Earth Notes: On Solar DHW For Our Home with UniQ and PV Diversion

Updated 2019-10-17 20:03 GMT.
Trying to actually get some solar hot water done now that electricity grid CO2 intensity makes the idea sensible...
immersion heater posterised
If I were applying for government funding I'd call this a cross-vector hybrid time-shifting storage-based DHW carbon-reduction scheme. Which means that I shift some load from the gas grid to the electricity system (though probably not to the grid itself) while cutting CO2 emissions.

Summary Questions and Answers

Why install a Sunamp UniQ heat battery?

To try and reduce our carbon footprint, reduce flows of energy from (and to) the grid, and understand how different forms of storage play together. Basically we get to burn less stuff and stay comfortable.

Will a heat battery save lots of money?

No. I expect it to be roughly a wash on our bills. Every gas unit that we don't pay to import will be balanced by about one electricity unit that we won't get paid to export.

What are some other names for heat battery?

Thermal battery, thermal store, thermal energy storage, heat store, hot water tank. A hot water bottle is a (portable) thermal store!

2019-09-27: What, and Why Now?

In this article I describe the process of selecting and installing a (Sunamp UniQ) heat battery to take some load off the gas grid for domestic hot water (DHW) while trimming CO2. The aim is not simply to shift that load to the electricity grid. Instead this should reduce outflows from our PV geneneration otherwise spilled to grid.

This makes sense now that the typical electricity grid intensity in CO2/kWh is similar to or below that from burning gas in our boiler. Also, the Sunamp UniQ is considerably more compact than a hot water tank, and leaks much less heat too. If we can keep much of that DHW energy in the house everyone's a winner.

This heat battery has to play nicely with our other behind-the-meter Enphase AC Battery, and the off-grid storage.

Our boiler is a Potterton Performa 24 gas combi and does not accept pre-heated water. (Though must be good to ~25C since I’ve measured that at the kitchen cold tap.) Thus I would prefer a CombolSol type arrangement for auto fallback to combi for DHW so that we can never run out of hot water. I would like the option to tweak control settings to allow it to become a stand-alone top-up-at-night job if we do ditch the gas entirely.

I’d also like options to add extra control/override and monitoring, since the data and experience writeup is the primary reason that I’m doing this.

I estimated that PV diversion could cover most of our DHW needs (~4kWh per day) most of the year. That works better with storage of a more than a single day's demand. The storage can then carry heat into a less sunny day from time to time. (That implies at least a UniQ 6 with a bit over 6kWh of storage.) I'd expect us to be automatically almost entirely using gas for DHW Nov/Dec/Jan.

2019-09-28: Basics and Nice-to-Haves?

What are the basics needed?

  • Must not cause any import from electricity grid (unless otherwise overidden).
  • Must not hamper DHW delivery (eg annoy my family) even mid-winter. Ideally should improve flow rate and temperature stability while using UniQ.
  • Must not uglify the kitchen!
  • Must play nicely with the Enphase AC Battery.
  • Must fit into the slightly cramped space and non-vanilla (two generation feeds) system that we have.

What would be nice to have, eg for better UX and grid integration?

  • A 'boost' button to manually allow us to top up enough for a bath, ASAP.
  • Indicator of how full store is.
  • Indicator of input and/or supplied energy to UniQ.
  • Ability to control/monitor remotely from RPi to gather stats such as DHW kWh and to try out more complex control strategies.
  • Ability to adjust 'spill' margin and delay before diversion starts.
  • Ability to adapt control to supply more DHW so eg ensures at least half full with wee-hours top-up from grid, maybe done slowly to reduce grid I2R (I2R) losses.
  • Ability to adapt control to supply all DHW if combi dies, so eg forced on when store nearly empty (maybe slowly at first to reduce grid I2R losses).
  • Autonomously stops diverting when grid frequency drops by near operational -0.2Hz limit, after a few seconds at most to be useful, to help avoid load shedding and other grid issues.
  • Ability to work alongside a possible future additional UniQ-based solution for central heating (CH) radiators charged at night.


  • It is not a goal to save money.

Some or all of the "nice-to-haves" are out of scope for an initial install. Avoiding making them hard later, if possible, is a goal.

2019-10-10: Space for the UniQ

kitchen boiler cupboard UniQ destination under combi

I took some yucky pics with a tape measure to establish the approximate depth (~60cm), height (~100cm), width (~55cm) available under the combi where the UniQ might live.

I don't yet have a design sketch. I did find some exciting reports of problems with UniQ 'e' models that won't recharge until either 50% or 90% depleted. Sunamp describes the reports as overblown. I am not troubled by them.

I can take a firmware update after install if need be. For my purposes it’s not critical, it just reduces the utility of the store if it doesn’t capture all that it could.

See some more non-pretty pics of the space where I'd like the UniQ to fit, under our combi. It will mean giving up some valuable (goods) storage space.