I don't remember offhand the exact output, but the stock 990's alternator is pretty wimpy.  Running all the lights will definitely exceed it's output, but I'm not too worried.  I ran the front ones for a couple of hours tonight while moving some dirt and then checked the alternator to make sure it wasn't getting too hot.  It wasn't even warm.  I think the worst case is that I'll drain the battery some, but I can always top it up with a charger if necessary.  I put an Optima in the tractor, so it should be able to take a bit more abuse than the stock battery.