To understand, there are a few constants. Coil = resistance. Battery = voltage. And also you need to know the current draw that your battery can handle.

For starters, an Ego VV(variable voltage) will suffice, the reason you go VV is to keep a constant voltage so you can maintain your vape production. In this case your coil will be a fixed resistance. Wattage = V^2 / R so you can calculate your wattage there.

For mech mod you really need to use a rebuildable atomizer and sub-ohm in order to get a decent vape production. This is because your battery fully charged is 4.2V but will keep on dropping with use, so you will need to find a certain ratio in order to maintain vaping until your battery runs flat(around 3.2v). Most people will run around 0.7 ohm and that will give you 25W at fully charged battery and around 14W when your battery is near zero. Of course that is ideal situation, if your mech mod is not properly constructed it will have high internal resistance and your wattage will run a lot lower.

So what most ppl do nowadays is to use onboard chip to maintain the wattage at a certain level, that way you are not bothered by the battery level and you can swap atomizer without compromising vape quality.