[raverbashing]

> Solar lights are many times more thermodynamically efficient, cheaper to make, smaller and much more robust (no moving parts!).

Beyond the "no moving parts" (and maybe cheaper to make), I think you're just spilling a marketing blurb.

> This "lifting weights" thing is a silly gimmick. A lifted weight stores a miniscule amount of energy compared to a battery that's been charged by the sun.

Potential energy = m.g.h So, lifting a 10kg weight at the height of one meter gives approx. 98J

Incident solar power is 100W/m2 (best case). A square solar cell 10cm of side hence gets 1W (or 1J/s), after efficiency losses this is even less (which are present on the GravityLight as well). Between 0.5W and 5W seems common on sale websites

So, lifting the weight above (which takes 1s approximately) gets you as much energy as 36s of solar energy (of course, you can't recharge it at night)

Also consider the cost of the battery

Edited for wrong math