Solar iBoost+ Solar Immersion Controller
The Solar iBoost+ by Marlec is the UK's best-selling solar PV immersion diverter, installed in over 125,000 UK homes. It automatically diverts surplus solar generation — power that would otherwise be exported to the grid for little or no return — directly to an immersion heater, providing free hot water from solar energy at no running cost. Using pulse width modulation (PWM) proportional control, it matches diversion exactly to the export level: never triggering grid import, never wasting excess generation. Supports up to 2 immersion heaters (up to 3kW each) with automatic cascading prioritisation. Built-in savings display logs energy diverted over today, 7 days, 28 days and total. Manual and timed boost for grid top-up when needed. Wireless sender and clamp — no signal cables between meter and tank. Compatible with battery storage via adjustable cut-in threshold. 2-year warranty. 125,000+ UK Homes Installed 3kW Max Per Immersion 2 Immersions Supported PWM Proportional Control 100W Export Threshold 2 yr Warranty How the Solar iBoost+ Works 1 Sender + Clamp at Meter The clamp fits around the live incoming supply cable at the utility meter and detects when the property is exporting electricity to the grid. 2 Wireless Signal to Controller The sender unit transmits export level data wirelessly to the Solar iBoost+ controller located at the hot water tank. 3 Proportional Diversion The controller diverts exactly the right amount of excess power to the immersion — proportional to the export level, not full power — so grid imports are never triggered. 4 Savings Logged Every kWh diverted is recorded. Press the display button to see savings: today, yesterday, last 7 days, last 28 days, and total. Key Features Proportional Diversion — Only Excess Power, Never Grid Import Solar iBoost+ uses pulse width modulation (PWM) to continuously vary the power delivered to the immersion heater in exact proportion to the current export level. If the property is exporting 1.4 kW, exactly 1.4 kW (minus a ~100W buffer) is sent to the immersion. If a kettle is switched on mid-diversion and export drops, the diversion reduces instantly — the grid is never drawn on to compensate. PWM proportional control100W export thresholdNever imports from grid Wireless Sender — No Meter Wiring, No Signal Cables Through Walls The sender unit transmits the meter export reading wirelessly to the Solar iBoost+ controller. The only wiring required is the clamp cable at the meter and the immersion connection at the controller. No signal cables between meter and airing cupboard. The sender and controller are factory paired — press no buttons on the sender or pairing will be lost. Factory-paired wirelessNo signal cables through walls Dual Immersion Support — Cascading Prioritisation Automatically detects and manages up to 2 immersion heaters (each up to 3kW). Excess solar is first diverted to the top (upper) immersion until its thermostat is satisfied. Energy then cascades to the lower immersion. Every 15 minutes the top immersion is rechecked — if it has cooled it is prioritised again, keeping the upper tank section hot for immediate use. 2 immersionsAuto-cascade15-min top-up check Manual Boost and Programmable Timed Boost Manual Boost — each button press adds 15 minutes of full-grid power to the immersion, up to 2 hours maximum. Stops automatically when the immersion thermostat is satisfied. Timed Boost — programmable on a 5-day/2-day schedule with two set points per day. Separate Winter/Summer programmes switchable at the press of a button. Manual boost 15-min stepsTimed 5/2 scheduleWinter/Summer modes Battery Storage Compatible — Adjustable Cut-In Threshold The default export threshold is 100W before diversion begins. Where a battery storage system is installed, the threshold can be raised in 50W increments up to 500W, ensuring the battery charges before the immersion does — the homeowner decides the priority split. 100W default thresholdAdjustable 50W stepsBattery storage compatible Built-In Savings Display — Today, 7 Days, 28 Days, Total The controller display shows diversion in real time (Heating by Solar 1.6 kW) and logs cumulative energy savings. Press the display button to cycle through: Saved Today, Saved Yesterday, Saved 7 Days, Saved 28 Days, Total Saved. Programme the current electricity tariff in the optional Buddy to see £-per-hour savings. Real-time display5 savings periods Compatible with All Domestic PV Systems — No Size Limit Works with transformer, transformer-less and micro-inverters. No strict PV system size limit — compatible with typical residential installations. One Solar iBoost+ can serve two separate cylinders (using two units with the same sender, offset thresholds). All PV inverter typesNo system size limit2 cylinders possible Technical Specifications Electrical Operating Voltage 220–240V AC Maximum Permissible Load 13A resistive (max 3kW immersion per circuit) Supply Connection 16A MCB or 13A fused outlet Maximum Immersion Rating Up to 2 × 3kW immersions (each must have working thermostat) Diversion Control Control Technology Pulse Width Modulation (PWM) — proportional Export Threshold (default) 100W (adjustable 50–500W for battery storage compatibility) Diversion Range 100W to 3,000W Boost Functions Manual Boost 15-minute increments, max 2 hours (grid power) Timed Boost 5-day / 2-day schedule, 2 periods per day Seasonal Programmes Winter / Summer settings switchable at one button press Wireless Sender / Controller Link Factory-paired wireless (do not press sender button) Sender Battery 2 × AA (not rechargeable) Box Contents Contents Solar iBoost+ controller, Sender unit (with cable and plug), Measurement clamp, 2× AA batteries, wall fixings, installation and user manuals Warranty Warranty 2 years from date of purchase (proof of purchase required) Manufacturer Marlec Engineering Co. Ltd, Corby, Northamptonshire ℹ Installation: The Solar iBoost+ must be installed by a qualified electrician. Connect via a 16A MCB or 13A fused outlet. Cable from Solar iBoost+ output to immersion heater must not exceed 3m and must not be interrupted by any switches, timers or electronic controls. Minimum 100mm clearance all round. Frequently Asked Questions Does the Solar iBoost+ ever draw power from the grid to heat the immersion? No — during normal solar diversion operation, the Solar iBoost+ only uses power that is already being exported to the grid. The 100W buffer ensures the system stays on the export side at all times. Grid power is only used during manual Boost or timed Boost functions when the homeowner deliberately activates top-up heating. Can I use Solar iBoost+ with a battery storage system? Yes — raise the cut-in threshold above the battery system's own threshold so the battery charges first. The threshold is adjustable from 100W up to 500W in 50W steps. Once the battery is full and export exceeds the threshold, Solar iBoost+ activates automatically. Will Solar iBoost+ replace my existing hot water timer? Yes — once installed, the Solar iBoost+ replaces any existing boost switch or hot water timer. The immersion cable from the Solar iBoost+ output to the heater must have no interruptions (no switches, timers or electronic controls between them). Does it work with my type of PV inverter? Yes — Solar iBoost+ is compatible with transformer-type, transformer-less and micro-inverters. There is no restriction on the size of the PV system. Note: mechanical meters with rotating disks can cause measurement distortions — position the clamp along the meter cable away from the base of the meter. How do I replace the sender batteries? The sender uses 2 × AA batteries (non-rechargeable). If the iBoost+ Buddy is fitted, battery status is permanently shown on the Buddy display. Do not use rechargeable batteries in the sender unit. 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