My motorcycle's headlight bulb specifications are 12v-35/35W. What does a 35/35w mean here.(I know w stands for watts but why 35/35). Also headlight draws current from alternator whose specs are 0.13kW/5000 rpm.Can I upgrade to 45/40W 12 V bulb?
It means there are two filaments in the bulb, both rated at 35W. Presumably, dipped beam uses one of them and full beam uses both together.
If you upgrade from 35/35 to 45/40, the headlight is taking 85W on full beam instead of 70W, compared with the alternator rating of 130W.
We can't be sure if that will overload your alternator since we don't know the power requirements for the rest of the bike, but IMO it will probably be OK. If you have an electric starter, there should be some margin in the alternator rating to handle recharging the battery after each engine start, even when you are riding at night with all lights on.