You're right. I did say it was a back of the envelope calculation. :-)
If you have a 50% efficient generator, just double the size of the ballast; 20kg is still reasonable. Or double the height, though that would be somewhat harder; you would need some kind of pulley arrangement. Or some combination of the two.
Also, I think low voltage DC generators can do considerably better than 50%; the numbers for this kind of application can be quite different than those for the kind of large scale AC generators we're used to.
If you have a 50% efficient generator, just double the size of the ballast; 20kg is still reasonable. Or double the height, though that would be somewhat harder; you would need some kind of pulley arrangement. Or some combination of the two.
Also, I think low voltage DC generators can do considerably better than 50%; the numbers for this kind of application can be quite different than those for the kind of large scale AC generators we're used to.