Out here the biffy gets less attention than it should. For illumination we've used the old kerosene lantern solution (smelly), candles (inflammatory with five kids), Aladdins (highly satisfactory but it takes them a long time to warm up and they're fragile). The kerosene lantern also heats the biffy in the winter, very effectively.
We've thought about running an underground feeder wire from the house to the biffy, but it's 100 feet, we move the biffy to a new hole every year or two, and we only have one empty fuse socket in our ancient 60 amp service panel. A more localized source of power would be better.
For thirty bucks and a visit to Cabela's, we can buy a 12v solar panel with some leads on it. It's designed to charge smaller batteries (motorcycle size) to run deer feeders and game cameras. No mention on the Cabela's website as to the wattage (low, I'm sure) or whether it has a diode to prevent nighttime leakage. Apparently it can't keep a larger battery charged, but a small one is okay for a few bulbs.
One of these on the roof, a lawn mower batter inside, and then either a 12v high-intensity LED light cluster or two, or even that old standby, the 1156 automotive blinker bulb, and we can see to find our anatomy in the dark with both hands.
And a switch, of course. Some wire.