A 30watt iron sould be fine,it'll get the job done anyway,same temp usually,just takes longer to heat back up after you make a joint than a more powerful iron.I find soldering so much easer since i got an antex soldering station and a fume extractor,worth every penny if you solder often.
If your going to solder batteries you at least a 50 watt iron so you dont over heat your batteries. I use the HAKKO 936 soldering station. A 30 watt iron will over heat batteries because you would have to leave it on so long to heat the solder. A 30 watt iron might be okay if your only soldering wires to plugs & speedos.