When I used the 20 watt halogens I could run one for 1 hour with out any noticeable dimming. If i used the second one as needed to give more light i could notice the light dimming real bad in 40 mins. This using a 5ah battery in Denver during winter. I figured rough math 20/6=3.3 so in theory I should be able to get at least 3hrs run time.