Whatever I buy, I has to work on rechargeable batteries alone. The Kindle set up is going into the boonies, where there will be no electric utility power, ever. To run appliances, I use an array of 50W solar panels, a bank of 12V 35Ah sealed lead-acid batteries, a voltage regulator, and either a power inverter (12VDC to 120VAC) or a car cigarette plug charger (12VDC to 4.2VDC) for 18650 Li-ion batteries.
Can the Raspberry Pi run on batteries alone? If so, it might be just the thing.