Copy and paste from net Hope it helps Nutsnbolts 1 year old but some info there.
Watts = Volts X Volts / Ohms
For example, a standard 3.5 ohm 801 atomizer on a 3.7V battery generates 3.7 X 3.7 / 3.5 = 3.9 watts.

[A standard 2.3 ohm 510 atty on its 3.2V battery does a bit better with its 4.5 watts: one of the reasons for its excellent vapor production and popularity.] However, most vapers find that 6-8 watts is the sweet spot.
Prior to 2010, only a 5V mod could deliver that sweet spot experience: 7.1 watts for the 801 atty. Thus the popularity of high-voltage mods (which won the 2009 Vapy Award for Best Innovation of the Year).
This year, LR (low-resistance) atomizers were introduced for the most popular models: 510, 801, and 901. [NOTE: They all need to be used on batteries of at least 450 mAh, e.g., the Fat Batts or Mods.] Most LR attys are 1.5 ohms, but alternatives (e.g., 2.0 ohms) have been appearing.
For example, a 1.5 ohm LR 510 atty on a 3.4V eGO generates 7.7 watts: sweet! [On a 3.7V mod, however, I prefer a 2.0 ohm atomizer: 6.8 watts ... rather than the 9.1 watts of the 1.5 ohm LR atty.]
All Greek to me.......