Rod said! Its not the voltage that dictates this, its the Current the lamp
So its impossible to say
The sums go something like this
Watts = Amps x Volts, so amps = watts/volts
Say it is a 100amp hr battery, that is 1 amp for 100 hours
So a 25W bulb uses around 2 amps, 25/12, so around 50 hours.
Chris said, Depends on the wattage of the bulb and the power of the battery
watt bulb on a 75ah battery should last about 40hrs
The sums go like this & prove him correct
75[AH battery]/(21[Watts]/12[Volts])B.8 hours
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