[VLM]

My limited experience with lasers is the heatsink problem is significant, so worry about dissipating a couple KW of heat in your hand or backpack, not carrying around a mere couple KWh of energy.

I imagine it would sound and feel much like holding an industrial heat gun for awhile. This might be a problem.

[Crito]

Hmm, now I'm imagining something like Reason (http://en.wikipedia.org/wiki/Snow_Crash#Reason). A backpack sized battery, but needs a water-submersed heatsink.

[Hello71]

right; 40% efficiency means 40% comes out as light; the other 60% comes out as heat.