Not if you build a 12v system are willing to use a simple but uncool sealed lead acid battery. 5ah battery is only about $11 at just about any electronics or battery store and will drive a 20W light for over 2 hrs. You can put it in a handlebar bag or in a box on a rack trunk. I used a Carlon 6x6x4 junction box with a 12v lighter plug and push button switch mounted on the box. I can actually take the box off and use it as a light duty 12v power supply for anything with a lighter type plug. The box was waterproof, but I drilled rows of tiny upward angled holes just under the lip of the cover for ventilation since most batteries say "Do not charge in a sealed container." As a precaution against splashing and to make the box blend with my panniers I cover it with a piece of waterproof black cordura just held in place by the staps that hold the box in place, leaving one corner loose enough to acces the lighter plug for charging. Every few months I open the box just to check on things and have never seen the slightest sign of water intrusion.

If sla is too unstylish you just may not be geeky enough to hang with this crowd.
FWIW,
Raymond