Depending on your latitude, in the winter on a cloudy day a small DIY system might be near useless. The panels might have a voltage when short circuited, but ge

Depending on your latitude, in the winter on a cloudy day a small DIY system might be near useless. The panels might have a voltage when short circuited, but getting enough power through the DC-to-DC converter with enough leftover to overcome the internal resistance in the battery would be a tough. At best, it might trickle charge the battery and keep it from losing charge from self-discharge. The advantage to the bike is that it not only covers the days where the solar is falling a little short (and where an extra panel would also work), but also on the days where the solar is failing to produce anything of significance (at which point you would either need a ton of extra panels, but even then you still might not be covered).