for Todd's example. Todd did not mention resistance. R = E / I -- R = 12 / 4 = 3 ohms.

Since 3 ohm 50 watt resistors are nether cheap or available at your local Radio Shack I suggest you take an automotive head light, measure the amperage draw and compute the battery capacity from there. Remember that the amp hour rating on batteries if often stated at the 40 hour rate. ie a 100 AH battery will supply 2.5 amps for 40 hours. Higher draw will result in lower total amp hours.