Before you tell me that the question has already been asked, only similar things have been asked.
I need to power a Raspberry Pi with some electrical equipment attached to it. The Pi needs to run for at least 48 hours and I need at least 2V and 900mAh to be used for the other equipment. The device must be portable, so it doesn't have to weigh too much. I thought about using more than one power source and somehow merge them together (for example using rechargeable Li-on batteries and a solar panel).
What would be the best way to accomplish it?
EDIT: I should be able to see how much power is left.
Add to your 2 V load at 1800 mWh the power consumption of the RPi. Find a good guess on that at RPi@SE. Given the amount the RPi consumes (20 to 45 Wh a day) your load seems to be negligible for your power supply. So the most simple solution would be to pick a suitable power supply for the RPi and use a switching step down regulator to provide the 2 V.
(Depending on the application the RPi might not be best suited for a portable solution.)