With power meters, communication often works but the values look wrong. The symptom narrows the cause: off by a constant factor means CT ratio or scale, complete nonsense means word order or data type, negative means direction or sign. The examples below are manufacturer manual contents collected in this site's device maps.
Causes by symptom
| Symptom | Check first |
|---|---|
| Current or power off by a constant factor (e.g. 40×) | CT ratio: the meter setting, and whether the register holds primary or secondary values |
| Off by 10, 100 or 1000 | Scale (÷10 kWh, ÷100 Hz, ÷1000 PF, …): the manual's conversion table |
| Nonsense values (huge or tiny numbers, floats like 1e-40) | 32-bit word order (ABCD, CDAB), integer vs float |
| Value from the neighbouring register | 0-based vs 1-based (40001) addresses, table 03 vs 04 |
| Negative active power or PF | Sign convention (import/export, leading/lagging), CT direction |
| Energy suddenly smaller | Rollover past the maximum, or a value cleared over communication |
1. Primary or secondary values?
It depends on the meter. The LS ELECTRIC GIMAC1000 device map says «all measurements are Float32 in actual units with PT/CT ratios applied», and Rootech Accura states that PT/CT multipliers are not applied separately. The Accuenergy Acuvim-L word table from 0130h, on the other hand, holds secondary values that must be multiplied as in the manual's Table 4-27.
Acuvim-L from 0130h (Table 4-27) Voltage U = Rx × (PT1 / PT2) / 10 [V] Current I = Rx × (CT1 / CT2) / 1000 [A] Power P = Rx × (PT1 / PT2) × (CT1 / CT2) [W] CT 200/5, I1 register = 3750 → 3.750 A × 40 = 150 A
So «40 times too big after multiplying by the CT ratio» means you multiplied a register that was already primary, and «40 times too small» means you used a secondary value as is. If the meter's own CT setting differs from the real CT, both the display and the Modbus values are off by that ratio; use the CT ratio calculator for the ratio.
2. Scale and units
Integer registers drop the decimal point. On the Acuvim-L, frequency is ÷100 (1388h = 50.00 Hz), power factor ÷1000 (−1000 to 1000) and energy ÷10 kWh. Use the manual's conversion table or the scale column in the device map. Some meters have both an integer table and a float table; decide which one you read first.
3. Word order of 32-bit values
Energy and float values take two registers, and the wrong order gives nonsense. Eastron SDM meters send floats as ABCD (high word first); the GIMAC1000 switches between ABCD and CDAB with its Swap setting. The Acuvim-L puts energy Dwords high word first: the manual's write example puts 0A9D 4089h into 0156h/0157h, which is 178077833, ÷10 = 17807783.3 kWh. Some devices, such as the Fuji Electric PXH, send the low word first. If unsure, put both registers into the float converter to see all four orders at once.
4. Sign: import/export, leading/lagging
- Acuvim-L phase powers are signed 16-bit (−32768 to 32767), and energy has separate import (Ep_imp) and export (Ep_exp) registers. Reading a signed value as unsigned 16-bit shows negatives as numbers in the 60,000s.
- The meaning of a negative power factor differs by meter (on the GIMAC1000, negative is leading). Check the manual's sign convention.
- Negative active power on a site with only loads means the meter measures in the export direction; compare the CT direction and phase wiring with the wiring diagram in the meter manual.
5. When energy goes down
Accumulated energy has a maximum (the Acuvim-L energy register range is 0 to 999999999, i.e. 99999999.9 kWh); past it, counting starts over. The Acuvim-L energy registers are also writable with FC16, so another program may have cleared or preset them. In logging, treat «smaller than the previous value» as a rollover.
6. If it still differs
Compare the meter display and the Modbus value at the same moment. If the display is wrong too, it is the meter setup (CT ratio, wiring type 3LN, 3LL, …); if the display is right and only Modbus is wrong, it is the address, type, scale or word order. Off-by-one addresses are covered in the address notation guide, 32-bit values in the word order guide.
Sources
- Accuenergy Acuvim-L Series Power Meter User's Manual V2.01 (1030E2201) — printed p.93 FC16 example, pp.135-136 Tables 4-26/4-27, pp.146-147 Tables 4-37/4-38
- LS ELECTRIC GIMAC1000 Modbus Map (Tech Data, 2024-02) — Float32 values with PT/CT applied
- LS ELECTRIC GIMAC1000 User Manual (2023-03) — p.42 Swap (ABCD/CDAB)
- Eastron SDM630-DI User Manual V1.02 — float registers (ABCD)
Published 2026-10-30