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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.

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

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.

One of these would provide 4 kW, too (although I don't know for how long), and is only 5.8 kg:

http://www.ev-power.eu/Winston-40Ah-200Ah/WB-LYP160AHA-LiFeY...

Well, Osborne 1 was 10.7 kgs and it was considered a laptop, so there is that.

My dad had one when I was a kid. They were called luggables, and weren't meant to be put on laps....

What if you had really strong legs? You're right, of course.