Wireless power transfer efficiency calculation
See this official WPC link for example. You can't know for sure unless you measure the Tx and Rx powers, not fields -- directly calculate the voltage and current, cos\$\phi\$ included (you're transmitting active power). As an informative number, readily found kits work with an efficiency of 50%-70%, due to the air core coupling and relative distance, but it can be made better with less distance and, perhaps, an opened magnetic core -- but don't forget that the frequency lies between 105kHz-210kHz (IIRC). For more details, it's always a good idea to see what the WPC has to say about it, it's a bunch of links with information, very useful.