The output wattage is not the key measure here. The important number is the input wattage.
21v is 25.2 watts
29V is 40.6 Watts
The total consumption is the sum of the heat given off by the SWiM switchgear plus the heat given off by the PI itself as part of the conversion to 29VDC.
You must measure the wattage (the amperage multiplied by the voltage) going in and use that number to multiply by your electricity rate. CONUS residential electricity rates vary from 8.75 CENTS/Kwh (Washington state) to 19.87 CENTS/Kwh (Connecticut). The average rate across all of the US (including AK and HI) is 12.31 CENTS/Kwh.