watts = volts x ampere
(8 watts )= (12 volts )x ampere
ampere = 0.67 ampere
0.67 ampere x 4 bulbs = 2.68 ampere
(50 ampere - hours) / (2.68 ampere) = 18.66 hours or 18 hours and 39.4 minutes kapag fully charged , theoretical, pero less than yan sa actual...
pakitama kung mali ako...hehe