PV monitoring system application note

This page accompanies the main OpenEnergyMonitor guide, which details the main step by step process of building and configuring an OpenEnergyMonitor system.

This page documents application specific notes on building a wireless web-connected PV system monitor to measure generation, and grid import/export.



As detailed in the main guide, an OpenEnergyMonitor system comprises wireless sensor nodes that periodically send data to a web-connected base-station. The wireless sensor node used to monitor the PV system is the multi purpose board called emonTx.

Connected to the emonTx are two clip-on CT sensors and an AC-AC plug-in voltage adapter. These are used to sense the PV system generation, and consumption. It is possible to monitor the LED pulses from a pulse output utility meter to measure the energy flow. However, CT sensors yield a more accurate measurement of instantaneous power. When monitoring power using the pulse counting method, the accuracy is determined by the meter's pulse rate. When minimal power is flowing, the duration between pulses can be extremely long.

Select the current and voltage emonTx firmware when you reach the upload the configuration specific firmware step of the main OpenEnergyMonitor guide. Note: the emonTx V2, emonTx V3.2 and emonTx V3.4 use different versions of firmware.

1. Install sensors & setup emonTx

Users in North America should consult the Building Blocks guide "EmonTx - Use in North America." This is predated by some Forum discussions on US installations at http://openenergymonitor.org/emon/node/711 and http://openenergymonitor.org/emon/node/3265.

Where the CT sensors are connected depends on how the PV system has been installed.
For reference, we call these systems Type 1 and Type 2. 


Type 1

Use this when the generation and consumption can be monitored separately. The amount exported/imported to or from the grid is simply the difference between generation and consumption. Knowledge of the direction of the current is not required, therefore a plug-in AC-AC voltage sensor adapter is not essential but is still recommended for accurate readings. For a Type 1 solar PV monitoring system, the grid import/export is calculated as follows:

Grid (import/export) = Consumption – Generation

Consumption and Generation should be positive, reverse the orientation of the clip-on CT sensor if either is not. Nett Grid Power will be positive when importing, and negative when exporting.

Type 2

Use this when the generation and consumption cannot be monitored separately, i.e the PV inverter output is fed into a spare MCB (circuit breaker) in the fuse box. Other household loads are connected to other circuits in the same fuse box. If this is the case, the output from the PV inverter and the grid import/export connection will need to be monitored instead. Knowledge of the direction of the current is required to determine the difference between power import and export, therefore an AC-AC voltage sensor adapter is required.

When the AC-AC voltage sensor is used, (see below) the sign of the grid import/export power reading will depend on the orientation of the CT. To be compatible with the software examples included with this documentation, it is desirable to orient the CT on the grid import/export cable so the power reading is positive when importing, and negative when exporting. The correct orientation can be determined by trial and error.

Household power consumption can be calculated in software:

Power consumption = Solar PV generation + Grid import ('Grid import' is negative when exporting)

AC-AC voltage sensor adapter

In order to determine the direction of the current flow (important for being able to tell the difference between grid import or export) an AC voltage reading is needed to provide a reference point. This is explained in detail in the Building Blocks article An introduction to AC Power.

The emonTx obtains a voltage sample using a 9 Volt AC-AC plug-in transformer. Using this adapter enables us to monitor the RMS AC voltage, and thus, calculate real power and power factor. For information about how the AC-AC adapter and AC RMS reading works, see the relevant section in Building Blocks. A nearby power socket is required for the AC-AC adapter.

Note: The emonTx V2 cannot be powered from this AC-AC adapter, but the emonTx V3 can. Special measures were taken in the design of the emonTx V3 power circuits to minimise the distortion caused by rectifying and loading the AC-AC adapter, and its affects on the sampling of the AC waveform. A separate 5V USB power supply, or batteries, must be used to power the emonTx V2.

CT sensor connections

General note regarding the installation of CT sensors: the clip-on CT sensors must be clipped round either the live (brown in the UK), or neutral (blue in the UK) wire. Not both. It is sometimes necessary to carefully remove plastic ducting (sheathing/insulation) to access the live and neutral wires. Live terminals should not be exposed but if in doubt, switch off the power before investigating. A shock from AC mains electricity can be fatal. If in doubt, consult the advice of an experienced electrician.

The CT sensors should be connected to the emonTx before being clipped round a live cable.


Above: CT installation on the live wire AC output from the PV inverter. Generation meter can be seen at the top right.

Solar Display

You might want to add a solar energy display to your setup. There is PV monitor specific firmware that can be used for that. Once you have installed the Arduino IDE, libraries and firmware following the Setting up the arduino environment guide, the PV monitor firmware for the display can be selected by going to:

File > Sketchbook > OpenEnergyMonitor > EmonGLCD > EmonGLCD_SolarPV



This emonGLCD was wall mounted in a convenient location. The purpose of the emonGLCD is to give the homeowner an instant reading of house power usage compared to PV system generation.

It has been suggested that a homeowner might attempt to reduce their consumption when a low amount of power is being generated, and perform non-essential tasks that consume more power such as laundry, dishwasher etc. when excess power is generated. To give a clear indication when excess power is generated, the tri-colour LED's on the top of the emonGLCD are green when power is exported, (excess power being generated) and red when power is being imported (power deficit).


Financial benefits aside, it was found homeowners felt personal satisfaction by keeping the lights green when possible. Behavioral change due to domestic PV system monitoring is an interesting topic. Several research papers have been written on the topic. The papers highlight the importance of monitoring and clear indication of both generation and consumption, to prompt changes in behavior. The monitoring system described here gives far superior user indication compared to those used in the research below

Behavioural responses to photovoltaic systems in the UK domestic sector, James Keirstead, Lower Carbon Futures, Environmental Change Institute, University of Oxford, UK, 2007.

Photovoltaics in the UK domestic sector: a double-dividend? , James Keirstead, Lower Carbon Futures, Environmental Change Institute, University of Oxford, UK, 2005.


Example of the kind of dashboard that can be created in emoncms to explore PV system energy production. See the emoncms user guide for instructions on using emoncms.