If it weighs less than 1kg and saves more than 3 minutes per day of plugging and unplugging things and/or opening packets of AAs then Chris White's calculations suggest it's a win.
Someone has done this (TCR or TABR?). I can't remember the outcome, I think it was a bit of a faff. Depends how fast you're moving and how much night riding you're doing. I like the fit and forget nature of a dynamo.
Solar panel ?
If it weighs less than 1kg and saves more than 3 minutes per day of plugging and unplugging things and/or opening packets of AAs then Chris White's calculations suggest it's a win.