Alternator rating of Royal Enfield Thunderbird

Generally it is 13.6 volts for any motorcycle.

By capacity it actually means the power that an alternator can deliver, not the rated voltage. Usually the voltage output of an alternator by itself is fluctuating and the regulator-rectifier does the job of providing steady dc.

If a lot of high power lighting and other types of accessories are used, that will drain the battery sooner and need a higher powered alternator. Well, the battery will drain only if the charging rate of the battery (depends on alternator output) is less than the discharging rate.