Energy Meter Pulse Calculator
Power (kW) and energy (kWh) from a meter's pulse constant (imp/kWh), pulse interval or count; CT/VT multiplier and the pulse rate a PLC counter must handle.
Formulas
- One pulse = 1 ÷ C kWh = 1000 ÷ C Wh (C = pulse constant, imp/kWh)
- Pulse interval T seconds → power P [kW] = 3600 ÷ (C × T)
- N pulses in t seconds → energy E = N ÷ C [kWh], average power = 3600 × N ÷ (C × t) [kW]
- Pulse rate at power P: f = P × C ÷ 3600 [Hz], period = 1 ÷ f
- On a transformer-connected meter with secondary-side pulses, primary value = secondary value × CT ratio × VT ratio (the multiplier)
Pulse interval by pulse constant (primary, multiplier 1)
| imp/kWh | One pulse | 0.1 kW | 1 kW | 10 kW | 100 kW | Rate at 100 kW |
|---|---|---|---|---|---|---|
| 100 | 10 Wh | 6 min | 36 s | 3.6 s | 360 ms | 2.78 Hz |
| 400 | 2.5 Wh | 1.5 min | 9 s | 900 ms | 90 ms | 11.1 Hz |
| 1000 | 1 Wh | 36 s | 3.6 s | 360 ms | 36 ms | 27.8 Hz |
| 3200 | 0.3125 Wh | 11.3 s | 1.13 s | 113 ms | 11.3 ms | 88.9 Hz |
| 10000 | 0.1 Wh | 3.6 s | 360 ms | 36 ms | 3.6 ms | 278 Hz |
The smaller the load, the longer between pulses, so a short-window average jumps by a whole pulse (e.g. 100 imp/kWh at 0.1 kW gives one pulse every 6 minutes). With a high constant and a large load, the rate climbs and the counter's input limit and the pulse width matter.
How to read it
- «Power from pulse interval» is the average between the last two pulses. With a fluctuating load, the «average power» over many pulses is steadier.
- Averaging pulse counts over one minute gives a resolution of one pulse per minute: at 1000 imp/kWh that is 1 Wh ÷ 60 s = 60 W steps.
- Applying the multiplier twice, or not at all, is a common mistake. Check separately in the manual whether the meter display is already primary (models with a CT ratio setting) and whether the pulses are too.
- If the pulse width (ms) exceeds half the period, pulses run together at full load and a counter may miss them. Also check the counter input's minimum ON and OFF times in the PLC manual.
Where it is used
- Counting meter pulses with a PLC high-speed counter, digital input or data logger — the conversion factor and input limits.
- Collecting per-machine energy from meters without Modbus, or reconciling Modbus readings with pulse counts.
- When logged values are off by a factor of tens after a CT change — checking where the multiplier goes.
FAQ
How do I calculate power from imp/kWh?
P[kW] = 3600 ÷ (pulse constant × pulse interval in seconds). At 2000 imp/kWh with a pulse every 1.8 s: 3600 ÷ (2000 × 1.8) = 1 kW.
How do I convert pulse counts to kWh?
Pulses ÷ pulse constant. At 1000 imp/kWh, 2500 pulses are 2.5 kWh. For secondary-side pulses, multiply by CT ratio × VT ratio.
What is the multiplier with a 200/5 CT?
200 ÷ 5 = 40. With a 6600/110 VT, multiply by another 60 for 2400.
Can a plain PLC input count the pulses?
Only if the pulse rate at maximum power is low enough for the input filter time and scan time not to miss pulses. Compare «Pulse rate at maximum power» above with the PLC input specification.
For CT ratio, secondary current and burden, use the CT Ratio Calculator. For kW, kVA and power factor, use the Three-Phase Power Calculator. For power meter addresses read over Modbus, see the device register maps.
Basis
These are definitions: 1 kWh is the energy of 1 kW for one hour (3600 s), and the hour h = 3600 s is listed in the SI Brochure table of non-SI units accepted for use with the SI. The multiplier is the product of the transformer ratios. Pulse constant, pulse width and primary/secondary basis vary by product, so enter the values from the meter's manual. BIPM SI Brochure.