The amperage taken to produce the watts is what kills the small batteries kinda like the circuit breakers in your house are rated in amps and trip if you exceed their limit. On a 3.7 volt battery a 2.4 ohm carto produces about 5.7 watts and draws about 1.5 amps of current. A 1.7 ohm carto on the same battery produces 8 watts but draws almost 2.2 amps and this is quite a strain. I try to stay at or below the 1.5 amp range. I read about people vaping upwards of 8 watts and wonder what kind of flavor they're getting but a lot of them are strictly dripping onto an atty which is yet another ball game and in my opinion only, outdated except for sampling flavors. Don't take my word for it. Do a search on "burnt taste" where some new member bought a high powered device after reading all the raves about 5 volts and 12 watt vaping. With a vv you can run a 3.0 ohm carto @ 4.1 volts and get the same 5.7 watts but only draw about 1.3 amps as opposed to the 1.5 in the example above. To get the 8 watts out of a 3.0 ohm you kick it up to about 4.9 volts but only draw 1.6 amps as opposed to the 2.2 amps in the example above with the 1.7 ohm. With the 1.7 ohm carto just say for example the battery has a 2 amp circuit breaker, which could be the switch on your battery, you're going to trip it trying to produce 8 watts. If it has a 2.5 amp breaker you're just going to run your power bill up. If a vv device like the Vmax is used you can subtract around 25% of the voltage as the PWM produces differently. It pulses full battery voltage in waves that simulate lower voltage. For myself only, in the evening if I feel like doing hobby vaping and becomming a flavor snob for a while, the clearos are key to getting the most flavor. Fruits taste best to me at or below about 5 watts in a clearo such as the SI CE4 style for fat bats or the Phoenix CE3 style for 78mm. For Tobaccos I like about 5.5/6 watts in the same carto. I can't wait for SI to come out with the smaller (CE3) size for the 78mm batteries. Other than that during the day when this ain't no hobby I just screw a filler carto on a 78, put a few more cartos and batteries in a flat hard shell case and out the door for all day. My only attempt at improving this resulted in a pocket full of liquid when a CE3 came apart lol. It's amazing what 1 little ml of niquid looks like on the front of your jeans.
If you want to go into vaping as a hobby, which there is nothing wrong with, PLEASE get an education before you buy. Kindergarten and First grade are over. Learn about the different battery chemistries, types of protection, stacking batteries, chargers etc. and buy a volt meter first. Taking the time to learn about the volt, ohm, amp relationship can save you a fortune in experiments. In the school of hard knocks the test always comes first, then the lesson. There have been several people severly injured and we're fortunate there haven't been any deaths. Don't give the antis any ammo.
One more thing to save you some math if you're going to try different cartos. A 2.0ohm carto @ 3.7 volts draws 1.85 amps and produces 6.8 watts. Pretty good for tobacco but working the battery pretty hard. I still do it sometimes just for the heckuvit. The're be more sales lol.
Here's a link you can play with that makes it easier to figure out what I'm talking about and save you a lot of shipping costs. It's also a lot of fun.
Ohm's Law Calculator Put in any two values and it will calculate the rest for you.