The P4460 is a bit more expensive but retains it's power usage data if power is turned off and will convert its readings into daily, weekly, monthly, and annual charges.
Scroll down for on this study. http://webelieveinmarriage.net/__media__/js/netsoltrademark.php?d=sites.google.com%2Fview%2Fhowmuchdoesanseoauditcost%2Fhome