The scanner would be scanning for 15 minutes each hour, and the raspberry pi would be always on or could be turned off between scans. What specs would I be looking for in a solar array?
Canoscan max power consumption: 15W
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The battery sizing will be dependent on a couple of things:
1) how much sun will this location get? Without that info you can't determine what your long-term average recharge (i.e. power input rate) is.
2) what your risk tolerance is. If you're going to "properly" engineer this beast, you'll need to figure out what you expect your worst case solar power is. Said another way: "What's the longest period of time that you expect little to no solar input?" If you're in a location (like I am) where you might well get 4 days of thick overcast in a row in the middle of december, that's a very different calculation than if you're in a mostly sunny climate near the equator. There are insolation charts for the USA here (and lots of other places): http://www.gogreenenergyusa.net/images/insolation-map.png
Some of this will be educated guessing: if you want to be able to go a week without a sunny day, you'll need a battery that can provide (10W * 24hr * 7 days --> 1680Whr). Note most batteries are actually specified in Amp/hours at a given discharge rate, and since you will likely be cycling yours quite a bit you probably don't want to deeply discharge the battery all that often because that shortens the effective life.
in short, you have more questions at the moment than you do answers. ;-)
I'd go for the most practical/cheap approach:
Use the cheapest car battery with a dashboard solar pannel battery-charger.
But I have no idea if you can supply your Canoscan with only 12V...
(If you prefer better looks... look for PowerGorilla on the net, it's a backup battery for laptops and they also supply solar chargers. You can select upto 24V output.)