Ok, can you read from the mysql database the last valid value for the feeds and then update the last value to reflect the correct value?
Are you using redis by any chance to cache the last value of the feeds?
Is there a particular reason you cant use the PHPFina and PHPTimeSeries feed engines? I would recommend using these engines, there are actually post processing tools for power to kWh feeds available for the PHPFina feed engine.
You might be interested in @nchaveiro’s recent post on the topic of converting mysql feeds to phptimeseries in his case: Convert mySQL feeds to what? - #2 by TrystanLea