Here's the main battery kWh comparisons of the plug-in hybrids in the US. Keep in mind that these numbers may refer to full capacity, not necessarily the usable capacity. Please correct me if I am in err:
2013 Chevy Volt = 16kWh
2013 Ford Fusion Energi = 7.6kWh
2013 Ford C-Max Energi = 7.6kWh...