I play myself with Stalker v2 with attached XBee S2 (standard with cons approx. 40mA, not Pro) sending data from balkony to another part of the flat, where is another “receving” Arduino Mega. I live in the middle of Europe and I started with one 1W solar panel without any solar tracking. Stalker actually sends data every cca 17 seconds and I don’t switch on/off Xbee between transmitions. Orientation of the panel allows to receive sun rays from early morning till approx. 1400hrs now during the summer time. If there is clear sky the 1W panel is able to re-charge the 1850mAh Lipo to full within above mentioned period. But if there is cloudly the Lipo voltage is kept so so during the day and it is dropping down during the night.
So the next step is actually 2pcs of 1W panels in parallel connnection which re-charge the Lipo’s lost energy within 2-3hrs when clear sky, and add 100-200mVolt during the day when ovecast. But this is now, in summer. I’m afraid it can’t work in the winter time independently from 2 solar panels without external re-charging.
So now I’m preparing the panel with 4pcs of 1W solar panels and we will see. Nevermind this is experience with one XBee module only. I know that I can still improve a bit Stalker’ consumptoin both in hardware and software way.
The consumption in your case will be higher with GPRS and energy balance will depend mainly on meteo conditions at sensor’s place.
So that’s my experience with Stalker and solar panels for the last two months …
Cheers for now