See http://www.472khz.org/pages/tools/antenna-simulator.php for a very useful tool to estimate the performance of antennas on 472kHz. This tool was produced by Rik ON7YD. Based on my own limited experience you do not need big ERP at all to have fun with WSPR on the band.
If you know your antenna current you can use it to work out your ERP or EIRP. The tool will show how much power you need to reach the ERP or EIRP limit for the band.