Well on my stock 20A alternator, I am running all the factory lights, the deere single rear work light, and a pair of 55W halogen fog lights mounted down low on the front so I can see under the loader bucket. Thats probably 5x55w or about 275+ watts load. In addition you may have the lights on the rops running and the instrument lights and the e-pto (but that is a high intermittent load, with low constant draw). 20A at 12v gives you 240w of capacity. So I may be drawing a little more from the battery than the alternator is replacing, but after running like this for 5 hrs in the dark last weekend, no ill effects and my charging light did not come on.
Iowa2210