Technology that helps us live within ecological limits

The OpenEnergyMonitor System

System Overview

The system is an open source monitoring system with the capability to monitor electricity, temperature, humidity and interface with pulse output meters such as water meter's and smart electricity meters.

The system is made up of five main parts: the emonTx, emonTH, emonGLCD, emonBase and emoncms.

These can be assembled and configured to work for a variety of applications from home energy monitoring to solar PV import/export monitoring and building fabric thermal performance monitoring.

The system is fully open source both hardware and software with documentation on everything from AC theory to sensor circuit design and application programming available on the website.


The emonTx is the main sensor node. It usually sits next to the electricity meter box and measures electricity being used or generated via clip on CT (Current Transformer) sensors and an AC-AC adapter to power the unit and provide an AC voltage sample for Real Power and Power Factor calculations. The emonTx can also be powered via 3 x AA batteries (Apparent Power readings only)

In addition to CT based electricity monitoring the emonTx has a pulse counting input which can be used for interfacing with smart meters and certain types of water meters. It also has a DS18B20 digital temperature sensing input which supports multiple DS18B20 temperature sensors on a single bus.

emonTx: Documentation, Shop


The emonGLCD is a table top LCD display, designed for showing information such as current power consumption in a glance-able fashion. The display is a standard 128px by 64px LCD and so is fully programmable. The standard firmware can either show a simple home energy monitor type display or a solar pv import vs export display with ambient LED's that glow green when a household is exporting and red when importing.

The EmonGLCD is currently only available in kit form and so requires manual assembly, a pre-assembled version will be available in 2014.

emonGLCD: Documentation, Shop


The emonTH is a long lasting, easy to deploy wireless temperature and humidity sensing node designed for use in building thermal performance monitoring. The emonTH is powered by 2x AA batteries and is available with a choice of either DS18B20 based temperature sensing or DHT22 based Temperature and Humidity sensing. An external DS18B20 temperature sensor can easily be wired into screw terminal connector to provide external temperature readings.

emonTH: Documentation, Shop


The emonBase is the internet gateway, relaying readings received wirelessly from sensor nodes to emoncms, which is an open source energy monitoring web application (see below). There are several basestation options but the recommended basestation is based on the Raspberry PI a popular low cost Linux computer. The RaspberryPI basestation can run a full LAMP web server with emoncms installed for local data-logging if used with an external harddrive or it can be configured to forward data directly to a remote server such as

emonBase: Documentation, Shop

emoncms is an open-source web application for processing, storing, visualising energy and environmental data. One common application of emoncms is to explore historical electricity consumption data, zooming in at any point, either to see instantaneous power over seconds, minutes or hours or to see daily kwh use and what percentage of a day is used by loads of different power ranges ie: 0-1000W, 2000-3000W.

Development is also under way to integrate building energy modelling and coheating test tools into emoncms with the recent first release of an implementation of the SAP model (The UK governments standard assessment procedure for assessing the energy performance of buildings).

emoncms: Documentation, Building energy modelling

RFM12B 433Mhz, 868Mhz radio

The system uses a commonly used HopeRF (RFM12B) on the ISM frequency bands 433Mhz (worldwide) or 868Mhz (Europe only) transceiver to allow low cost two way communication between the hardware modules in the system. Up to 30 wireless nodes can be connected to a single emonBase on a single network group. Up to 250 different network groups are possible.

The line of sight RF range is up to 300m. In practice with walls and other obstacles in the way; the RF range is very similar to the range from a standard domestic WIFI router: usually sufficient to get around a standard house.

rfm12b: Documentation

Arduino and Raspberry Pi open platform

The emontx, emonglcd and Raspberry Pi wireless receiver board (RFM12Pi) is based on the Arduino platform, a popular open-source micro controller platform with a large community. This makes these modules fully configurable and adaptable for different applications with a little programming knowledge. The emonBase uses the Raspberry Pi popular low cost Linux computer platform.

OpenEnergyMonitor Online Shop

The OpenEnergyMonitor system is available through the openenergymonitor shop. The system is currently primarily used by individuals, education institutions and companies with proficiency in electronics and software development. The system is a mixture of through hole electronics which are provided in kit form and pre-assembled surface mount boards. It is sold as a development kit, not yet a full commercial product.

Full pricing including volume pricing is available on the openenergymonitor shop:

The OpenEnergyMonitor shop is run by Megni energy monitoring.

Megni and OpenEnergyMonitor

Megni energy monitoring is run by Glyn Hudson and Trystan Lea. We are based in Snowdonia, North Wales. We are both also founders and core developers on the OpenEnergyMonitor project. The OpenEnergyMonitor project is its own entity separate from Megni it is an online open source community project with many developers from around the world contributing and using the technology for all sorts of different applications.

Our Values

Technology that helps us live within ecological limits.
The development of the system and its application is guided by the motivation to make the transition to sustainable energy in our own lives, we started by optimising electricity use in our own homes and understanding the contribution of solar pv generation, increasingly development is on measuring the performance of building fabric and sustainable heating systems and optimising the use of heating systems.

We use open-source technology developed by ourselves and others as a part of the OpenEnergyMonitor project. We believe open-source is a better way of doing things. You can learn more about this, how the technology works and how to build, repair and improve it, on the project website:


We are based in the mountains of Eryri (Snowdonia) North Wales and we stayed here because of the mountains, we are keen climbers and mountain bikers. Thanks to the internet we enjoy developing open hardware and software and running a technology business from here too.

Glyn's climbing blog: Adventures + North Wales

Twitter: @trystanlea, @glynhudson