Heating water accounts for around 17% of a typical UK home's energy bill, according to the Energy Saving Trust. A smart hot water cylinder — or a smart controller fitted to your existing one — can cut that significantly by shifting heat to cheap off-peak tariff windows, soaking up spare solar generation, and giving you real-time control from your phone. Whether you're upgrading alongside a heat pump, getting more from your solar panels, or simply tired of running out of hot water, the options below are worth knowing.
What makes a cylinder 'smart'?
A smart hot water system does at least one of the following:
Time-of-use tariff scheduling. Heats overnight on cheap rates (e.g. Octopus Go or Agile) and switches off before peak pricing kicks in. See our guide to Octopus smart tariffs for how much you can save.
Solar diversion. Instead of exporting surplus solar electricity at a low feed-in rate, a diverter controller routes it into the immersion heater. This is typically far more valuable than the export payment — see our solar monitoring guide for more on measuring generation vs. consumption.
App and remote control. Boost hot water before you arrive home, check current temperature, and set holiday modes without touching the cylinder.
Heat pump compatibility. ASHP and GSHP systems need a well-insulated indirect cylinder with a large coil; the best smart cylinders are designed to work at the lower flow temperatures heat pumps produce. Our heat pump buyer's guide explains the cylinder sizing rules in detail.
Key specs to compare
When choosing, look at: capacity in litres (a family of four typically needs 180–250 L), heat-up time, standing heat loss (kWh/24 h — lower is better), smart features, and compatibility with your heat source. Prices vary by retailer and installer, so treat the figures below as a guide.
The best smart hot water cylinders and controllers
1. Mixergy Smart Cylinder — best all-in-one smart cylinder
Capacity: from 150 L to 300 L indirect models. Typical price: around £1,200–£1,500 installed depending on size and installer.
Mixergy is the only UK-made smart cylinder with patented top-down or layered heating. Instead of heating the whole tank, it heats the top portion first — so you always have some hot water available within minutes, even if the full tank hasn't reached temperature. This is a genuine efficiency gain for solar-diversion setups: you can capture a short burst of surplus PV and have usable hot water immediately rather than waiting for the whole volume to warm.
The companion app shows real-time temperature by zone, energy used, and estimated hot water available in minutes of showering. It supports Octopus Agile tariff integration, meaning it can automatically schedule heating to coincide with the cheapest half-hours of the day. Mixergy cylinders are built for heat pump pairings — the large-surface-area coil suits the lower flow temperatures (around 45–55°C) that an ASHP or GSHP produces. See our Vaillant Arotherm Plus review for a real-world example of heat pump and cylinder pairing.
If you want a single product that handles scheduling, solar diversion, app control, and heat pump compatibility out of the box, Mixergy is the benchmark.
2. myenergi Eddi — best solar diverter
Price: around £280–£320 (diverter unit only; installation extra). Works with any existing hot water cylinder.
The myenergi Eddi measures surplus solar (or wind) generation in real time and diverts it to an immersion heater rather than letting it flow back to the grid at the low export rate. It supports two immersion heaters (e.g. top and bottom elements in a twin-element cylinder), prioritises one then the other, and keeps a log of energy diverted. The Eddi has a published open API and integrates natively with Home Assistant — useful if you're already using Home Assistant for energy monitoring.
myenergi also makes the Zappi EV charger, and the two devices communicate: if your car is plugged in, the Zappi gets priority over the Eddi, then any remaining surplus goes to hot water. For solar homes with an EV, this coordinated approach is hard to beat.
3. Marlec iBoost Solar Diverter — simplest solar diverter
Price: around £230–£270 (unit only). Works with any standard cylinder with an immersion heater.
The iBoost has been on the UK market longer than most rivals and is known for reliability and straightforward installation. A wireless sender unit clips onto the incoming mains cable to measure export current; the receiver controls the immersion relay. There is no app, but there is a simple display showing how much energy has been diverted. For homeowners who want a set-and-forget solar diverter without smart-home complexity, the iBoost is a solid choice. It lacks the Eddi's API and EV integration, but it costs slightly less and has a long track record.
4. Sunamp UniQ Thermal Battery — best for small properties
Price: typically £2,000–£2,500 including installation, varies significantly by installer.
The Sunamp UniQ uses phase-change material (PCM) rather than water to store heat. PCM stores far more energy per litre than water — the UniQ e9 model stores around 9 kWh in a unit roughly the size of a large suitcase, compared with the 200-litre cylinder it replaces. This makes it ideal for flats, conversions, or any property where a traditional cylinder simply won't fit.
The UniQ charges from any heat source — boiler, heat pump, immersion element, or solar diverter — and releases heat on demand. Smart scheduling is available through the Sunamp app. The main drawback is cost: at twice or more the price of a Mixergy, it makes financial sense primarily where space is the binding constraint.
5. Vaillant uniSTOR — best for Vaillant system boiler owners
Price: around £600–£900 for the cylinder; installation extra.
If you already have a Vaillant system boiler or an arotherm heat pump, the uniSTOR unvented cylinder is the natural pairing. It connects to the eRELAX or sensoCOMFORT app — the same app that controls your heating — giving you unified hot water and heating schedules from one interface. Heat-pump-optimised variants have an oversized coil to work efficiently at lower flow temperatures.
The uniSTOR is not a smart-first product in the way Mixergy is: there's no tariff integration or solar diversion built in. But for simplicity and brand compatibility in a Vaillant ecosystem, it earns its place here.
6. Standard cylinder + smart immersion controller — best budget option
Price: from around £50–£150 for a controller such as the Drayton Aube TH 143 or Heatmiser neoAir, plus any immersion installation costs.
If your existing cylinder is in good condition, you don't necessarily need to replace it. A smart immersion controller replaces the standard timer on the immersion circuit and adds app scheduling. You won't get solar diversion or layered heating, but you can time heating to coincide with cheap tariff windows. This is the lowest-barrier entry point to smart hot water management.
Comparison at a glance
Mixergy Smart Cylinder — best overall; solar diversion, tariff scheduling, layered heating, heat pump ready. Around £1,200–£1,500 installed.
myenergi Eddi — best solar diverter; open API, HA integration, EV coordination. Around £280–£320.
Marlec iBoost — simplest solar diverter; reliable, no app needed. Around £230–£270.
Sunamp UniQ — best for tight spaces; PCM thermal battery, very compact. Around £2,000–£2,500.
Vaillant uniSTOR — best for Vaillant system boiler/ASHP owners; unified app control. Around £600–£900.
Smart immersion controller — best budget upgrade; adds scheduling to any existing cylinder. From around £50.
Things to consider before buying
Cylinder insulation. A poorly insulated cylinder loses heat overnight; this undermines the whole point of cheap-rate heating. Look for a standing heat loss below 1.5 kWh per 24 hours. Modern factory-insulated cylinders exceed this; older copper tanks with loose jacket insulation often do not.
Heat pump compatibility. If you have or are planning an air-source or ground-source heat pump, choose a cylinder with a large indirect coil rated for low flow temperatures. Both Mixergy and Vaillant uniSTOR offer heat-pump-specified variants. Check our heat pump guide for cylinder sizing rules.
Tariff optimisation. Smart cylinders deliver the biggest savings on time-of-use tariffs. If you're on a flat-rate tariff, the case is weaker. Check whether your energy supplier offers an EV or off-peak tariff — the Octopus smart tariff guide covers several options available to UK households.
Solar diversion payback. A solar diverter such as the Eddi or iBoost typically pays back in two to four years for homes with a 3–6 kWp solar array, based on typical summer generation figures and current electricity and export rates — figures vary significantly by location, system size, and usage. See our solar panels guide for more on yield expectations.
Related: best smart immersion heaters UK, Octopus Energy smart tariff UK, and best solar panels UK.
