A smart immersion heater controller can cut your hot water bills significantly — especially if you have solar panels, a time-of-use electricity tariff such as Octopus Agile, or access to off-peak Economy 7 rates. Rather than running your immersion heater on a fixed timer or switching it on manually, a smart controller automatically diverts cheap or free electricity to heat your hot water tank at the lowest possible cost.
This guide covers the best options available in the UK in 2026, from retrofit controllers that bolt onto an existing cylinder to complete smart hot water systems. If you are also considering solar battery storage UK options, a solar diverter for your immersion heater is often the most cost-effective first step before committing to a full battery installation.
How smart immersion heater controllers work
Traditional immersion heaters run on a fixed timer or are manually switched — they draw power from the grid regardless of whether cheaper electricity is available. Smart controllers change this in two main ways:
- Solar diverters monitor how much surplus solar generation you are exporting to the grid and redirect that otherwise-wasted electricity into heating your hot water. You heat water with solar power that would have earned you only a few pence per unit under the Smart Export Guarantee (SEG), instead of spending 20–25p per unit buying grid electricity to do the same job.
- Time-of-use programmers schedule your immersion heater to run during off-peak periods — typically midnight to 7 am on Economy 7 tariffs, or during cheap half-hourly slots on Octopus Agile. Economy 7 night rates are typically 7–12p/kWh compared with 20–25p/kWh in the daytime, so shifting hot water heating overnight can halve the running cost.
Most households use around 1,500–2,500 kWh per year to heat domestic hot water with an immersion heater. At 24p/kWh daytime rates that is roughly £360–£600 per year; shifting that load to off-peak or solar can bring it down to £100–£200 or lower.
Best solar diverters for immersion heaters
myenergi Eddi — best overall solar diverter
The myenergi Eddi is widely considered the most capable solar diverter for UK homes with solar PV. It monitors your generation and consumption every second, diverting surplus power to your immersion heater (or other resistive loads such as underfloor heating). Unlike simpler diverters, the Eddi works with as little as a few hundred watts of surplus generation rather than requiring the full element wattage before it activates, which makes it effective even on cloudy days.
The Eddi 2.1 has built-in Wi-Fi and Ethernet, integrates with the myenergi app, and works with Octopus Energy's Intelligent tariff to schedule heating from the grid during cheap overnight slots when solar generation falls short. It also pairs with the myenergi Zappi EV charger and libbi battery, making it the natural choice if you plan to expand your smart energy ecosystem. Expect to pay around £400–£500 for the unit, with professional installation typically adding £150–£250.
Marlec Solar iBoost+ — best budget solar diverter
The Solar iBoost+ from Marlec has been installed in over 125,000 UK homes and remains one of the most popular solar diverters on the market. It uses a wireless clamp at your meter to detect surplus export and diverts that power to up to two immersion heaters in sequence. A small display unit shows how much energy has been diverted — today, yesterday, over the past seven days, and cumulatively — so you can see your savings at a glance.
The iBoost+ lacks app connectivity out of the box (a separate Wi-Fi add-on is available) and does not integrate with smart tariffs, but it is simple to install and reliable. Customer reviews frequently cite savings of 1,500–2,300 kWh per year from typical 3–5 kWp solar systems, equating to roughly £300–£500 in avoided grid electricity purchases. Prices are typically £200–£280 for the unit.
Best Economy 7 and smart-tariff controllers
Secure E7+ — best for Economy 7 homes
The Secure E7+ (formerly marketed under the Horstmann brand) is the leading Economy 7 immersion heater controller in the UK. It replaces older Economy 7 Quartz and Electronic 7 units using the same mounting plate, making it a straightforward retrofit for millions of UK homes still on Economy 7 hot water circuits.
The E7+ has Bluetooth app control within range of the property, a 7-day programmer with up to three on/off periods per day, and a 30/60/120 minute boost. An optional plug-in Wi-Fi adaptor extends full remote control from anywhere. For landlords, a tamper-proof programme lock prevents tenants from overriding schedules. The unit costs around £75–£90 and comes with a 7-year manufacturer's warranty.
If you are on Octopus Agile or another half-hourly tariff, the E7+ cannot automatically track price fluctuations — for that level of automation you would need a solar diverter with tariff integration such as the myenergi Eddi, or a full smart hot water cylinder such as Mixergy. You can also use a home energy monitor alongside a basic timer to identify your cheapest heating windows manually.
Best smart hot water cylinder — Mixergy
Mixergy is a UK-designed smart hot water cylinder that replaces your existing tank rather than adding a controller to it. Its patented top-down heating technology heats only the top portion of the cylinder first, delivering usable hot water up to ten times faster than a conventional cylinder heating from the bottom up. The manufacturer claims energy savings of up to 40% compared with a standard cylinder.
The Mixergy app allows you to set schedules and monitor usage, and the cylinder supports demand-side response — it can automatically shift its heating schedule to off-peak grid periods when electricity is cheapest, including integration with smart tariffs. It is compatible with gas boilers, heat pumps, solar thermal, and solar PV via an immersion element. Available in capacities from 150 to 300 litres, Mixergy cylinders carry a 25-year cylinder warranty and a 3-year electronics warranty.
Mixergy is best suited to homeowners replacing an ageing cylinder, new builds, or social housing providers looking for measurable energy reduction at scale. Pricing varies by size and supply chain — expect to pay £800–£1,500 for the cylinder, plus installation, from specialist plumbing merchants.
Premium option — Sunamp heat battery
Sunamp Thermino heat batteries replace a conventional cylinder entirely with a compact phase-change material unit, roughly a quarter of the size of an equivalent hot water cylinder. Sunamp units are particularly well-suited to flats and homes with limited airing cupboard space, and they work with solar PV, Economy 7, heat pumps, and grid electricity. Supply and installation from a Sunamp installer typically costs £3,000–£5,000, placing this firmly in the premium category — but it may be relevant if space is a constraint alongside a desire for smart hot water control.
How to choose the right controller
Have solar panels? A solar diverter is almost always the most cost-effective investment. Start with the myenergi Eddi if you want app control and smart-tariff integration, or the Solar iBoost+ if you want a simpler, lower-cost option. Both pay back in three to four years in a typical 3–4 kWp solar installation.
On Economy 7 with no solar? The Secure E7+ is the standard choice — affordable, easy to fit, and reliable. Add the Wi-Fi adaptor if you want remote app control.
Replacing an ageing cylinder? Mixergy is worth serious consideration. The energy savings over the lifetime of the cylinder can offset the higher upfront cost, and tariff-aware scheduling means the cylinder works with Octopus Agile and similar products without any additional hardware.
Considering Octopus Agile or Intelligent Octopus Go? Read our guide to the Octopus Energy smart tariff to understand which tariff pairs best with your setup. The myenergi Eddi's direct integration with Intelligent Octopus is particularly useful: during cheap overnight slots, the Eddi can automatically top up your tank from the grid at rates as low as 7–9p/kWh, blending solar diversion during the day with cheap grid charging overnight.
Installation and safety
Solar diverters such as the Eddi and iBoost+ connect to your immersion heater element and your meter tails or consumer unit. In the UK, this work is notifiable under Part P of the Building Regulations if it involves new circuits in a bathroom or affects the main fuse/meter tails, so you should use a registered electrician (NICEIC or NAPIT registered). Installing a retrofit controller such as the E7+ onto an existing fused spur circuit is generally a like-for-like replacement and does not trigger notification requirements, though it is still advisable to have any electrical work checked by a competent person.
All products listed here are designed for standard 3 kW single or dual-element immersion heaters. Check your cylinder's existing element rating before purchasing — some older tanks have 1 kW or 6 kW elements that may need replacing or that will affect diverter performance.
