Disclaimer: This is the most simplistic explanation and is addressed to new vapers or vapers who are happy within the recommended "just right" power zone (4.5-8 watts) and use stock coils.
Ohm's Law as it pertains to vaping is really not that complicated--and it's very useful when you want to know what you're doing.
Voltage and wattage are often misunderstood by new vapers. Wattage is the power (heat, sweet spot) that your PV (battery and atomizer) generates. Wattage = Voltage (of your battery) squared divided by Resistance (Ω) of your atomizer [P=V[SUP]2[/SUP]/R]. If you're not good at math, don't worry, use this easy calculator:
Online Conversion - Ohm's Law Calculator
Of course, if you own a VW (variable wattage) device, you don't really need this calculator because your device will do the math for you.
The wattage you want, especially at the beginning of your vaping career, should be somewhere between 4.5 and 8.5 watts per coil. Anything lower than 4.5 watts may not vaporize your juice properly and will not produce enough warmth and vapor. Anything above 8.5 watts increases the risk of burning the filler in your cartomizers (if you're using them) or dry with your clearomizers.
There are, of course, other variables, like eliquid and JDD (juice delivery devices) that you're using on your batteries. Seven watts on a filler type cartomizer may feel different than the same 7 watts on a fillerless clearomizer or a dripping atomizer. The same is true for different eliquids; tobaccos, chocolate and coffees generally require more wattage (heat), while fruit and other delicate flavors do better with less heat. Everyone's sweet spot is different--those are just very general guidelines.
If you are using dual coil atomizers, things get a bit more confusing. Dual coil atties consist of two coils configured in parallel, which means that a 2.1Ω atty is really two 4.2Ω coils--so you have to calculate your wattage based on the 4.2Ω number--not 2.1Ω--roughly. Dual coil atties require more wattage than singles, but not quite twice as much. They produce more vapor due to increased surface. I usually increase the power (wattage) by ~50% when using dual coils; for example, if I like 6 watts with a single coil atty, I start at 8-9 watts with a dual coil atty. That's just my preference--YMMV. When in doubt--start low and adjust up as needed.
If you are interested in high wattage (and/or sub-ohm)vaping, that's a different conversation altogether--and not my area of expertise.
Experiment and you'll find your own bliss in no time!
The chart below is a good guide to safe vaping, even though some think it's a bit conservative.
