Ok, here's the thing....the reason why you need at least 20-30 watts to fire a sub ohm coil is because of its LACK of resistance. The coils in are atomizers are heated up by the friction of the electrical current passing threw the wire. The thicker the wire and/or the less wraps a coil has the easier the current passes threw the wire creating less friction and there for less heat. So therefore to properly heat up a lower resistance coil you need more current (amps) to make up for the lack of resistance to heat the wire up.
On a mech mod, there is no regulation so the battery is going to just dump whatever amount of power it has left in it at the wire and the wire is going to in turn heat up as much as the current passes threw it.
On a regulated device you set your desired power level (watts) and the chipset in the mod draws the power from the battery and converts it to the necessary amount of current to put threw the wire to get the desired result threw a process called "DC to DC power conversion." Otherwise known as a "buck/boost circuit" or "step up/step down conversion" where depending on the design of the chipset the power is drawn from the battery and run threw a series of inductors, transformers or capacitors depending on design and temporarily store and then release the energy at a higher current for step up conversion or limit the output for step down.
That's the reason why most people prefer a regulated device as apposed to a mech mod for these sub ohm tanks, myself included....because the regulated device puts out a constant power output regardless of the charge left in the battery till it's dead because the output is being regulated by the chipset in the mod as apposed to just running it on a mech mod, which will work, but the quality of the vape will constantly change as the battery drains and has less and less power output till it's dead.
Vaping is not only fun but educational too

good luck with your shopping and happy vaping!