Performance Equations (PE) are appropriate for a great varierty of usages but sometimes other technologies are more suitable. It looks to me like PI Totalizer would be more appropriate for what you are trying to achieve. Just like PE, using Totalizer is done via PI Points which can be configured using a plug-in for SMT3 (Points > Totalizers, as opposed to Points > Performance Equations).
I invite you to read the "PI Totalizer Subsystem" chapter in the PI Server Applications User Guide, which is available on the vCampus Library (under vCampus PI Products Set > PI Server). Should you require more assistance on creating and configuring those, please do not hesitate to request assistance: our regular Technical Support team (firstname.lastname@example.org - +1-510-297-5828) for regular PI Points management topics and the vCampus discussion forums for programming/scripting matters (should you want to deal with Totalizers in programmatic manner).