Here is the email I got back from my friend. He's pretty gear savy. Hopefully this helps.

"At 180 miles, depending on what your average speed is and when you start, you should be able to get by with about 4 hours of lights. Unfortunately, 4-5 hours means either multiple batteries or one huge battery or HID system. Most HID systems are over 300. I guess a lot of it depends on how much lighting you are looking for. I have an older performance system that I was really impressed with. It was fairly light, used a nmhi battery and had almost 3 hour run time on low beam which is not really that low. Enough to be riding with. The light has changed since then and now is rated at 4 hour run tiem on low beam which is 10W (same wattage as before). It is only 150 buks…..i know, I hate performance too…..but I bought the light a while ago cuz it was cheap and I have to say, it’s a good light. Definitely a lot of value for the money. The cheapest HID system I know of is around 350….and goes up from there. The L&M lithium ion setup is around 450-460. but it’s only rated at 3.5 hours. They chose to go lighter on the battery instead of increased run time. From what I hear it’s a very light system though. If you know of anyone else who already has a light maybe you can barrow the battery if you buy a compatibly system. Sometimes on the long rides they will haul your battery and lights to one of the later check points….so you don’t have to carry it for 150 miles in the middle of the day. Good luck….. "