accurate power consumption at the wall should employ a kill-a-watt. the SP readings are only as accurate as the PSU provides, whcih likely has a +/-5% range.
If low temps are an issue, why not reduce fan speed to make the unit a bit hotter internally?
My Kill-A-Watt is only working for 125V, has a 5%-15% variance in accuracy -- 15% at the ends of the measured range and is good up to 15A.
The unit has a quality check but is not calibrated in any way. IMHO, the PSU readings are as good as Kill-A-Watt readings.
More expensive Wattmeters might give you what you need, but do NOT rely on a $20.00 piece of electronics for high accuracy measurements. Look for units which come with NIST calibration certificates, as they usually will check the whole instrument span and tell you the expected variation in readings (3 points, 5 points, 9 points certificates are usually available) under the form of a curve.
Funny, my SP30 started to suffer from the 2.5TH drop recently .. where the bottom board completely shuts off >>
http://powerprice.info/SPT/graphic.phpLOL Now it died..