Solar PV monitoring
An independent meter on the PV output, another at the grid connection, and the inverter's own registers beside them for context.
- Measure
- PV AC output, site import and export, inverter state, and irradiance where performance has to be normalised
- Calculate
- Solar generation, self-consumption and export, from monitoring that reconciles
- Sensors
- ZEM-65, Class 0.5S to IEC 62053-22, import and export in both directions ZEM-63 datasheet (PDF, opens in a new tab)
- On site
- ZGW-20 Gateway, the energy balance, calculated where the meters are ZGW-20 datasheet (PDF, opens in a new tab)
Hardware for commercial solar monitoring
Two meters make the balance: one on the generation and one at the connection. The inverter adds detail that the meters cannot see.
The PV AC output, and the site's grid connection
- Choose the CT rating for each point: 120 A split-core, or 300 A, 1 kA and 3 kA Rogowski coils
Class 0.5S to IEC 62053-22 with its CTs, 110 to 480 V line to line
Holds both meters and the inverter data, and calculates the balance on site
Inverter registers: DC strings, AC output, state and alarms
Modbus RTU over RS-485, cable runs to 1,000 m
If performance ratio is being calculated
An irradiance or module-temperature transmitter, where performance is normalised
ZMB-31 and ZIO-20 share one enclosure with different terminals inside, so the ordering code is what you specify.
The measurements a solar energy balance needs
Generation, import and export are three numbers that have to agree. Measure two of them and the third is arithmetic.
| Position | What it tells you | Reference | Sensor |
|---|---|---|---|
| PV AC output | What the plant generated, measured independently of the inverter | State whether the meter covers one inverter, a group or the whole plant ZEM-63 datasheet (PDF, opens in a new tab) | ZEM-65 (on this page), PV output |
| Grid connection | Import and export, and the sign convention everything else is read against | Confirm CT direction under both import and export | ZEM-65 (on this page), connection |
| Inverter registers | DC strings, operating state and alarms that no AC meter can see | Register availability is model and firmware specific ZMB-3X datasheet (PDF, opens in a new tab) | ZMB-31 (on this page) |
| Irradiance | Whether a low day was the plant or the weather | The transmitter's own range, scaling and maintenance plan ZIO-2X datasheet (PDF, opens in a new tab) | ZIO-20 (on this page) |
Inverter and meter will differ. They measure at different points with different losses between them. A stable difference is normal; a changing one is worth investigating.
Sign convention, once. Decide that import is positive and export negative, write it down, and check every meter against it during a known export period.
The energy balance in Edge
Self-consumption is not measured directly. It is what the balance leaves over, so the balance has to be right.
- Self-consumption is derived
- A calculated sensor takes generation minus export, on the same interval, with both inputs explicit.
- A plant that has stopped
- A limit rule on generation during daylight, or a no-data rule on the inverter, catches a plant that stopped without anybody noticing.
- Both directions are kept
- Import and export stay as separate points rather than one net figure, because a net number hides both.
- Onward wherever it is needed
- Edge exports the balance over MQTT or HTTP to a portfolio system, without the site depending on it.
Commissioning checks
Most solar monitoring arguments are about signs and boundaries. Settle both at commissioning.
-
Each meter is on its stated boundary
Trace both meters on the single-line diagram and write what each one includes.
Pass when the boundaries are on the drawing, and nothing is measured twice.
-
Signs are right in both directions
Watch the connection meter during a known import period and during a known export period.
Pass when import is positive and export negative, on every phase.
-
Meter and inverter are compared
Compare the independent AC meter with the inverter's own total over a full day.
Pass when the difference is stable and explained by the measurement points, not drifting.
-
Night and full output both look right
Check the readings at night, at low generation and at full output.
Pass when night generation is zero rather than a small positive or negative number.
-
Derived numbers are labelled
Look at where self-consumption, performance ratio or avoided carbon appear.
Pass when each one says what it was calculated from, and which system calculated it.
What this measurement does not tell you
A meter reports energy at a point. The rest is interpretation, and it needs its method stated.
- Inverter data and an independent AC meter can legitimately differ, because they measure at different points.
- Monitoring does not replace inverter protection, grid-code controls or a revenue meter's obligations.
- Performance ratio and avoided-carbon figures need declared methods, reference data and matching periods.
- A string-level fault may not be visible in the AC total at all.
Sources and related guides
Sources
- IEC 61724 photovoltaic system performance (opens in a new tab) IEC. Monitoring classes, measured quantities and the reference conditions a performance ratio depends on.
- ZEM-63 Wireless 3-Phase Electricity Monitor datasheet (opens in a new tab) EpiSensor. Specifications, ranges and ordering codes.
Related
- Battery demand response Application guide Measure the connection and the battery separately, read state of charge and limits from the BMS, and record events at the resolution the programme wants.
- Generator remote monitoring Application guide Independent electrical measurement of what the generator carried, controller states for why, and fuel level, with the protection boundary left alone.
- Three-phase power calculator Engineering tool Work out active, reactive and apparent power for a balanced three-phase load from voltage, current and power factor.
- Interval demand calculator Engineering tool Turn the energy recorded in one demand interval into average demand in kW.
Reviewed by EpiSensor Engineering on . Revision 4.
Balance generation, import and export on one site
We will place the meters, check the inverter can be read, and write the balance with you.