Open Energy Monitor Explained | Glyn Hudson & Trystan Lea

Open Energy Monitor Explained | Glyn Hudson & Trystan Lea

In this episode of Energy Unwrapped, I’m joined by Glyn Hudson and Trystan Lea, the founders of OpenEnergyMonitor, to explore the story behind one of the most influential open-source projects in home energy monitoring. We go right back to the beginning, from building circuit boards by hand to creating a platform now used by thousands of homeowners and installers.

We discuss how OpenEnergyMonitor grew alongside the Arduino and Raspberry Pi revolution, why the project has remained open source from day one, and how an early interest in electricity monitoring eventually led to HeatPumpMonitor.org. Along the way we talk about John Cantor’s influence, the importance of high-quality performance data and how monitoring has helped transform heat pump design and commissioning.

As someone who’s been using OpenEnergyMonitor since my solar installation in 2018, this was a conversation I’d wanted to have for a long time. If you’ve ever wondered where HeatPumpMonitor came from, why monitoring matters or how open-source hardware helped shape today’s renewable community, you’ll enjoy this one.

What we cover in this episode

• how OpenEnergyMonitor started
• meeting through childhood electronics projects
• inspiration from Sustainable Energy – Without the Hot Air
• the early Arduino community
• building the first circuit boards by hand
• turning a hobby into a business
• why OpenEnergyMonitor has always been open source
• the arrival of the Raspberry Pi
• local monitoring versus cloud platforms
• the evolution of EmonCMS
• monitoring solar PV, batteries and EVs
• John Cantor’s influence on heat pumps
• the origins of HeatPumpMonitor.org
• why accurate heat pump monitoring matters
• learning from real-world heat pump performance
• commissioning versus hardware design
• weather compensation and flow temperatures
• accurate heat loss calculations
• simple open-loop heating systems
• how monitoring is improving the entire heat pump industry
• the future of OpenEnergyMonitor

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Full Transcript

Below is the full transcript for this episode, lightly edited for readability.

Meet Glyn Hudson and Trystan Lea

Glyn and Trystan explain how a shared interest in electronics, mountain biking and sustainability eventually led to the creation of OpenEnergyMonitor. What began as a hobby project between childhood friends has grown into one of the best-known open-source energy monitoring platforms in the UK.

The Birth of OpenEnergyMonitor

Inspired by David MacKay’s Sustainable Energy – Without the Hot Air, Trystan wanted to understand exactly how much electricity his own home was using. At the time there were very few internet-connected energy monitors, creating the opportunity to build something from scratch.

Building Circuit Boards by Hand

The first OpenEnergyMonitor kits weren’t manufactured products. Every PCB was assembled by hand before demand eventually pushed the team towards professionally manufactured hardware and an online shop.

How Arduino and Raspberry Pi Changed Everything

The timing couldn’t have been better. As Arduino made electronics more accessible, Raspberry Pi arrived just as OpenEnergyMonitor needed a reliable local computer to collect and process energy data. That combination became the foundation of the platform many people still use today.

Why Open Source Still Matters

One of the recurring themes throughout the episode is openness. Every hardware design and software project has been released as open source, allowing users to modify, improve and maintain their own systems without depending entirely on a manufacturer.

Monitoring Solar, Batteries and EVs

Although many people now associate OpenEnergyMonitor with heat pumps, the platform originally focused on electricity monitoring. Today it can monitor almost every major electrical circuit in a home, including solar PV, batteries, EV chargers and household loads.

John Cantor and the First Heat Pump Monitoring

The conversation then turns to John Cantor, whose decades of heat pump experience helped inspire the next stage of OpenEnergyMonitor. His own high-performing heat pump showed that far better results were possible than many of the early industry trials suggested.

Why HeatPumpMonitor.org Was Created

HeatPumpMonitor.org grew directly out of the OpenEnergyMonitor community. Initially built as a simple way to share public dashboards, it has since become one of the largest collections of independently monitored heat pump performance data available anywhere.

Learning From Real-World Heat Pump Data

Having thousands of monitored systems allows installers and homeowners to compare designs, identify best practice and understand why some systems consistently outperform others. The discussion highlights how powerful shared data has become for improving installations.

Commissioning Matters More Than You Think

One of the biggest findings from the monitoring data is that many underperforming heat pumps aren’t limited by hardware. Instead, commissioning, weather compensation and system settings often make the biggest difference to seasonal efficiency.

Heat Loss, Radiators and Better Design

The conversation also explores how improved heat loss calculations, simpler hydraulic designs and better optimisation have changed the industry’s understanding of what’s possible. Accurate design is helping more homeowners install heat pumps with less disruption than previously thought.

Why Open Data Benefits Everyone

By sharing performance publicly, installers can demonstrate the quality of their work, homeowners gain confidence in the technology and the wider industry benefits from genuine evidence instead of assumptions. It’s a powerful example of community-driven learning.

Open Source in a Cloud-First World

As more energy products rely entirely on cloud platforms, Glyn and Trystan explain why local control and open-source software remain just as important today. Homeowners retain ownership of their data and their systems continue working even if a manufacturer disappears.

The Future of OpenEnergyMonitor

The episode finishes by looking ahead to the next generation of hardware, larger commercial monitoring projects and the continued growth of HeatPumpMonitor.org as more installers and homeowners contribute real-world performance data.

About the Energy Unwrapped Podcast

Energy Unwrapped is my podcast about renewables, home electrification and how energy technology works in the real world. You can browse all episodes, platform links and future updates on the main Energy Unwrapped Podcast page.

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