Determine the output voltage of an integrated ciruit
It's not a very clear datasheet, but the answer is in this paragraph of the IC's datasheet:


  To prevent rechargeable batteries from being exposed to excessive
  charging voltages and to prevent over charging a capacitive storage
  element, the over-voltage (VBAT_OV) threshold level must be set using
  external resistors. This is also the voltage value to which the
  charger will regulate the VSTOR/VBAT pin when the input has sufficient
  power. The VBAT_OV threshold when the battery voltage is rising is
  given by Equation 3:


Tie the load to VSTOR. The device will then try to maintain the voltage given by that equation applied to the programming resistor network. For the evaluation board, it looks like this is 3V.

On the other hand, that means you need a rechargeable battery with a characteristic voltage of about 3V, which is inconvenient. It would be better if the "wsn" device could handle up to 4.2V then you could use LiPo batteries or similar.