My plan is to use 2x nimh batteries. Nimh can need up to about 1.5V before they are fully charged (depends on how fast the solar panel can charge them but for a small panel charging will probably be less than 0.1 so 1.45V should be enough to fully charge) These lights usually charge directly from the solar panen to avoid having to add a charging circuit, to keep cost and complexity down. 3x1.45V=4.35V which is more than most 3.3V components can handle. The atmega328 works down to 1.8 with lowest BOD. Rfm69 works down to 1.8. Nrf24 works down to 1.9. At 1V a nimh is almost totally exhausted. So 2x nimh gives enough voltage at the low end for the components I usually use. This avoids adding a step up/down which means lower complexity and cost, and no loss of efficiency due to conversion. These are just my thoughts so far. Hopefully improvements can be made