A 2220 bulb is not 21 candlepower. A 2220 bulb is 6.4 volts, 3.28 amps. Volts x amps = watts. Therefore, a 2220 bulb is 21 watts.
There is no conversion from watts to candlepower. Watts is input energy, whereas candlepower is output. There is no direct correlation between the two as bulb type, design, and execution all effect output.