Smart Solar Diverter vs EV Charger Priority UK: How to Share Surplus Generation
Once you have solar panels generating more electricity than your home is using, you face a surprisingly nuanced question: where should that surplus go first — the hot water tank or the car? This matters more than it sounds. A solar immersion diverter and a solar-aware EV charger are both trying to claim the same spare electrons, and without intentional configuration, most systems simply give priority to whichever device got there first. This guide explains how surplus solar is shared by default, which smart systems actually let you set priorities, and how to build a scheduling strategy that makes sense for your household across the seasons.
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The Default Problem: First Come, First Served
A standard solar installation without smart energy management has no awareness of competing loads. When surplus generation appears — say, 1.5 kW of excess after your household appliances are accounted for — a basic immersion diverter will claim that surplus for the hot water tank. If your EV charger is also waiting for solar, it will either remain on a timer schedule (running off grid power) or compete for the same surplus by drawing it independently.
The result is that two devices that are each individually “solar aware” may not be collectively optimised. You can end up with a full hot water tank at midday and a flat EV battery — or vice versa — depending on which device has priority in your setup. The fix is an ecosystem approach where a single energy manager monitors generation and allocates surplus intelligently.
Understanding Typical Daily Surplus: Summer vs Winter
Before designing your priority logic, it helps to understand how much surplus solar you actually have to work with in practice:
- Summer (June–August): A 4 kWp system in the south of England might generate 18–22 kWh on a good day. With a typical household drawing 8–10 kWh over the same period, surplus could be 8–12 kWh. This is enough to fully heat a 200-litre hot water cylinder (roughly 4–5 kWh), and add 30–40 miles of EV range (at ~4 miles/kWh for a typical EV).
- Spring / Autumn (March–May, September–October): Daily generation drops to 8–14 kWh. Household consumption may rise. Surplus is typically 3–7 kWh — usually enough for hot water, sometimes enough for partial EV charging (15–25 miles).
- Winter (November–February): Daily generation averages 2–5 kWh. After household loads, genuine surplus may be 0–2 kWh per day. Hot water diversion may be only partial; EV solar charging from surplus is minimal.
This seasonal picture matters enormously for priority strategy. In summer, there is usually enough surplus for both the hot water tank and partial EV charging. In winter, you may need to make an active choice about which takes priority — or accept that both rely on grid power during that period.
When to Prioritise the Hot Water Tank
Giving the immersion diverter first call on surplus solar makes sense in these situations:
- You heat water electrically (immersion only, no combi boiler): Hot water is a daily essential. If your diverter does not top up the cylinder, you will draw grid power for hot water every evening. At 28–35p/kWh for grid electricity, this adds up quickly.
- You have a heat pump for space heating: Heat pump systems typically use domestic hot water tanks as a buffer. Keeping the tank warm via solar reduces heat pump run time, which saves electricity more efficiently than heating the tank from scratch with the heat pump.
- Your EV rarely runs below 50% charge: If you drive less than 20 miles per day, a brief top-up charge overnight is sufficient. Hot water priority in this case delivers reliable daily savings on grid bills.
Our detailed guide on solar immersion diverters explains the products and economics in full — including which diverters have prioritisation controls built in.
When to Prioritise the EV Charger
EV charging priority makes more sense when:
- You drive a high daily mileage (50+ miles): If the car needs a meaningful charge every day, not charging from solar means drawing expensive grid power later. The financial case for EV priority is strong if your car is a daily workhorse.
- You have a gas boiler: If hot water is produced by a gas combi boiler or gas-fed immersion, the marginal cost of heating water from gas is around 5–8p/kWh (at typical 2026 gas prices). Diverting solar to the EV instead of the immersion saves 28–35p/kWh on EV charging — a much better financial outcome. There is no immersion diverter return worth having when the alternative fuel is cheap gas.
- Summer surplus is large: With 8+ kWh of daily surplus, there is no meaningful trade-off — the diverter can run first, then the EV charger picks up whatever remains.
Smart Ecosystems That Actually Let You Set Priority Logic
Most off-the-shelf solar installations do not come with built-in priority management across multiple devices. You need either a dedicated energy management ecosystem or a single brand that covers both the diverter and EV charger. Here are the three main options available to UK homeowners in 2026.
myenergi — Eddi + Zappi (Best UK Ecosystem for Diverter + EV)
myenergi is a UK company based in Lincolnshire that makes both the Eddi immersion diverter and the Zappi EV charger. When used together with the myenergi hub (a small device that connects to your router), the two devices coordinate in real time — you can set which gets surplus first, define minimum charge levels for the EV before the hot water gets priority, and review daily energy flows through the myenergi app.
The Zappi operates in three modes: Fast (charges at full rate from the grid, ignores solar), Eco (tops up solar with a minimum of 1.4 kW from the grid if needed — practical for days with intermittent cloud), and Eco+ (only charges from surplus solar — zero grid draw). The Eddi operates on a similar basis, with a configurable boost mode for mornings when you want the tank topped up from the grid before solar generation begins.
The Zappi costs around £700–£900 installed and the Eddi around £300–£400 installed. The myenergi hub adds around £100. Total ecosystem cost for a new installation: approximately £1,100–£1,400, which is excellent value for the capability.
GivEnergy — Battery + EV Charger + Immersion Management
If you have a GivEnergy battery system, their EV charger (the GivEnergy EV Charger, rated at 7.4 kW) and their inverter can manage priority between battery charging, EV charging, and immersion diversion from a single portal. GivEnergy’s “Eco Mode” allocates surplus solar to a priority queue that you configure — typically battery first, then EV, then immersion (or vice versa depending on your preference).
The advantage of the GivEnergy approach is that the battery acts as a buffer: the system charges the battery from solar during the day, then uses the battery to charge the EV overnight (avoiding peak tariff rates). The immersion diverter operates on surplus during the day before battery is full. This three-way management works well for homes with solar + battery + EV — which is an increasing proportion of UK homes.
SolarEdge — Home Hub + EV Charger + Immersion
SolarEdge’s Home Hub inverter (successor to the StorEdge) can manage solar, battery, EV charger, and immersion heater from a single unified platform if you use SolarEdge-compatible hardware throughout. The SolarEdge EV Charger integrates directly with the inverter for solar-first charging. Immersion diversion can be configured via the SolarEdge modular energy meter.
The SolarEdge ecosystem is most compelling for new-build or full system replacement — it does not retrofit as flexibly as myenergi onto existing non-SolarEdge solar. But for those building a system from scratch, the integrated management is clean. For smart home automation enthusiasts who want to integrate solar energy management with broader home systems, the team at Smart Home Wise have covered SolarEdge and other platforms in depth.
Ecosystem Comparison: EV + Immersion Management
Here is how the three main ecosystems compare for managing surplus solar between an EV charger and an immersion diverter. All prices are approximate installed costs in the UK as of October 2026. The Ofgem energy saving checker can help you understand what incentives may apply to your setup. Savings vary depending on property size, usage, and energy tariff.
| Feature | myenergi (Eddi + Zappi) | SolarEdge (Home Hub + EV) | GivEnergy (Battery + EV + Immersion) |
|---|---|---|---|
| Surplus priority control | Yes — configurable in app | Yes — via Home Hub settings | Yes — via GivEnergy portal |
| Works with any inverter brand | Yes (AC-coupled) | SolarEdge only | Best with GivEnergy; some AC coupling possible |
| Battery integration | No (separate device needed) | Yes (SolarEdge battery) | Yes (native GivEnergy battery) |
| Immersion diverter product | Eddi (~£300–400 installed) | Via energy meter config | Via inverter/portal relay |
| EV charger product | Zappi (~£700–900 installed) | SolarEdge EV Charger (~£900–1,100) | GivEnergy EV Charger (~£750–950) |
| App quality | Very good | Good | Good |
| Best for | Retro-fit to any solar system | New SolarEdge installs | GivEnergy battery owners |
| Total ecosystem cost (EV + immersion) | £1,100–£1,400 | £1,500–£2,000+ | £1,000–£1,500 (if battery already installed) |
Practical Scheduling Strategies
Beyond hardware priority settings, scheduling is the other lever available to you. Here are three strategies that work well in practice:
Strategy 1: Morning Gas Boost + Solar Eco Mode (All Year)
Set the Eddi or GivEnergy to boost the hot water tank from the grid at the cheapest overnight tariff rate (e.g. Octopus Go at 7p/kWh overnight) for 30 minutes each morning. This ensures the tank is warm before the day begins. Then set both the EV charger and immersion diverter to solar Eco mode during daylight hours. Whatever surplus arrives, the diverter and EV share it proportionally. This approach avoids paying peak rates for either and works year-round regardless of solar generation level.
Strategy 2: Summer Diverter Priority (April–September)
In summer, run the Eddi with higher priority than the Zappi during the first 3 hours of peak generation (roughly 10am–1pm). Once the tank reaches temperature (the Eddi detects this via clamp sensor), priority automatically switches to the Zappi. Result: free hot water daily, plus substantial EV top-up in the afternoons.
Strategy 3: EV Priority With Minimum SOC Threshold (Winter)
In winter, set the EV charger to take priority when generation exceeds 1 kW (the minimum for meaningful charging). Set the Eddi to only activate when the EV reaches its minimum state of charge (e.g. 80%). This ensures the car is ready for daily use, with whatever solar remains going to hot water. Overnight grid top-up on a cheap tariff handles any remaining shortfall. The Energy Saving Trust’s guidance on solar self-consumption confirms that smart scheduling is one of the highest-return optimisation steps available to solar homeowners.
Getting It Set Up
If you are starting from scratch, the myenergi ecosystem (Eddi + Zappi + hub) is the most installer-friendly option and works with virtually any solar setup. If you already have GivEnergy, adding their EV charger is the path of least resistance. If you are building a full SolarEdge system, integrate the EV charger and immersion at the design stage.
Use the MCS installer finder to locate EV charger and solar specialists in your area. Not all solar installers are OZEV-certified for EV chargers — check both accreditations before commissioning the work. Our solar battery payback guide and solar monitoring guide cover the additional financial case and monitoring options available once you have the hardware in place.