If you are planning to use a solar panel to power your Android device to do time lapse capture, it is advisable that you do some measurements and calculations to make sure the solar panel you use provides sufficient power for the environment you will be subjected to. This is especially important if you are doing long term remotely time lapse photography.
Tip: If you are going to use solar panel to power your device, it is best to reduce your screen time out to the lowest setting, i.e. 15 seconds or less to reduce LCD power usage when the device repeatedly wakes up from solar panel power supply fluctations.
In order to find out the amount of power your Android device uses, the easiest way would be to fully charge the device and then fully drain it under the same condition it will be used.
Example A, in our comparison graph, HTC G1 was able to operate for 1575 minutes (26.25hr) under airplane mode before the battery is flat.
Given the fact that HTC G1 has a 1150mAh battery, this means this phone have a current drain of 1150mAh / 26.25h = 43.81 mA
In other words, we have a daily power requirement of 43.81 mA x 24 h/day = 1051mAh/day = 1.051Ah/day
The typical lithium in Android phones are rated at have 3.6V, for HTC G1, that means it consume 1.051Ah/day x 3.6V = 3.78 W/day
Example B, in our comparison graph, Nexus One was able to operate for 1860 minutes (31hr) while both WIFI and GSM radio are on before the battery is flat.
Given the fact that Nexus One has a 1400mAh battery, this means this phone have a current drain of 1400mAh / 31h = 45.2 mA
In other words, we have a daily power requirement of 45.2 mA x 24 h/day = 1085mAh/day = 1.085W/day
i.e. Nexus One consume 1.085Ah/day x 3.6V = 3.9 W/day
We need to make sure your Android device never turn itself off during its time lapse capture operation, because the phone can't turn itself on even when solar power is resumed. If your device continues to do time lapse capture 24/7 even when the solar power is off, in example A, HTC G1 was able to continue capture for 24.2hr assuming its battery was full; in example B, Nexus One was able to continue capture for 31hr assuming its battery was full.
Assuming we have 12hr of daylight and the phone battery was fully charged when sun sets (solar power is off), that means the phone MUST operate for at least 12hr using its own build-in battery. Allowing for non-fully-charged build-in battery and cloudy conditions, etc. It would be advisable to have at least 24hr battery allowance. In both example A and B, both devices satisfy this requirements.
If you require 24/7 time lapse capture and you device cannot operate for at least 24hr alone on its own build-in battery, you should consider adding a solar panel charged battery in your setup.
For referencing purpose, in our test, a 8700 mAh external battery, consisting of three 2900mAh lithium cells in parallel was only able to power/charge HTC G1 continuously for around 70 hours while the HTC G1 is capturing 100% black photos at 1minute interval into the 2GB microSD card. This means that the presence of this 8700mAh battery is only able to increase the capture duration by two times, from 34.5 hr using internal battery to 70hr using external battery. Theortically, assuming there is no lost in energy in the process, this battery should increase the capture duration by 8700/1150 = 7.5 times. This apparent significant lost of energy means that one needs to pay attention to the energy lost by the setup and preferably run some test before deploying it to the field. In this case, the low level of current drain (33.3mA) over long period of time probably contributed to the low conversion efficiency as these battery is probably designed for efficient (88%) current drain at around 500mA.
Daylight capture only
If you are using a solar panel without a solar panel charged battery and you don't need to capture photos at night, you can configure 24/7 timelapse to start/stop capture when power supply is connected/disconnected. That means when the sun comes out and your solar panel is supply power, 24/7 time lapse app will start capturing photos. Whereas when the sun goes down and the solar panel is not supplying power, 24/7 time lapse app will stop photo capture to conserve power. This means that doing so wiill allow your device to operate/standby a lot longer.
Example A, HTC G1's standby time is rated at up to 406 h (17days).
Example B, Nexus One's 3G standby time is rated at up to 250 h (10.4days).
In both example A and B, this would allow them to standby for a week or more. Meaning that even if it was cloudy/raining for a week, your device should still have power to continue operation when the sun comes out and your solar panel resumes power supply. In other words, you don't need to add external battery to your setup.
Solar Panels power generation is typically given in Watts and this is the maximum amount of power a solar panel will generate in bright sunlight. To calculate the energy it can supply, multiply the Watts by the hours exposed to sunshine.
In summer (winter) months, you can a usable average of around 5 (4) peak sun hours per day.
For a 1.5W Solar panel in 4 hours of Full Sun per day, 1.5 W x 4 = 6 W
Note: for simplicity, we are ignoring natural losses and partial sun in the calculations.
This 6W daily power output of a 1.5W solar panel is larger than the 4W daily power consumption by HTC G1 or Nexus One when they are capturing photo 24/7. This means it is possible to power HTC G1 or Nexus One for 24/7 time lapse photo capture using a 1.5W solar panel.
However, in practice, a 5V 1.5W rated solar panel only provides up to 300mA of current output, lower than the 500mA requirements of the USB specifications. That means a solar charged battery is likely necessary to act as buffer to ensure that the solar panel can provide a stable 5V USB power for charging the device. A small capacity battery will likely suffice for this purpose. In any case, it is advisable to use a solar panel with higher rating that you required to ensure you don't run out of power due to rainy/cloudy days.
For devices/setup that consumes more power (>4W/day) or to avoid having the solar charged battery, you will need to use a higher power solar panel. These days, you can purchase 5V, 5/6W solar panels which provides USB output port for charging. Since these panels can easily meet the 2.5W (5V, 500mA) requirements of USB specifications even in cloudy days, you should not require a solar charged battery is used to act as buffer and you should be able to rely completely on the device' build-in battery if your device can last through the night with its build-in battery. However, when using these panels, you should make sure that you have a stabilizer curcuit in place such that your solar panel will provide stable 5V USB power. Some solar panels are sold with such stabilizer circuit in place or you can purchase such stabilizer circuit separately. Without the 5V stabilizer, you may experience charging problems or cause damage to your device. Even with the stabilizer circuit, we noticed some Android device will reboot by itself if the power supply is unstable and fluctuates, e.g. as the sunlight is dimmed repeatedly by overcloud. If you intend to use such device, it is best for you to have a solar charged battery in between to avoid your device rebooting.
In any case, it is always advisable to test your setup to ensure it works as expected before you deploy it in the field.
Before you put your device in your setup outdoors, you need to make sure your setup have features that will protect your phone from rain water. Most phones in the market are not water proof and will fail if submerge in water or subject to rain.
Even if your phone is water proof, there is a good chance that repeated heating/cooling from sunlight will destroy the manufacturer's water proofing.
Therefore, we would advise that you put your phone under shade rather than in direct sunlight. Ideally, use your solar panel to provide the shade. In addition, you should water proof your phone even if your phone is water proof. Kitchen cling wrap is probably a good material to wrap your phone without affect camera's visibility.
Plastics will degrade under direct sunlight. If you put some plastic outdoor, after some months, you will see the plastic becomes brittle and will crumple. I suppose you don't the same to happen to your device's casing which is probbaly made of plastic.
If you intend to put your device outdoor for months, we would suggest you to protect your phone against UV damage. You could put a replacable case/cover on your device before you apply water proofing or you could do use privacy window film which can block UV and infra red radiation. UV is easy to block by any opaque material, so there are room for you to invent your own method here.