You mean a eMate?
On topic, you'll want a solar panel that can provide twice the total wattage that the computer will ever need. This ensures ample power to charge the battery and run the computer at the same time, whilst still having enough for non-ideal circumstances, such as:
- Winter time
- Low sun angle
- Cloudy days
- Non-optimal angle relative to sun or strong light source
- Dirt on the panel
- The fact that you'll have to have a plastic cover over the panel because glass won't work very well, despite its much better light transmission capabilities
- The panel's decay over time (solar panels are typically rated to supply a certain power wattage, but after a certain time, they drop to about 80% of their original output, and that's about the time the warranty runs out)
- Stickers that uninformed users will put on the panel
... and so on. Ideally you'd use 3rd (4th in design phase yet?) generation panels but those are costly. Most solar panels of the monocrystaline form have an efficiency range of about 12-15% at best. Polycrystaline panels are cheaper but are not as efficient and take up more space.
It's certainly doable. We've been making ultra-low power gadgets and other stuff like that for 30 years now. (Not I, just the industry at large.)