Let's check by calculating the ET for the last hour in your database, a time when, hopefully, there is a meaningful ET. Use this select statement:
select min(datetime), max(dateTime), max(outTemp), min(outTemp), avg(radiation), avg(windSpeed), max(outHumidity), min(outHumidity), min(usUnits), max(usUnits) from archive where dateTime>(select max(dateTime)-3600 from archive);
We'll also need the values of ET:
select ET from archive where dateTime>(select max(dateTime)-3600 from archive);
Send me the results, and I'll do the calculation.
-tk