In practical terms, a bulb of the same wattage consumes the same amount of current, whether it be ac or dc - but it is worth bearing in mind the route it takes to get there from the battery, and the losses involved

For argument's sake - taking the same state of battery charge, and assuming that you're running it down to the same level each time

if you power it directly from the battery - no losses - 100 hours

inverter (85% efficient) - 85 hours

switch-mode voltage converter - 93 hours

- this is NOT precise, but gives a fairly good comparison

The other practical point to bear in mind is cable size - a 12 or 24v system requires far heavier cables/fuses/switches than mains voltage (10 to 20 times heftier!) due to the amperage!

Another point worth mentioning is that you should NEVER equate the rated amp/hrs of a battery to what you'd ever dream of taking out of it........even the very best batteries have their lives considerably shortened by deep discharge!