I would recommend a wire directly to the battery and a toggle switch - us a 16 gauge wire (or heavier) and fuse the wire as close to the battery as possible. Also run a separate ground wire of the same gauge from the light to the battery negative terminal.
The alternator is rated at 147 watts - layman's terms, a little over 10 amps - at 5000 rpm
The stock lights and battery consume 3.25 amps - the battery consumes .9 amps to charge. The battery can draw a maximum of 4 amps if low or in poor condition. If all the lights are on and battery drawing 4 amps, the total demand on the alternator would be appx. 7.25 amps
As long as the battery is in good condition, you could add 2, 30 watt lights and probably be safe (total draw of all lights and the battery on the alternator appx. 8 amps) - exceed that and you will most likely need to charge your battery after riding. Also, you could expect the alternator, the rectifier/regulator or both to have shortened lives due to the higher output demanded from them.
Happy motoring
