I saw the PowerHouse eMonitor on Ask This Old House, and it looks like a very neat setup. It uses small, relatively inexpensive current transducers ($20 each) to measure each line. They clip over the wire just like a clamp-on ammeter, as seen below.
Have you seen any other sources for these in the 15-20A range? And pretty accurate? Considering the cost of transducers, their packages actually are pretty reasonably priced… I’d just rather create something myself.