How to Stop Your Solar Battery Feeding Your EV Charger

If you have solar panels, a home battery, and an EV charger, you may have noticed something frustrating: your expensive battery storage system drains its charge into your electric vehicle instead of powering your home through the evening and overnight. This is a common issue that affects many households with combined solar, battery, and EV setups. The good news is that it is entirely fixable with the right configuration. In this guide, we explain why it happens and how to solve it for the most popular battery and charger combinations.

Why Does Your Battery Feed Your EV Charger?

To understand the problem, you need to know how a home battery system decides when to discharge. Most battery systems, including the Tesla Powerwall and GivEnergy, monitor your home's electricity consumption via a current transformer (CT) clamp on your incoming supply. When the battery sees that your home is drawing power from the grid, it discharges to meet that demand and reduce your grid import.

The issue arises because an EV charger is a very large electrical load, typically drawing 7 kW on a single-phase supply. When you plug in your EV and it starts charging, the battery sees a sudden spike in home demand and responds by discharging as fast as it can to meet it. A fully charged 13.5 kWh battery can be completely drained in under two hours by an EV charger running at 7 kW. The battery was intended to power your lights, fridge, and other home loads through the evening and overnight, but instead it has emptied itself into your car.

This is problematic for two reasons. First, you lose the overnight home backup that the battery was supposed to provide, meaning you import expensive grid electricity for your evening and morning household consumption. Second, the round-trip efficiency of charging a battery and then discharging it into an EV adds approximately 10-15% losses, meaning you waste energy in the process. It is far more efficient to charge your EV directly from solar or the grid than to route the energy through your home battery first.

Solutions for Tesla Powerwall

If you have a Tesla Powerwall, there are several approaches to prevent it from feeding your EV charger. The simplest method is to set a minimum reserve percentage on the Powerwall. In the Tesla app, navigate to the Powerwall settings and increase the backup reserve to a level that preserves enough energy for your evening and overnight home loads. For example, setting a 50% reserve on a 13.5 kWh Powerwall keeps 6.75 kWh protected from the EV charger, ensuring your home still has stored energy available.

A more sophisticated approach is to schedule your EV charging for a specific time window, either through your EV's built-in scheduler or your charger's app. Set the EV to charge during the overnight off-peak period (typically midnight to 5am on time-of-use tariffs). During this window, the Powerwall can be set to charge from the grid as well (if you are on an off-peak tariff like Octopus Intelligent Go), so both the car and the battery charge at the cheapest rate. The Powerwall then has a full charge ready for the next day's home consumption.

Solutions for GivEnergy

GivEnergy systems offer granular control through their app and portal. The most effective approach is to use the charge and discharge time slots to prevent the battery from discharging during your EV charging window. For example, if your EV charges between midnight and 5am, set the GivEnergy battery to "charge only" mode during those hours. This prevents the battery from discharging into the EV while allowing it to charge from cheap off-peak grid electricity.

CT clamp placement is also critical with GivEnergy systems. If the CT clamp is positioned on the main incoming supply and the EV charger is connected downstream of the clamp, the battery will see the EV charging load as household demand. Some installers place the EV charger on a separate circuit upstream of the CT clamp, so the battery does not see the EV load at all. This hardware-level solution prevents the issue regardless of software settings, but it requires careful electrical design during installation.

The myenergi Zappi: A Natural Solution

If you are using a myenergi Zappi EV charger, the solar diversion issue is largely solved by the charger's design. In Eco mode, the Zappi only draws surplus solar energy for EV charging. It monitors the same CT clamp data as your battery system and intelligently coordinates with it. When the Zappi detects genuine solar surplus, it diverts that energy to the EV before the battery absorbs it. In the evening, when solar generation has stopped, the Zappi stops charging in Eco mode and the battery is free to discharge into your home loads without interference.

For overnight EV charging, you can schedule the Zappi to charge from the grid during off-peak hours independently of the battery system. The myenergi ecosystem, particularly when paired with a myenergi Libbi battery, offers the most seamless integration because all devices communicate directly and share CT clamp data. However, the Zappi also works well alongside Tesla Powerwall and GivEnergy batteries when configured correctly.

General Best Practices

Regardless of your specific equipment, there are several general principles for managing a solar, battery, and EV charger setup effectively. Schedule EV charging during off-peak hours rather than plugging in immediately when you arrive home. Use your battery's discharge scheduling to prevent it from emptying during EV charging windows. Ensure your installer configures CT clamps correctly so that the battery and charger have accurate data about generation and consumption. Consider a solar-diverting charger like the Zappi for daytime solar EV charging, and use scheduled grid charging overnight.

At Premier Electrical Renewables, we configure every solar, battery, and EV charger installation to avoid this common problem from day one. If you have an existing system that is not set up correctly, we can also carry out a reconfiguration visit to optimise your setup. Proper configuration makes the difference between a system that works against itself and one that delivers maximum savings across your home and vehicle.