Dargo, in the real tractor world, a 15amp alternator probably would suffice. But here in the TBN tractor world, many of us with small tractors tend to also watch the TV show PIMP MY RIDE and we load up our little toys with all sorts of goodies. So for that very small minority of tractor owners who put extra lights on their tractors, add a Pat's Easy Change, weld on bucket hooks, add extra toolboxes, chain holders, GPS units, XM radios, multiple cup holders, remote starters, spinner hubcaps, rear view mirrors and dare I say color coordinated fuzzy dice . . . well to us folks who do that stuff, then we need to know the output rating for our alternators, we also need to know the pump flow of each of our hydraulic pumps, and the wall thickness or our ROPS. Because we are tractor geeks.
Talk to any dealer who has been around for quite a while and ask him what % of people add lights, they will say it is about 2 or 3%, ask about how many people weld chain hooks on their buckets, the answer I've gotten is about 10%, ask about a Pat's easy change and they will ask you what the heck you are talking about.
What I am trying to point out is that some things you read about commonly on TBN are not common at all in the real world of tractors.
Heck some of the brands you read about on TBN are rare as hens teeth in the real world but seem relatively common from our little warped view of the tractor universe.
Whenever I talk to a tractor dealer I constantly have to remind myself that to him, a tractor is a tool, but to me a tractor is a hobby. Very similar to talking to cops about guns. Many only shoot their gun to 'qualify' and then never again, but many gun nuts (like me) buy bullets in bulk and load our own or have highly customized weapons at our fingertips. Same thing applies to all parts of society.
IslandTractor wrote: <font color="green"> Why not just do some clever electrical wiring so the headlights go out when the forward worklights go on? Wouldn't the standard alternator/dynamo be sufficient to run one set of lights at a time?</font>
Yes. In some cases you might start to draw your battery down a bit but it would recover the next time you ran without the lights. For anyone other than a commercial operator, it would be fine. There are dozens of examples of little BX series Kubota owners who do just that with their tiny dynamos and have extensive lights and report no problems.
IslandTractor also wrote: <font color="blue"> And speaking of boys and toys, what about just using LED based worklights instead?? I get an amazing amount of light with three LED bulbs running off two AAA batteries which last 24hours. I imagine you could light up a football field with 20-30amps at 13-14 volts! </font>
Ahhh, my next project . . . .