Low Voltage.
I suggest you very carefully experiment and verify what your PIC will do with a 1.5 volt supply. The datasheets are silent about such operation. I had a bunch of data loggers where the 3.7 volt battery went completely flat and the processor code was all erased - not scrambled - erased. The first 60 +/- bytes looked valid but from there to top of memory all locations read FF. Normally my loggers come back while the 3.7 volt Lithium Thionyl still has some charge left but I was shocked to find that all 6 loggers with the flat batteries had erased the program code and scrambled the same three EEROM locations as well. Fortunately the data stored in the external flash memory was intact. There must be a low voltage erase mechanism in the 18F4620 as the only voltage source in my loggers is the 3.7 volt button cell. LVP was turned off in the config fuses but the pin was floating.

Current draw.
Getting the "nano-watt technology" to draw nanowatts is quite a challenge. So many registers must be set/cleared and powering down the peripherals like external memory must be sequenced just right or you lose data or get excessive current spikes.
My data loggers have a PIC18LF4620 with two M25P64 for 16 MByte storage plus an ECG amplifier, accelerometer, pressure transducer, RTC and temperature sensor. The RTC and PIC are permanently powered up but the M25P64, amplifier and other sensors are power switched through PNP transistors. The PIC has a 4 MHz crystal.

The lowest current draw I can get down to is a sleep at around 3 microamps or about 10 microwatts. Processor, amplifiers and peripheral current is 3 mA and during the flash write to the M25P64 the current jumps to 12 mA for less than a second. Hardly nanowatts by my definition.

If your logger draws less curent I would like to know how you did it.

Cheers
Brian