Apple provides an 85 watt MagSafe charger with 15 inch MacBook Pro - so if you were to expect the normal 4 hour recharge time from empty from your solar panel, you'd need to carry 25+ pounds of solar panel and several pounds of inverters since MagSafe charging assumes you have 110/220 VAC and not direct current to charge. (i.e. don't do this.)
There are vendors that make exactly what you ask - I would contact the and ask if you can expect better performance from USB-C charge MacBook instead of MagSafe MacBook if you don't buy my quick summary below.
What I would recommend is going iPad Pro and keep a Mac mini somewhere else for things you simply cannot do locally. RunningSince you are looking at the new models that accept USB C charging, you have lots more flexibility to use solar / iOS type chargers than before. The early 2017 lineup has 4W to 35W processors instead of the much higher wattage ones from 2016 and older.
Even so, running LTE network and remote access to where you have AC power for your Mac keeps your bicycle build light and reduces your cost for solar 10x or more. If you absolutely need a MacBook on the road, get newer MacBook and MacBook Pro support USB-C power delivery which opens you up to more reasonable power sources. The 2017 lineup has 4W to 35W processors instead of the much higher wattage ones from 2016 and older.
I personally wouldn't bike with anything bigger than the MacBook with USB-C charging or iPad, but you could be thinking of a trailer where 35 pounds of gear and Arizona type sunlight would make a MacSafe MacBook possible. Also, the MacBook come with 29W, 61W, or 87W USB-C Power Adapters so you can pick a solar panel that would mimic those recharge rates. It's possible you could get away with a 29W charge on the biggest MacBook if you didn't run down the battery but 1/5 of the way each day and had good times to charge it (i.e. all day on the back of the bike in clear conditions).
I have heard of people using the 13" and 15" MBP for light work and running all day on a 10W range iPad charger. They lost battery charge all day and watched CPU intensive tasks, but for email, light web, no video or no encoding work - they made it most of the day with less than 10% battery loss and regained 25% battery charge with the Mac sleeping all night on the iPad charger. Now you have USB-A to USB-C charge capability with a small cable instead of lugging a MagSafe AC to DC transformer.
