Like pretty much everyone else is saying, good idea, bad implementation.
I would have definitely used a laptop, but, if I was going to do a desktop system, I would have definitely used an underclocked processor, and 2.5" hard drive. Since you wouldn't reasonably be using this system for anything too extreme, an underclock and undervolt would be reasonable, and lower power consumption a LOT.
2.5" hard drives aren't exactly hard to come by either, and it's not like they'd be storing a ton of data, so I think one of those should have been used as well. The argument that they wanted to use off the shelf parts doesn't work, because the PSU wasn't either.
As for the battery issues, you guys are all pretty much right in saying that that battery isn't gonna last long at all, a little more research in that department would have went a long way.
I think the concept of a computer running of nothing but sunlight is pretty cool, but it's completely unreasonable to ever use in any circumstance (at least until solar cells get WAY more efficent and way cheaper.)
Sapit, I leave my computer on 24/7 simply for convenience, and in case anyone wants to leave me messages via MSN. It's hardly costing me any money and it's near silent, so why not? I do underclock it overnight and when I'm gonna be gone all day, at 400mhz and 0.9v the cpu pulls about 4w full load, the hard drives shut off after a certain amount of time, monitor shuts off, etc, so it's really not pulling much power. This may also sound strange, but I can't sleep without the ever so faint hum of my computer running, without it on it's just too quiet and I can't sleep. Doesn't make much sense but yeah...