Why do current or power readings appear to be off by a factor of ten?

SunSpec uses paired scale-factor registers. Read a_sf alongside a, then compute the real value as a * 10^a_sf. The same pattern applies to every measured quantity. If your client ignores scale factors, all readings will be systematically wrong.

The gateway connects but all registers return zero or NaN. What now?

Confirm the meter has Modbus TCP enabled and is actively measuring an energised circuit. Check that the Modbus unit ID in the portal matches the meter configuration. Some meters only expose SunSpec registers when the metered circuit carries live voltage.

Why does the ts register never change between polls?

The timestamp and seq registers are updated only when the meter generates a new signed snapshot. If the meter's signing interval is longer than your poll interval, ts will appear static. Check the meter's signing or logging period in its configuration interface.

What does the alg register value represent, and can I ignore it?

The alg register identifies the cryptographic algorithm the meter uses to sign the reading snapshot. It is informational for most monitoring setups, but if you are using the signature for compliance or billing verification, record it alongside the signature blocks and the ts value to reconstruct a verifiable audit trail.

Free Modbus template: SunSpec Model 220 — Secure AC Meter Selected Readings

Name: SunSpec Model 220 — Secure AC Meter Selected Readings
Brand: SunSpec
SKU: 121e96f4-c53d-464d-8589-7035f588e9fa

SunSpec Model 220 is a standardized AC meter information model defined by the SunSpec Alliance, used by any SunSpec-compliant energy meter that supports cryptographically signed readings. The template reads instantaneous measurements, including current, phase voltage, grid frequency, real power, apparent power, reactive power, and power factor, each paired with its scale factor. It also captures four-quadrant cumulative energy totals (active, apparent, and reactive), plus tamper-evident security fields: a Unix timestamp, millisecond offset, sequence number, cryptographic algorithm identifier, and signature block count. This template enables verifiable, audit-ready energy monitoring over Modbus TCP.

Registers (33)

33 read / 0 write

Name	Address	Function	Type	Unit	Access
id SunSpec model identifier, fixed value 220 for this model.	•••	•••	•••		R
l Length of the model block in registers.	•••	•••	•••		R
a Total AC current, sum of all phases, amperes.	•••	•••	•••	A	R
a_sf Scale factor for AC current register.	•••	•••	•••		R
phv Phase voltage, line-to-neutral, volts.	•••	•••	•••	V	R
v_sf Scale factor for voltage register.	•••	•••	•••		R
hz Grid frequency in hertz.	•••	•••	•••	Hz	R
hz_sf Scale factor for frequency register.	•••	•••	•••		R
w Total real (active) power, watts, positive is import.	•••	•••	•••	W	R
w_sf Scale factor for real power register.	•••	•••	•••		R
va Total apparent power, volt-amperes.	•••	•••	•••	VA	R
va_sf Scale factor for apparent power register.	•••	•••	•••		R
var Total reactive power, volt-amperes reactive.	•••	•••	•••	var	R
var_sf Scale factor for reactive power register.	•••	•••	•••		R
pf Power factor, expressed as a percentage.	•••	•••	•••	Pct	R
pf_sf Scale factor for power factor register.	•••	•••	•••		R
totwhexp Cumulative active energy exported, watt-hours.	•••	•••	•••	Wh	R
totwhimp Cumulative active energy imported, watt-hours.	•••	•••	•••	Wh	R
totwh_sf Scale factor for cumulative active energy registers.	•••	•••	•••		R
totvahexp Cumulative apparent energy exported, volt-ampere-hours.	•••	•••	•••	VAh	R
totvahimp Cumulative apparent energy imported, volt-ampere-hours.	•••	•••	•••	VAh	R
totvah_sf Scale factor for cumulative apparent energy registers.	•••	•••	•••		R
totvarhimpq1 Cumulative reactive energy imported, quadrant 1, varh.	•••	•••	•••	varh	R
totvarhimpq2 Cumulative reactive energy imported, quadrant 2, varh.	•••	•••	•••	varh	R
totvarhexpq3 Cumulative reactive energy exported, quadrant 3, varh.	•••	•••	•••	varh	R
totvarhexpq4 Cumulative reactive energy exported, quadrant 4, varh.	•••	•••	•••	varh	R
totvarh_sf Scale factor for cumulative reactive energy registers.	•••	•••	•••		R
evt Meter event bitmask indicating fault or status flags.	•••	•••	•••		R
ts Unix timestamp of the signed reading snapshot, seconds.	•••	•••	•••		R
ms Millisecond offset for the timestamp, 0 to 999.	•••	•••	•••		R
seq Sequence number incremented with each new signed snapshot.	•••	•••	•••		R
alg Identifier of the cryptographic algorithm used for the signature.	•••	•••	•••		R
n Number of digital signature blocks that follow in the response.	•••	•••	•••		R

Loading registers…

SunSpec Model 220 — Secure AC Meter Selected Readings

Overview

Use cases

Installation

Things to watch for

Registers (33)

Working with Modbus in your projects?

Question about your setup?

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