The Vaping Power Chart (Click to Enlarge)
Power is measured in watts, calculated by V2/R
Basically, stay our of red because it's dangerous, stay out of blue because it's useless.
Stay in green you should be fine, stray outside and you may or may not like the result.
Deep Red: TOO HOT, burnout pretty much certain
Red: Too hot, higher is risking coil burnout
Purple: Too warm, some juices may fry
Green: Just right
Deep Green: A little cool, vapor production is lessened
Cyan: Too cool, very little vapor production
Deep Blue: Cold, for all intents not functional
Related article: Watt Mapper: The Vaping Voltage Chart