>>535477497
NP level and NP overcharge are two different things.

NP level refers to the level of that servants NP, dictated by how many times you've obtained them and combined them. For most damaging NPs, this is usually just a raw damage increase up to NP5. For non-damaging NPs it will usually increase one of the effects of the NP. The effect in question is usually dictated by a "[Lv. 1]" next to the specific effect in the description. Additionally, if your servants NP level is more than 1, they gain the ability to gain NP gauge past 100%.

NP Overcharge is a mechanic every NP has where they have one or two effects tied to how "overcharged" the NP is. This is NOT the same as level. The things that impact Overcharge levels are

- Using an NP with 200% or 300% NP gauge (+1 for each 100% extra)
- Using an NP while having an Overcharge buff (amount will depend on the buffs source)
- Using an NP after another NP in a chain (+1 for each one before it in the attack chain that turn)

Take my Saber here for example. I've rolled her four times, so I have her at NP Level 4. That has made the damage output of her NP greater, while you can see that the Overcharge effect is a seperate effect.

>>535478089
Oh yeah, true. So I'd say Black Grail is marginally better then since you probably want the 400 extra ATK more than you want the 500 HP per turn if you're running BG anyway.