In this episode of Energy Unwrapped, I’m joined by Pete Armstrong, CEO and co-founder of Mixergy, to explore why hot water cylinders could become one of the UK’s biggest untapped energy storage assets. We look back at how Mixergy grew from an Oxford University spin-out into one of the UK’s most innovative renewable technology companies, and why the team believes smart hot water will play a major role in the future electricity grid.
Having owned a Mixergy cylinder since 2022, this is a conversation I’ve wanted to have for a long time. We discuss state of charge, top-down heating, stratification, grid flexibility, solar self-consumption, dynamic tariffs and why knowing exactly how much hot water you have available is a huge improvement over a conventional cylinder. We also explore Future Homes Standard, local control, connected devices and what’s next for Mixergy.
If you’ve ever wondered why I often describe a hot water cylinder as a battery, this episode explains exactly why.
What we cover in this episode
- how Mixergy started at Oxford University
- the story behind the Mixergy name
- Robert Llewellyn’s role in Mixergy’s early success
- how a smart hot water cylinder works
- top-down heating and stratification
- state of charge for hot water
- why smart cylinders outperform traditional cylinders
- using hot water as grid flexibility
- storing surplus solar energy
- smart tariffs and intelligent hot water heating
- Future Homes Standard opportunities
- SSES and connected energy devices
- local control versus cloud connectivity
- heat pump hot water optimisation
- why tariff prices matter more than COP
- thermal storage compared with home batteries
- the future of grid flexibility
- Mixergy’s next generation of products
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Full Transcript
Below is the full transcript for this episode, lightly edited for readability.
Meet Pete Armstrong and the Story Behind Mixergy
Pete explains how he and co-founder Ren met while studying engineering at Oxford University before entering a climate technology competition that eventually became Mixergy. What started as research into energy storage and electricity networks evolved into a smart hot water company focused on making thermal storage flexible and connected.
How Mixergy Got Its Name
The discussion explores the engineering concept of “exergy” and how it inspired the Mixergy name. Pete also shares the amusing story of the company’s first choice of name and why it had to be abandoned.
Robert Llewellyn and the First Mixergy Cylinder
Pete tells the story of meeting Robert Llewellyn at a book signing and convincing him to install one of the very first Mixergy cylinders. That opportunity helped introduce the technology to a much wider audience through Fully Charged.
What Makes a Mixergy Cylinder Different?
We explain the key differences between a conventional hot water cylinder and a Mixergy cylinder, including top-down heating, intelligent controls, state-of-charge monitoring and the ability to heat only the water you need.
Why State of Charge Changes Everything
One of my favourite features is knowing exactly how much usable hot water remains. Rather than relying on thermostats and hysteresis, Mixergy measures the thermal energy stored in the cylinder and displays it as a simple percentage.
Optimising Hot Water for Heat Pumps
We discuss my own Mixergy installation and how understanding heat transfer, compressor output and cylinder charging led to significant improvements in hot water efficiency. That experience ultimately became the basis for many of the heat pump optimisation articles published on Energy Stats UK.
Why Hot Water Is the UK’s Hidden Battery
One of the biggest themes of the episode is flexibility. Millions of hot water cylinders already exist in UK homes, yet they receive far less attention than EVs and home batteries despite representing an enormous amount of energy storage.
Using Solar PV Instead of Exporting
As export prices continue to fall during sunny periods, Pete explains why storing surplus solar energy as hot water is becoming increasingly attractive. Rather than exporting electricity for very little value, homeowners can improve self-consumption by heating water instead.
Future Homes Standard and Smart Hot Water
We explore how Future Homes Standard could accelerate the adoption of connected hot water cylinders. New homes with heat pumps and solar PV create the perfect opportunity to increase self-consumption while supporting a smarter electricity grid.
Smart Energy, SSES and Connected Homes
Pete explains how new regulations around Smart Secure Electricity Systems (SSES) will encourage appliances such as heat pumps, batteries, EV chargers and hot water cylinders to become connected energy assets capable of responding to grid signals.
Local Control and the Future of Smart Homes
Following recent industry events around cloud services, we discuss the importance of local control. Pete outlines Mixergy’s plans for local connectivity while maintaining the benefits of cloud-based optimisation and grid services.
Why Tariffs Matter More Than COP
A fascinating discussion looks at the relationship between heat pump efficiency and electricity prices. Pete argues that, in many situations, using cheaper off-peak electricity has a greater impact on running costs than chasing the highest possible COP.
The Future of Grid Flexibility
The episode finishes by looking ahead to the next decade. As heat pumps, batteries, EVs and smart hot water become connected, homes will increasingly help balance the electricity grid while reducing household energy costs.
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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