As far as I know, max preheat wattage is the same as the device's max wattage, which varies depending on the number of cells in the device. 200w for 2 cells, 300 for 3, and 400 for 4.
Preheat isn't totally useless either, though replay almost makes it(and, really, TC in general) obsolete.
Say you're using an RDA. You figure out exactly what the max temp is you can use to avoid dry hits and set your wattage high enough for a quick ramp up, but the temp is too hot for your taste. You turn the temp down a bit, which works, but only until your wicks start to dry out. See, the device measures coil temperature, not vapor temperature, and the two aren't the same. If your coils are at 420 degrees for example, the vape will be hot with a fully saturated wick, but cooler the drier the wicks are. This is how TC works. Now you can either re-drip or crank the temp back to your max-without-burning temp to maintain the vape.
On the other hand, you can keep your temp at max and turn down the wattage. This way, when your wicks are fully saturated the wattage is just high enough to give you the vape you want without TC kicking in. Then once the wicks start to dry the device can continue to give the same vape a bit longer until the coils reach the temp limit. The problem here is that you sacrifice ramp up speed.
That's where preheat comes in. Set your preheat wattage to 50% higher than your preferred wattage, use the punch settings to set how close to your set temp you'll get before preheat deactivates, and the preheat will apply the extra wattage until you're close(based on the punch) to your max temp, then drop back to your normal set wattage.
This way you get the same vape every time until your wicks go dry. This method works very well once you've got it dialed in. As I mentioned though, replay gives basically the same result with much less tinkering. If you're having issues with replay though, or just like to tinker, you have that other option. That's what I've loved about Evolv boards from the beginning. Options.
Hope that made sense.
