I've just measured two different but conflicting readings using an off the shelf watt meter. My quandary is that these measurements conflict and seem untrustworthy.

My question is: whether measuring from a power board or the wall plug matters or if there is something else at play here that I've missed?


  • Reader: Arlec Electrical Meter (PC222)
  • Power Board: HPM 12 Socket with surge protection and indicator light
  • Device 1 & 2 & 3 Connected to power board. Power board connected to wall plug.

Notes: Device 2 runs under seemingly constant load/utilization for both readings.


1st Reading: 820W measured at power board plug from Device 2. ~247V. PF of 97%

2nd Reading: 220W measured at wall plug of power board with Devices 1&3 on and consuming.

3nd Reading: 630W measured at wall plug of power board with all devices consuming. ~247V. PF of 87%

Edit: These devices are electronic and 2 have computer PSU's.

  • \$\begingroup\$ PF has a value no greater than 1. \$\endgroup\$ – Andy aka Jan 11 '18 at 8:58
  • \$\begingroup\$ You have provided no links to the datasheets for these devices. We don't know whether you are exceeding the ratings, or trying to measure a few hundred watts with a device intended for hundreds of kilowatts. \$\endgroup\$ – Transistor Jan 11 '18 at 9:25

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