Disclaimer: This is the most simplistic explanation and is addressed to new vapers mostly or vapers who are happy within the recommended "just right" power zone (4.5-8.5 watts).
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, your VW (variable wattage) APV 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. 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) and even some juices, especially the delicate ones.
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 want to know more, this is a good read:
http://www.e-cigarette-forum.com/fo...-stuff-i-double-dog-dare-you.html#post9112897
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.
