rules) or a Full Caster like a Wizard (meaning that each of its levels count for a caster level in the multi classing rules).
To answer that we are going to use the alternative rule for spell points explained at the page 289 of the DMG which says that each spell slot is equivalent to a certain number of spell point and that those spell point can be used to cast a spell.
|Spell level||Spell point|
Which means that a Wizard would gain spell points in the following way:
|Class Level||Spell Points||Max Spell Level|
As you known a warlock has all its spell slots of the same level but it has less slots than any other class. For example it has only two spell slots at level 10 but they are all level 5. It also has Invocations and Arcanums. Invocation seem to be about the power level of a spell slot from you current level and since you can replace invocations you got at a lower level every time you get a warlock level we could say that they are all of your current level but for the sake of the argument let’s say that each invocation count for one spell slot.
The ratio I/S is the Invocation over Spell ratio meaning how many spells an invocation is worth. If we count an invocation as half a spell the I/S ratio is 0.5, if we count an invocation as 3/4 of a spell the I/S ratio is 0.75.
|Level||Known spells||Spell slot||Spell slot level||Invocations||Arcanum level||Warlock Equivalent Spell Point (ratio I/S = 0)||ratio I/S = 0.25||ratio I/S = 0.5||ratio I/S = 0.75||ratio I/S = 1||Wizard Spell Points|
On this graph you can see the evolution of the Warlock spell point relative to the Wizard.
Since we can at least count 1 short rest per day and since Warlocks get their spell slots back after a short rest we can see that even without counting invocations (“I/S = 0” column and line) a Warlock has at least as much spell points as a Wizard. And if we count two short rests and an Invocation to Spell slot ratio of 50% (I/S = 50%) we blow the Wizard out of the water.
Of course those data doesn’t take into account the lack of versatility of the Warlock. Especially at level 10 or lower when it has only two spell slots. So … I don’t know. I would rule that the Warlock is some kind of weird half-caster, maybe even full-caster? What do you think?
The data I computed in this post are available in ods format.