Hhhhhmmmmm, couple of problems on this thread. Power doesn't determine battery life, amps do, which is affected by the coil resistance. So, a higher resistance coil is going to give you longer battery life using the same power. Also, Current MULTIPLIED by resistance equals voltage. For the new vapers on here, I found this website extremely simple to use:
Watts - volts - amps - ohms conversion calculator
Check that out after you watch this video on Ohm's law and VAPING:
https://www.youtube.com/watch?v=xc90pdATEM0
The precise scientific understanding of these concepts is far less important than understanding their relationship, and how these variables affect your vaping experience. From a practical standpoint, if the juice tastes burnt, turn the voltage/power down. If you have variable wattage, start at 8 watts and adjust up or down to taste. If you have variable voltage only, add 2.0 to the Ohms on the coil, and adjust up or down to taste. If you don't have an Ohm meter, i.e., either a variable wattage mod or a multimeter, consider getting one. Many replaceable heads have the resistance (in Ohms) stamped on them, but even if they don't, start low and increase voltage until you find the vape you prefer. If you can't change voltage in you battery, then try and buy coils that have Ohm's approximately 2.0 less than your battery power voltage. For example, a fixed battery may have an output voltage of 3.7 volts, try to use a coil with about 1.5 - 1.8 Ohms.
If none of this makes sense, keep vaping until you find a setup that works for you. That's what most of us did in the beginning, and for the most part, what I do now. The important thing is that you're not smoking now, or in the process of quitting, and you're just trying to figure this stuff out and have the best experience possible. With you on that, and good luck! Power up and Vape On!