The average 12-volt automotive battery powers a 250-watt bulb for 3.36 hours. Typical automotive batteries have about 70 amp-hours of capacity, and therefore have 12V � 70Ah = 840 watt-hours of capacity, and 840 watt-hours � 250 watts = 3.36.
The amp-hours rating can vary considerably from battery to battery, and old batteries have fewer amp-hours than newer batteries. Energy-efficient bulbs actually consume much less power than traditional bulbs. The brightness of a light bulb is actually measured in lumens, not watts, and an LED bulb consumes fewer than one-fifth as many watts as a traditional bulb that produces the same number of lumens. An LED bulb that only consumed 47 watts would run for almost 18 hours on a 70Ah battery.