CT clamp placement with EV chargers solar, battery, and EV — getting all three right
An EV charger draws 7–32 amps. If your inverter's grid CT clamp sees that as household demand, the battery will discharge to supply the car — draining your stored solar in minutes. Where each CT sits determines whether the system works correctly. This guide covers the three main configurations and how to test yours.
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A typical house without solar draws 0.5–1.5 kW at any given moment. An EV charger draws 7.4 kW (single-phase) or up to 22 kW (three-phase). That is a massive spike in load — and every CT clamp in the system reacts to it.
Your battery inverter uses a CT clamp on the meter tails to measure grid import and export. When it sees import, it discharges the battery to offset it. When it sees export (surplus solar), it charges the battery. This works perfectly for normal household loads. But when a 7.4 kW EV charger switches on, the grid CT sees a massive spike in import — and the inverter reacts by discharging the battery at maximum rate to "help."
Most smart EV chargers (Zappi, Ohme, Pod Point, Hypervolt) have their own CT clamp on the meter tails. This CT monitors total house demand so the charger can reduce its rate if the combined load approaches the supply fuse limit (typically 60A or 100A). If the EV charger's CT and the inverter's CT are both on the same section of meter tails, they are both reacting to each other's behaviour — creating a feedback loop.
With incorrect CT placement, a 9.5 kWh battery can be fully drained in under 90 minutes of EV charging — energy that was meant to power your house through the evening peak. You end up importing from the grid to run the house while the battery's stored solar goes into the car. The financial impact is significant: you are replacing 5–7p/kWh stored solar with 24–30p/kWh peak-rate grid electricity.
The three CT clamps in a typical solar + battery + EV system
Each device needs its own current measurement. Understanding what each CT does — and what it should and should not see — is the key to correct placement.
Positioned on the meter tails (the thick cables between the electricity meter and the consumer unit). Measures net import or export. The inverter uses this to decide: charge the battery (when exporting surplus solar), discharge the battery (when importing from the grid), or do nothing.
Critical point: This CT should only measure loads you want the battery to respond to. If the EV charger is downstream of this CT, the battery will try to supply the car.
Positioned on the AC output cable of the solar inverter (or on the solar input if DC-coupled). Measures generation only. Some hybrid inverters measure this internally and do not need a separate external generation CT.
EV relevance: The generation CT is not directly affected by EV charger placement. However, if you have a Zappi or other solar-diverting charger, it needs to know generation to calculate surplus — so the generation CT feeds both the inverter and (indirectly) the EV charger's logic.
Positioned on the meter tails, usually as close to the meter as possible. Measures total site current so the EV charger can reduce its charge rate if total demand approaches the supply fuse limit (load balancing / dynamic load management). This is a safety requirement — BS 7671 requires that connected load does not exceed the supply capacity.
Critical point: The EV charger's CT must see all loads (house + solar + battery + EV) to calculate headroom correctly. It therefore always goes on the meter tails before any branch point.
Configuration A: Grid CT before the EV charger spur
This is the default installation in most homes where solar and battery were fitted first, and an EV charger was added later.
Configuration B: Henley block split — grid CT after the EV spur
This is the recommended configuration for most solar + battery + EV installations. A Henley block splits the meter tails so the inverter only sees household load.
A Henley block (also called a service connector or cut-out connector) must be installed by a qualified electrician. It must be rated for the supply fuse size (typically 60A or 100A). The Henley block is typically installed in the meter cupboard or next to the consumer unit. It must be accessible for the DNO if needed. The total connected load across both outputs must not exceed the supply fuse rating.
A Henley block typically costs £20–40 for the part. Installation labour is 1–2 hours. The total cost is usually £100–200 including labour and certification. Compared to the ongoing financial loss from battery drain (potentially £200–400/year), this is a straightforward payback. Many EV charger installers now fit a Henley block as standard when solar is present.
Configuration C: Separate consumer units
In some retrofit installations, the solar and battery are on a separate consumer unit (sub-board) from the main house circuits. The EV charger may be on either board or fed directly from the meter tails.
CT clamp arrow direction and polarity
Every CT clamp has an arrow moulded into the housing. This arrow must point in the direction of normal power flow. Getting it wrong inverts the reading — the inverter sees export as import and vice versa. For a full explanation of how CT clamps work, see our CT Clamps Explained guide.
Power normally flows from the grid (meter) into the house (consumer unit). The arrow follows this direction. When you export solar, current flows the opposite way — the CT registers this as negative current, which the inverter interprets as export. If the arrow is backwards, the inverter sees import as export and will charge the battery when it should discharge (and vice versa).
Solar power flows from the inverter into the consumer unit. The arrow follows this direction — from inverter towards the consumer unit. If your inverter measures generation internally (common on hybrid inverters like GivEnergy), you may not have an external generation CT at all.
Same as the grid CT — the EV charger needs to see total import. The arrow points from the meter towards the loads. If the CT is on the neutral cable instead of the live, reverse the arrow direction (or configure the charger to invert the reading in software, if supported).
Brand-specific CT clamp considerations
Each inverter and EV charger brand has its own CT clamp naming, configuration, and quirks. Here are the most common combinations we encounter in UK installations.
GivEnergy uses an ID1 CT clamp for grid measurement and an ID2 CT for generation (on older models — newer hybrids measure generation internally). The ID1 CT must be repositioned to the consumer unit feed after a Henley block if an EV charger is added. GivEnergy's portal also allows you to set a minimum SoC reserve and discharge schedules as a software workaround, but the hardware fix (Henley block + CT repositioning) is always preferred.
See: GivEnergy EV charger problems · CT clamp direction settings
The Zappi is unique because it can solar-match — diverting surplus solar to the car. It needs a grid CT on the meter tails and a generation CT on the inverter output. When a battery is present, the Zappi also needs to know battery charge/discharge current. Without this, the Zappi misinterprets battery discharge as solar surplus and charges the car from the battery. Solutions include: an additional CT on the battery circuit connected via a Harvi wireless sender, or configuring the Zappi to read the generation CT net of battery flow.
The Zappi's grid CT must be on the meter tails before the Henley block (it needs to see total site demand for load balancing). The inverter's grid CT must be after the Henley block on the consumer unit feed only.
Non-solar-diverting EV chargers (Ohme, Pod Point, Hypervolt, Wallbox) use their CT clamp for load balancing only — they reduce charge rate to stay within the supply fuse limit. They do not attempt to solar-match. The CT placement principle is the same: the EV charger's CT goes on the meter tails before any split, and the inverter's grid CT goes on the consumer unit feed after the EV spur is separated.
Ohme chargers can optionally use Octopus or other tariff APIs to schedule cheap-rate charging. This does not change CT placement requirements.
Symptoms of incorrect CT clamp placement
If you are seeing any of these symptoms, CT clamp placement is the likely cause. These can often be diagnosed remotely by reviewing your monitoring data — see our CT clamp problem page for diagnosis steps.
The most obvious symptom. You start EV charging and within minutes the battery SoC drops sharply. The inverter portal shows high discharge power matching the EV charge rate. The grid CT is seeing EV demand as household load.
Your monitoring shows grid import during sunny hours while the battery is charging. The inverter is confused about the direction of power flow — typically caused by a reversed CT arrow. It thinks it is exporting (so it charges the battery) when it is actually importing.
The EV charger's CT is seeing battery discharge current as additional house load. It thinks total demand is higher than it actually is, so it reduces charge rate to stay within the fuse limit. This is a false positive caused by the EV CT being on the wrong side of the battery inverter output.
If the grid CT arrow is reversed, the inverter reports grid import as export and export as import. During EV charging (a large import), the monitoring shows a large export value — which is clearly wrong. The fix is to physically reverse the CT clamp or invert polarity in software.
The Zappi's eco or eco+ mode diverts surplus solar to the car. If it cannot distinguish between solar generation and battery discharge, it treats battery discharge as surplus and charges the car. This happens when the Zappi has no CT or Harvi on the battery circuit to subtract battery flow from the surplus calculation.
How to test your CT clamp placement
You can verify correct CT placement without any tools — just your EV charger, your inverter monitoring app, and 10 minutes. This is an adapted version of the kettle test from our CT clamps guide.
Make sure the EV charger is off or paused. Open your inverter monitoring app (GivEnergy portal, Solis Cloud, myGrowatt, etc.). Note the current grid import/export figure. With no EV, you should see a small household import (0.3–1.5 kW typically) or solar export if panels are generating. Also note the battery status — is it charging, discharging, or idle?
Start your EV charger at normal rate (typically 7.4 kW single-phase). Watch your inverter monitoring for 2–3 minutes. Focus on two things: (a) does the battery start discharging? and (b) what does the grid import figure show?
There are three possible outcomes:
If your battery is discharging to the EV charger, do not attempt to move CT clamps yourself — they are on the meter tails which carry the full supply current. Contact a qualified electrician or your original installer to install a Henley block and reposition the grid CT. Alternatively, book a remote diagnostic — we can confirm the issue from your monitoring data and provide your electrician with a clear specification of what needs to change.
Frequently asked questions
The most common cause is CT clamp placement. If the inverter's grid CT is on the meter tails before the EV charger spur, the inverter sees the EV's 7 kW draw as household demand and discharges the battery to offset it. The fix is to install a Henley block and reposition the grid CT so it only monitors the consumer unit feed — not the EV charger circuit. See Configuration B above for the recommended layout.
A Henley block is a service connector that splits the meter tails into two feeds. It allows the EV charger to take power directly from one output and the consumer unit from the other. This creates a separation point so the inverter's grid CT can be placed on the consumer unit feed only — preventing the inverter from seeing EV charger current. It costs £100–200 installed and is the standard solution for solar + battery + EV systems.
The arrow should point in the direction of normal power flow. Grid CT: arrow from meter towards consumer unit. Generation CT: arrow from inverter towards consumer unit. EV charger CT: arrow from meter towards consumer unit. If the CT is on the neutral cable instead of the live, reverse the arrow. A reversed arrow causes the inverter to read import as export — the most common cause of inverted monitoring data.
Yes, but the Zappi needs to know the battery's charge and discharge current to calculate true solar surplus. Without a CT on the battery circuit (connected via a Harvi wireless sender), the Zappi may misinterpret battery discharge as surplus solar and charge the car from the battery. The recommended setup is: Zappi grid CT on meter tails (before Henley block), generation CT on inverter output, and a battery CT on the battery inverter output connected via Harvi.
The EV charger itself typically does not require DNO notification if connected to the existing consumer unit supply. However, if the installation involves changes to meter tails or if total connected load exceeds 100A single-phase, your installer should check with the DNO. Your existing solar G98 or G99 registration is not affected by adding an EV charger — but if you modify the solar or battery system at the same time, the usual notification rules apply.
The hardware fix is to install a Henley block and reposition the GivEnergy ID1 CT so it only monitors the consumer unit feed — not the EV circuit. As a temporary software workaround, you can set a discharge schedule in the GivEnergy portal that prevents discharge during your normal EV charging hours, or set a minimum SoC reserve. The hardware fix is preferred because it works automatically regardless of charging schedule. See our GivEnergy EV charger problems page for detailed steps.
Related guides
What a CT clamp is, how it works, the kettle test, and why direction matters. Start here if you are new to CT clamps.
How CT placement affects export limit enforcement and what happens when the limit is misconfigured.
Diagnosis steps if your CT clamp is reversed, on the wrong cable, or causing incorrect monitoring data.
Battery draining to your EV charger?
A remote diagnostic confirms the issue from your monitoring data and provides your electrician with a clear specification of what needs to change — CT repositioning, Henley block, and arrow direction. Most CT placement issues diagnosed in a single session.