Overview

WattNode® BACnet and Modbus meters measure several power values once per second in raw units. The raw measurements are then scaled based on the meter’s calibration, the nominal line voltage, and the current transformer (CT) rated amps. Next, the meter averages the power readings depending on how the Averaging feature set. The default averaging is five seconds, but it can be set to 1 second, 5 seconds, 20 seconds, or 60 seconds. The computed results are stored internally as 32 bit floating point values. When we refer to “power values” we include the following:

PowerSum PowerA PowerB PowerC
PowerReacSum PowerReacA PowerReacB PowerReacC
PowerAppSum PowerAppA PowerAppB PowerAppC

As with any instrument, the power measurements have noise and systematic errors. With the averaging set to one second, the noise will be more apparent. Longer averaging periods will reduce the noise. Systematic errors are not affected by averaging, such as errors due to imperfect calibration, drift, temperature variations, and current transformer phase angle errors.

Raw Measurement Resolution

At full-scale power (nominal line Vac, rated CT amps, and unity power factor), the raw measurement will report approximately 260,000 counts per second, resulting in the following equation for raw resolution: R_R mbox{(watts)} = frac{V_n cdot I_n}{260000}

  • R_R = raw measurement resolution in watts
  • V_n = nominal line AC voltage (120, 230, 277, etc.)
  • I_n = rated CT amps

The following table shows the nominal Vac values for different WattNode BACnet and Modbus models and the corresponding raw measurement resolution using the smallest (5 amp) and largest (6000 amp) current transformers we sell.

Model Vn Nominal VAC 5A CT—RR Raw Resolution in Watts 6000A CT—RR Raw Resolution in Watts
WNC-3Y-208-(BN or MB) 120 0.00231 2.769
WNC-3Y-400-(BN or MB) 230 0.00442 5.308
WNC-3Y-480-(BN or MB) 277 0.00533 6.392
WNC-3Y-600-(BN or MB) 347 0.00667 8.008
WNC-3D-240-(BN or MB) 120 0.00231 2.769
WNC-3D-400-(BN or MB) 230 0.00442 5.308
WNC-3D-480-(BN or MB) 277 0.00533 6.392

Note: The effective resolution may increase due to averaging. We estimate approximate resolution increases of 2X for 5 second averaging, 4X for 20 second averaging, and 6X for 60 second averaging.

Output Resolution

The power measurements of WattNode BACnet and Modbus meters are available as floating point numbers in units of watts. Modbus meters also support integer power registers in units of PowerIntScale.

Floating Point Power Registers

The floating point power registers have a varying resolution, depending on the power value. The WattNode meter uses a 32 bit IEEE-754 floating point number representation. This has a 24 bit mantissa (including the hidden bit), so the effective resolution will be between one part in 223 (eight million) and one part 224 (16 million). This supports six or seven decimal digits of resolution. At larger power values, the resolution progressively drops (in watts). For example, the following table shows the output resolution for several different power values. The second column shows the effective seven digit decimal resolution as bold digits.

Power Seven Digit Resolution Approximate Resolution (W) Approximate Resolution (% of reading)
100 W 100.0000 W 0.0001 W 0.0001%
500 W 500.0000 W 0.0001 W 0.00002%
1000 W (1 kW) 1000.000 W 0.001 W 0.0001%
5000 W (5 kW) 5000.000 W 0.001 W 0.00002%
10,000 W (10 kW) 10,000.00 W 0.01 W 0.0001%
50,000 W (50 kW) 50,000.00 W 0.01 W 0.00002%
100,000 W (100 kW) 100,000.0 W 0.1 W 0.0001%
500,000 W (500 kW) 500,000.0 W 0.1 W 0.00002%
1000 kW (1 MW) 1000.000 kW 0.001 kW (1 W) 0.0001%
5000 kW (5 MW) 5000.000 kW 0.001 kW (1 W) 0.00002%

The raw internal measurement resolution with five second averaging is about one part in 1.3 million for a nominal power; dropping in proportion to the power. So at 1% of nominal power the resolution would drop to about one part in 13,000. The floating point resolution of a 32 bit float is between one part in eight million and one part in 16 million, regardless of the power.

Integer Power Registers

The WattNode Modbus integer power registers normally auto-scale to one of the values in the following table so that 120% of the nominal full-scale power won’t overflow. For example, if you are using 50 amp CTs with a WNC-3Y-208-MB, the full scale power is 120 V * 50 A * 3 phases = 18000 watts, so PowerIntScale would be 1, and the resolution would be one watt.

PowerIntScale Power Resolution Maximum Power Reading
0 (default) Auto-configure
1 1 watt ±32767 W
10 10 watt ±327.67 kW
100 100 watts ±3276.7 kW
1000 1000 watts ±32767 kW


If the power reading is large, then the resolution as a percentage of the reading will be better. As the reading gets smaller, the resolution will get worse. For example, the following table shows the resolution as a percentage of reading for different power levels. In all cases, PowerIntScale = 1 watt.

Power Resolution (% of reading) Resolution (watts)
20000 W 0.005% 1 W
10000 W 0.01% 1 W
5000 W 0.02% 1 W
2000 W 0.05% 1 W
1000 W 0.1% 1 W
500 W 0.2% 1 W
200 W 0.5% 1 W
100 W 1.0% 1 W

Summary

Depending on the WattNode model and the CT rated amps, the power resolution will normally be limited by the raw internal measurement if you are using the floating point registers. Whereas if you use the integer power registers, then the integer resolution will be much lower than the internal resolution. For floating point values, you can somewhat increase the effective resolution increasing the averaging to 20 or 60 seconds. In any case, WattNode meter accuracy (~0.5% or one part in 200) is normally the primary limitation, and also the noise will generally be higher than the floating point resolution.

See Also