A 2000 mAh (milli-amp) battery is supposed to be able to deliver 2000 mAh (=2 amp) continuously for the duration of 1 hour. How realistic this number is depends on the age/wear of your battery and the rate at which you discharge it. If you were to pull twice that (4 amp) you would not be able to do so for the full (theoretical) 30 minutes. The gentler you discharge your battery, the closer to the full hour you would get.
This may not be 100% exact, but I hope you get the drift
As for wattage; consider it a measure of heat. Or how much liquid you are able to vaporize in a set amount of time. There is one thing, tho. What you are thinking of as 30w and 50w batteries are not the batteries, but the mods the batteries go into. How much strain a battery is capable of handling is measured in Amps, not Watts. Watts is just a measure of the
effect of volts and amps at work, not something happening
in the battery.
This is were I should point you towards some kind of
Ohms Law calculator. To find out if your battery is capable of delivering what your mod needs, you to plug some numbers into that calculator. Set voltage to 4.2v as that is the voltage of a fully charged li-ion battery. Then add Amps - press play on tape - and you'll get the wattage your battery is capable of delivering. If you don't know the Amp limits of your batteries, please do ask around on ECF.
Sidenote: In Norwegian, watts are sometimes referred to as 'effekt'. Which is your 'effect' (duh). Sort of makes it easier to understand. Maybe...