Watts = Amps * Volts
For DC circuits, voltage times amperage equals wattage.
W = VA
A = W/V
A = 20 / 12
A = 1.67 A
Amps hours is the number of amps drawn times the hours of continuous usage.
1.67(9) = 15 Ah of capacity
A fresh and fully charged 22 AH battery will drive a 20 W 12 V lamp for
22 / 1.67 = 13.17
or a little over 13 hours
Forgot what my question was…lol