Lipo rider pro + poly li-ion + solar panel

HI i am planning to make a solar panel charger for poly li-ion battery. I have look at the seeedstudio website and found the lipo rider pro and the poly li-ion 6600mAh battery. But for the solar panel, 3W solar panel 138x160, it says “To prevent any damage to boards that accept a narrow range of input voltage, like Lipo Rider, it’s recommended to check whether the OC voltage is safe before any connection” what does it mean to check OC voltage??

According to the description of the rider pro product, it always output 5V right? because I will need to power a 5V sensor and 3.5V microcontroller together (the microcontroller accepts 5V too) and the total current drawn by them is about 55mA or convert to power is 55mA x 5V = 275mW/hour. seems like there is only usb output from this product, but i need to have a Vdd and Ground to power my sensor and microcontroller, What can do here?

Based on the estimation of 4 hours of sunlight/day, do you think the ‘3W solar panel 138x160’ and poly li-ion 6600mAh battery is good enough? The sensor and microcontroller will run 24hours a day