If you're putting a smart EV charger (a Zappi, an Ohme, a hypervolt and so on) onto a house that already has solar panels and a home battery, there's a classic gotcha that catches even good installers. Left unaddressed, your battery discharges to power the car — so instead of charging the EV from cheap off-peak grid or free daytime sun, you drain a battery you paid thousands for, then refill it later at a worse price. Everyone loses except the meter.
The good news: the fix is standard, cheap (roughly £100–200 of parts and labour on top of the charger install), and the diagram below is genuinely all there is to it. The whole thing comes down to where you clip the little sensor rings.
First, what is a CT clamp?
A CT clamp (current transformer) is a small plastic ring that clips around a live cable and measures how much electricity is flowing through it — and in which direction. It doesn't touch the wire; it just senses it. Three different gadgets in a modern home each rely on one:
- your solar & battery inverter uses a CT to see whether you're importing from the grid or exporting spare solar, so it knows when to charge or discharge the battery;
- your smart EV charger uses a CT to see your total household demand, so it can throttle the car and not trip your main fuse (and, in solar mode, only sip surplus sunshine);
- some homes also have a CT just for metering generation.
Each CT only "knows" about the cable it's clipped to. That single fact is the whole story — what a CT is allowed to see is decided by where you put it. Put two of them looking at the same stretch of cable and they start reacting to each other, like two thermostats fighting over one room.
Why the battery ends up feeding the car
Picture the naïve setup: the car charger is simply wired into your existing fuseboard, downstream of everything. Now the car starts pulling 7 kW. The inverter's CT sees a big new lump of "household demand" appear — it can't tell it's the car — so it does exactly what it's designed to do and discharges the battery to cover it. You wanted to charge the car; instead you've emptied the battery into it.
The fix is to arrange the wiring so the inverter's sensor never sees the car's cable. Then, as far as the battery is concerned, the EV charger doesn't exist — and it leaves the car to charge from the grid (or genuine solar surplus) as intended.
Figure 1 — The correct order. The EV CT goes on the incoming supply before the split, so the charger sees the whole house. The EV charger takes its own feed off the first Henley block. The solar CT sits after that branch — so the inverter and battery are blind to the car and won't drain to feed it. The second Henley block is only needed if your house and solar run on separate consumer units. Each CT clamp's arrow must point toward the load it feeds.
The rule in one line
If you remember nothing else: the car's cable must branch off before the solar/battery sensor. The EV sensor sees everything; the solar sensor sees everything except the car. Get that order right and the two systems simply ignore each other.
Why the EV sensor still sees the whole house
It's deliberate. A smart charger needs to know your total household draw so it can dial the car back on a cold evening when the oven, kettle and heat pump are all on — that's how it avoids tripping your main fuse (and how "charge only from solar surplus" modes work). So the EV CT lives right up at the incoming supply and watches the lot. The charger already knows how much it is sending to the car, so it isn't confused by seeing its own load.
What to ask your installer
You don't need to wire this yourself — and you shouldn't (see the safety note below). But knowing the right questions tells a good installer you understand the job, and flags a weak one early:
- "Will the EV charger have its own feed off a Henley block, ahead of the battery's CT?" The answer you want is yes. If they plan to just spur the charger off the existing consumer unit, ask how they'll stop the battery discharging into the car.
- "Where exactly will each CT clamp sit, and which way do the arrows face?" Every CT has a direction arrow that must point toward the load. A reversed clamp reads backwards and breaks solar/export logic.
- "Is there physical room in the meter cupboard for the Henley block and the extra clamps?" Space and cable routing are the real-world constraint — worth checking before install day, not during it.
- "Does my inverter or charger support the setup directly?" Some ecosystems (e.g. myenergi's Zappi with certain hybrid batteries) have their own settings or a second CT to solve this in software — sometimes that's tidier than extra tails.
If you already have the problem
Notice your battery percentage dropping every time you plug the car in? That's the tell. It usually doesn't mean ripping anything out — an electrician can often fix it by adding a Henley block and moving the existing CT clamp to the right side of the branch, or by changing an inverter/charger setting. Worth a call before you write off the battery as "not working".
Safety — this is not a DIY job
Everything above happens on the meter tails: the thickest, live-all-the-time cables in your home, upstream of your main fuse. This is notifiable electrical work and must be done by a qualified, registered electrician (Part P in England & Wales; equivalent elsewhere). Adding solar or a battery also requires notifying your network operator (G98/G99). This article explains the principle so you can have a smarter conversation with your installer — it is not a wiring guide, and you should never open a meter cupboard or work on live tails yourself.
The bottom line
An EV charger, solar and a battery all get along fine — they just need to be introduced in the right order. Branch the car off a Henley block before the battery's sensor, point every CT arrow the right way, and each system stays in its lane: the charger balances the whole house, the battery ignores the car. It's an hour of an electrician's planning that saves you years of quietly cycling your battery for nothing.