Tim, you are right on the money. I really don't like the way battery manufacturers use "Amp Hours" as a measure of the storage capacity. They should use Watt hours since Watts are the measure of energy. Amps are really for measuring circuit capacity since they are a product of volts and watts.

"Marcus is dead on with the 27 minute calculation."

George did the math based on 51 Amp Hours and I cut it to a third or half due to the way high loads decrease the total output of a battery. I say no more than 15 minutes with two 650W fresnels on this battery. Switch to lower wattage lights and you will get the full draw.

BTW don't forget the lowly computer UPSs. I've used them on location before to power instruments (don't forget a hand truck - those little suckers are heavy!) - various capacities available, and when it's time for post you can stick them back under your desks. Frys or any stationery/office supply store, which makes them even more useful in the event a small genny packs up...