Saturday morning. Phone goes off. Customer’s voice on the other end: “The car and batteries charged during the cheap rate perfectly. Just tried charging the car and it’s pulling from the battery, not the grid.”
Fair shout from his perspective — he’d plugged the car in at lunchtime to top it up, and the dashboard on his Solplanet app was showing the battery emptying out. He was paying tea-time prices on a battery he’d just paid for. Not great.
The short version: the system was working exactly as designed. The battery’s CT clamp can’t tell the difference between an EV charging and a kettle boiling — to it, both are just “house demand”. Two fixes: change how you charge (only ever during the cheap-rate window), or change where the EV is wired (upstream of the battery’s CT clamp so it never sees that load). We were back on site Saturday morning and did the proper hardware fix at no extra cost.
Was the Install Actually Wrong?
Short answer: no. The install was textbook. Here’s what was on the wall:
- 1 × Solplanet ASW5120-LB-G3 hybrid inverter (5 kW)
- 3 × Solplanet 5 kWh battery modules (15 kWh total usable storage)
- 1 × Solplanet floor-standing base unit with feet
- 1 × Projoy 125 A DC isolator with enclosure
- 1 × Eastron SDM120CT energy meter and 100 A / 40 mA CT clamp
That CT clamp at the bottom of that list is the bit that matters for this story — so hold that thought.
So What Was Actually Happening?
When we dug into the timeline with the customer, the picture cleared up fast. He’d been doing exactly what we’d recommended: charging the car overnight on the cheap-rate window of his smart tariff. About 7p a unit between 12:30 and 5:30. Lovely.
Trouble was, his car was only getting to about 70% by morning — the off-peak window wasn’t quite long enough for a full charge from low. So when he saw it sitting at 70% with the sun out, he was plugging it back in mid-morning to top it up. Force-charge button. Done.
And every time he did that, the battery he’d just had installed would dutifully empty itself into the car. At 30p+ per equivalent unit of stored energy.
Crucially, the system wasn’t doing anything wrong. It was doing exactly what a solar battery is supposed to do: when it sees the house calling for power and the sun isn’t quite covering it, it discharges. That’s the whole point of having one.
Why Is the Battery ‘Fooled’ Into Running the EV?
Here’s where the CT clamp comes in. CT stands for “current transformer” — it’s the little donut-shaped clip that goes round one of the live tails coming into your consumer unit.
The job of that clamp is to tell the battery how much power the house is using. The bigger the number, the more the battery knows it needs to discharge to cover the load. Simple, clever, works brilliantly 99% of the time.
The catch: that clamp can’t tell the difference between things. To it, a kettle drawing 3 kW and an EV charger drawing 7 kW look identical. Both are just “demand”. So when the customer hit “boost” on his car app, the battery saw a fat new load appear on the house side of the meter and went, “right, I’ll cover that” — and started feeding it.
The CT clamp sits on the grid side. Everything past the consumer unit — appliances and the EV — looks the same to it. That’s the whole problem.
So the system did exactly what it was designed to do. The customer was running an extremely demanding “appliance” at the worst possible time of day, and the battery was bravely covering it like a champ. Just not in a way that saved any money.
What Are the Two Fixes?
This is where it gets interesting. There are two genuinely valid answers, depending on what you want.
Fix 1 — Behaviour: charge only during the off-peak window
If the customer is happy charging the car overnight on cheap rate, the answer is: don’t manually start a charge during the day. Let the smart tariff and the EV’s scheduling do the work.
- Plug in whenever — but only let it charge in the cheap window
- Never use boost / manual / force-charge during peak rate
- Accept that if you do hit boost during the day, the battery will discharge into the car. That’s not a fault, that’s physics
Fix 2 — Hardware: move the EV upstream of the CT clamp
The proper engineering answer. Split the Henley blocks so the EV charger is wired before the battery’s CT clamp instead of after it. Now the battery genuinely cannot see the EV — even if you force-charge in broad daylight.
- Battery still works normally for the rest of the house
- EV charger pulls only from the grid, never from the battery
- Customer can hit “boost” whenever they like — no surprises
Both are valid. Fix 1 costs nothing and works perfectly if the customer sticks to the routine. Fix 2 is a small bit of rewiring at the meter end and removes any need to think about it ever again.
If you’ve got solar, a battery, and an EV charger — ask your installer where the battery’s CT clamp will sit relative to your EV charger. The right answer for most homes that want to dodge the trap above is: battery CT clamp downstream of the EV branch. Tell them you’ve read about Henley-block splitting if they look puzzled.
What Did We End Up Doing?
We picked Fix 2. We were back on site Saturday morning with the kit, dropped the cover off the meter cabinet, split the Henley block, ran a fresh feed for the EV charger that lived before the battery’s CT clamp, and reseated everything. Good as new.
About an hour and a half on site. Customer can now plug his car in at any time of day, hit boost if he likes, and the battery quietly carries on doing what it’s supposed to do — running the rest of the house.
No charge for the visit. The system was technically working as designed, but if a customer’s confused about what’s happening, that’s our problem to fix — not theirs to live with.
— The fitter on the day
What Should You Know If You’ve Got Solar + Battery + EV?
If you’re already running this combo or about to, here are the bits people usually find out the hard way.
Your battery doesn’t know what an EV is. It just sees demand. If you drive that demand up during peak hours by manually starting an EV charge, the battery will pay for it on your behalf at peak rates. That’s not a bug — it’s the design.
Smart tariffs make all of this work. Octopus Go, Cosy, Intelligent Octopus, OVO Charge Anytime — these all give you the cheap overnight slot that makes EV charging economical. Without one of those, you’re paying around the same to fuel an EV as a small petrol car. The battery and panels make it cheaper than that, but only if you let the cheap window do the heavy lifting.
If your overnight charge isn’t enough, that’s a sizing question, not a force-charge question. If you’re regularly arriving at morning with too little range, the answer is usually a longer cheap window (some tariffs offer 6 hours instead of 5), a slower car-side charging speed paired with an earlier plug-in, or — if you’re running a really chunky car — a second cheap-rate session. Force-charging during peak rate to “catch up” is the most expensive way to do it.
Mention the EV at survey time. If you’re fitting battery storage for the first time and you’ve got an EV charger already (or planning one), tell your installer up front. We can plan the metering and Henley arrangement around that, instead of unpicking it later. Get in touch if you want a survey that thinks about the whole picture, not just the panels.
Tyler Crocker is Electrical Supervisor at Amp Renewables. Based in Washington, Tyne and Wear — call us on 0191 535 2711 or visit amprenewables.co.uk.