Solar powered iPhone for remote monitoring
I am working on a very similar project with a Samsung S4 phone. We use a 12 Volt 15 watt solar panel, a 3 amp PWM charge controller, a 7 Ah SLA battery, and a 2 amp DC/DC converter with 5 V USB outlet.
Right now we are running the app (which takes photos and uploads them to an ftp site once every hour) continuously. This works fine if the weather is sunny, but the SLA battery discharges after ~2 days of overcast weather. Running the phone continuously wastes power, so my intention is change the app so it sleeps most of the time. According to my calculations it will have no problem staying charged if we can get the average power consumption down to 1/4 of what it uses now.
If you can get your app to sleep between taking readings then your existing setup may work OK. If that is not possible then the only solution is a bigger panel and more battery capacity. A 60 watt solar panel, 5 amp controller, and 30 Ah SLA battery will probably be enough.