Floating Point Resolution

What is the resolution of the floating point numbers reported by the WattNode® meter?

The WattNode for LonWorks® (WNC series) and WattNode Modbus® (WNC series) both can report floating point values for power, energy, voltage, current, power factor, etc. They both use 32 bit IEEE-754 floating point numbers (single precision). This format has a 24 bit mantissa (if you count the hidden bit), so the effective resolution is between one part in 223 (eight million) and one part in 224 (16 million). This supports six or seven decimal digits of resolution.

With the exception of energy values (kWh), the measurement accuracy and resolution of the WattNode will generally be far lower than one part in eight million. For example, the frequency measurement has an internal measurement resolution of about one part in 250; therefore, there isn’t much value in displaying more than one or two digits to the right of the decimal point for frequency.

Power measurements have a very high internal resolution, but the accuracy is only roughly 0.5%, so five significant digits are usually sufficient. See BACnet and Modbus Power Measurement Resolution.

Energy measurements are more complex because of the way energy accumulated over time. See BACnet and Modbus Energy Resolution for details.

Note: Computers support single (32 bit) and double (64 bit) precision floating point numbers, and they may convert the single precision floating point values from the WattNode meter into a double precision value internally, but this will not change the underlying precision or accuracy of the values from the meter.

See Also