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.	0	FC03	uint16		R
l Length of the model block in registers.	1	FC03	uint16		R
a Total AC current, sum of all phases, amperes.	2	FC03	int16	A	R
a_sf Scale factor for AC current register.	3	FC03	int16		R
phv Phase voltage, line-to-neutral, volts.	4	FC03	int16	V	R
v_sf Scale factor for voltage register.	5	FC03	int16		R
hz Grid frequency in hertz.	6	FC03	int16	Hz	R
hz_sf Scale factor for frequency register.	7	FC03	int16		R
w Total real (active) power, watts, positive is import.	8	FC03	int16	W	R
w_sf Scale factor for real power register.	9	FC03	int16		R
va Total apparent power, volt-amperes.	10	FC03	int16	VA	R
va_sf Scale factor for apparent power register.	11	FC03	int16		R
var Total reactive power, volt-amperes reactive.	12	FC03	int16	var	R
var_sf Scale factor for reactive power register.	13	FC03	int16		R
pf Power factor, expressed as a percentage.	14	FC03	int16	Pct	R
pf_sf Scale factor for power factor register.	15	FC03	int16		R
totwhexp Cumulative active energy exported, watt-hours.	16	FC03	uint32	Wh	R
totwhimp Cumulative active energy imported, watt-hours.	18	FC03	uint32	Wh	R
totwh_sf Scale factor for cumulative active energy registers.	20	FC03	int16		R
totvahexp Cumulative apparent energy exported, volt-ampere-hours.	21	FC03	uint32	VAh	R
totvahimp Cumulative apparent energy imported, volt-ampere-hours.	23	FC03	uint32	VAh	R
totvah_sf Scale factor for cumulative apparent energy registers.	25	FC03	int16		R
totvarhimpq1 Cumulative reactive energy imported, quadrant 1, varh.	26	FC03	uint32	varh	R
totvarhimpq2 Cumulative reactive energy imported, quadrant 2, varh.	28	FC03	uint32	varh	R
totvarhexpq3 Cumulative reactive energy exported, quadrant 3, varh.	30	FC03	uint32	varh	R
totvarhexpq4 Cumulative reactive energy exported, quadrant 4, varh.	32	FC03	uint32	varh	R
totvarh_sf Scale factor for cumulative reactive energy registers.	34	FC03	int16		R
evt Meter event bitmask indicating fault or status flags.	35	FC03	uint32		R
ts Unix timestamp of the signed reading snapshot, seconds.	38	FC03	uint32		R
ms Millisecond offset for the timestamp, 0 to 999.	40	FC03	uint16		R
seq Sequence number incremented with each new signed snapshot.	41	FC03	uint16		R
alg Identifier of the cryptographic algorithm used for the signature.	42	FC03	uint16		R
n Number of digital signature blocks that follow in the response.	43	FC03	uint16		R