Based on three hours of tracking I did today with AGPS disabled, running MfE and 3G enabled, I used about 25% of the battery capacity. This is based on using battery-eye, not the little battery guide at the top which is useless. Now the device never obtains full charge (100%), it only ever gets to 95% on battery-eye and I don't know what the bottom of the range is. Also you need to be able to save the tracking file. Let us assume you need to switch batteries with 10% remaining. That gives you a working capacity of 85%. This gives you a working time of 10 hours. This will still need to be verified and I am sure many other factors will come into play. I really don't recommend the solar chargers for charging phones or some have an internal battery that is then used to charge the phone. Most of these require 10 or 12 hours of full sun to charge their internal battery. You cannot even get this on a clear day in summer in Australia, because full sun is not what you get in morning or late afternoon. At best you would get 8 hours. This all assumes you are angling the panel at the sun correctly, there is no cloud, your not walking under trees etc. For the weight and cost you are better to just buy several of the 3rd party batteries and charge them up before going.