This might help you --
Ohm's law | Steam Engine | free vaping calculators
The battery will charge to 4.2v. If the mod is not a variable/regulated mod, then that 4.2 is your baseline for voltage, and as the charge depletes with use, the voltage will drop. If you want to change how it vapes, the wattage, you change the resistance of the coil -- the greater the resistance, the lower the wattage, and vice-versa. Also, the greater the resistance, the less amperage it will draw from the battery, making the battery last longer.
Now I know one of you math geeks will probably pipe up with your "yeah buts", so chill already; I'm trying to explain this in basic terms to someone who is just learning it. No need to make it more complicated!
If you use that Steam Engine calculator, you can set the voltage to 4.2, for a charged battery, plug in the resistance of the coil you're using, and it will tell you what your wattage (power) is. What you said earlier, 5.1v, is probably actually 5.1 watts. The field marked "I", or current, is amperage -- I have no clue why they use "I" or "current" to describe it, because it is amperage; I suspect it's because of some weirdness in the french language, which is what makes Ohm's Law so utterly mystifying -- they use "I" instead of "A". They also use "R" instead of "O" for ohms, but that is a little better, because "R" stands for "resistance," which is what ohms measure.
If I had to try and figure out any of this without that Steam Engine calculator, I would be utterly lost, because I don't speak french, and I cannot remember variables that don't correspond to an English word. Forget trying to remember formulas, such as Ohm's Law, because my mind doesn't work that way.
Hope that helps a little.
Andria