Yes, they are Will saves (that part I was not correct about). A stun or sonic grenade has a DC of 15, which can be brought down to 13 or 11 if the target has the Uncanny Dodge 1 or Uncanny Dodge 2 feats. So, if the target does not have either feat, they have to roll a 16 or greater to not be affected.
The Stun force power has a DC of (5 + spell level + CHA Mod + WIS Mod) of the caster, so I assume the spell level would be 1 (the first tier of the power) or the level of your character plus the other numbers.
Other things that affect the DC are below:
+2 if you have the Force Mastery form active,
-2 if the target has the Makashi or Focus form active,
-1 if the target has the Niman form active,
+4 if the target has the Juyo or Mastery form active.
This modified DC is then checked against a Fortitude Save involving the target, the DC, a saving throw against mind-affecting, and involving the caster (literally, it looked like this:
nSave = FortitudeSave(oTarget,nMod, SWFP_PRIVATE_SAVE_VERSUS_TYPE);
). If the Fortitude Save returns 0, the save failed and they are stunned. If it returns 1, it succeeded and they're safe. If it returns 2, they are immune to it and safe.
So...
Let's assume you're level 25 at Malachor, a Jedi Weaponmaster with no mastery bonuses, with a Charisma of 16 and a Wisdom of 16. That gives you a DC of 5 + 25 + 3 + 3 = 36. And the best they could do is 20 + 2 + their fortitude save bonus = 22 + ???(which could be anywhere from 10 to 20).
While a Concussion or Sonic Grenade is a straight 15 DC against a will save + their will save bonus.