Why Your Home Battery Keeps Draining Into Your EV
If you have just had a home battery installed alongside your EV charger, you have probably noticed something frustrating: every time you plug the car in, the battery you paid thousands for empties straight into the car. By morning the house is back on grid import, and the savings you expected from your storage system have vanished.
This is one of the most common questions we get from customers at ALPS Electrical, especially those running a Tesla Powerwall, GivEnergy or Sigenergy battery with a Zappi, Ohme or Tesla wall charger. The fix is not complicated, but it does require the right charger settings, the right inverter logic and sometimes a CT clamp moved to the right place.
The Root Cause: How Smart Chargers See Power
A smart EV charger like the Zappi or Ohme uses a CT clamp on your incoming main tail to measure house consumption versus grid import or export. In Eco or Eco+ mode the charger watches for solar surplus and only pulls what is being exported. So far so good.
The problem is that the charger does not know — or care — where the surplus comes from. If your battery is set to discharge to support the house, and the house load suddenly increases when the car plugs in, the battery starts discharging to cover that load. The smart charger sees house consumption drop (because the battery is now supplying it), interprets that as surplus solar, and ramps up the car.
The result: the battery drains into the car at full speed, exactly the opposite of what you wanted.
Fix 1 — Set Your Charger to True Solar-Only Mode
Most modern smart chargers have a mode that only charges from genuine solar export, not from battery discharge. The exact name varies:
- Myenergi Zappi: Use
Eco+mode but configure the export margin so the car only starts when solar export exceeds 1.4 kW. Then set the battery to not discharge while the car is plugged in (see Fix 2). - Ohme Home Pro / ePod: Use the smart charging schedule and let the app pull from the cheapest off-peak slot rather than trying to chase solar. Disable solar mode entirely if you have a battery — it is the simpler fix.
- Tesla Wall Connector: Tesla chargers do not have native solar diversion. Schedule charging during off-peak hours via the car or the Tesla app, and lean on the Powerwall's own logic (Fix 3) to keep house battery and car battery separate.
- Easee / Wallbox / Hypervolt: Each has its own surplus-only setting. Check the export threshold is set high enough that battery discharge cannot trigger it.
Fix 2 — Tell the Battery to Stop Helping the Car
This is the step most people miss. Whatever charger you have, the battery itself needs a rule that says: "do not discharge to support EV load."
- GivEnergy: In the GivEnergy app go to Settings → Battery Charge/Discharge Schedule and create a discharge pause window during typical EV charging hours. Better still, use the GivEnergy EV Charger which integrates with the battery and prevents the cross-feed natively.
- Sigenergy SigenStor: The Sigenergy app has a dedicated "EV charging from grid only" toggle. Switch it on and the battery will hold its state of charge while the car is plugged in.
- Tesla Powerwall: In the Tesla app set Storm Watch off and use the Self-Powered mode with a reserved minimum (e.g. 50%). The Powerwall will not discharge below the reserve, so the car has to pull from the grid once you cross that floor.
- FOX ESS: Use the work mode "Backup" while charging, which forces the battery to hold its charge.
Fix 3 — Check Where Your CT Clamps Are Fitted
A surprising number of these problems are physical: the CT clamp on the EV charger is fitted on the wrong cable. It should be on the incoming grid tail, between the meter and the consumer unit, upstream of any battery or solar connection. If it is fitted downstream of the battery, the charger cannot tell the difference between solar export and battery discharge.
If your installer fitted everything in one visit they probably got this right. But if you added the EV charger after the solar and battery, double-check the clamp position. We see this misplaced about one in five times when we audit other installers' work in Teesside and Yorkshire.
Fix 4 — Use Time-of-Use Tariffs Properly
Once the charger and battery are not fighting each other, the most cost-effective approach is to charge the car overnight from the grid on a cheap EV tariff (Octopus Go, Intelligent Octopus, EDF GoElectric) and reserve your battery for daytime house consumption. This is almost always cheaper than trying to direct daytime solar into the car battery.
The maths: charging a 75 kWh car battery from 20% to 80% takes about 45 kWh. At Octopus Intelligent's 7p off-peak rate that costs £3.15. The same charge from your home battery would mean 45 kWh of stored solar that is then not available for the house, forcing you to import 45 kWh of grid electricity at peak rates (around 30p) — roughly £13.50. The savings on cheap tariffs are an order of magnitude better than self-supply.
What ALPS Does on Every EV + Solar + Battery Install
When we install all three together, we configure the system from day one to keep the car and the house battery on separate energy paths. That means:
- CT clamp fitted in the correct upstream position.
- EV charger set to off-peak grid charging by default, with a manual override for "surplus-only" days when the homeowner wants pure solar miles.
- Battery set to self-consumption with a 20% reserve and an EV-charging discharge pause.
- Documentation handed over showing exactly which app does what and how to change modes seasonally.
If your existing installation is fighting itself, we can audit it and reconfigure. Give us a call on 01642 790489 or request a callback and we will run through your setup over the phone first — most problems can be fixed without a site visit.