Flexible loads
How to charge an EV from solar when you also have a home battery.
Start with when the car must be ready. Then share the expected solar surplus between the car and the home battery, with a fallback if the clouds change the plan.
The common failure
A homeowner adds solar, then a battery, then an EV charger. Each product works. The system does not. The charger may start when solar is high, but then cloud cover arrives. The battery may discharge into the car. The inverter may export when another device could have used the energy. None of these devices is broken. They are simply not coordinated.
The right questions
- When does the car need to be ready?
- How much reserve should the home battery keep?
- Will tomorrow have better solar?
- Is exporting energy valuable right now?
- Is grid energy cheap enough to charge without regret?
- Will charging now create a comfort or peak-load problem?
Why static rules fail
"Always charge the car from solar" sounds rational until the car is needed early, the weather turns, or the battery reserve becomes more valuable than the extra EV range. Good energy management is contextual.
Look for surplus while the car is actually home.
A 24 kWh daily solar forecast does not mean 24 kWh is available for the car. Some production arrives while it is away; cooking, heating and other loads use part of the rest. Read the hourly solar production forecast, then subtract demand in the same hours. Avoid allocating the same surplus to both EV charging and the home battery.
Worked example · not a measured home
The car needs 18 kWh at the charger by 07:00.
Assume the car is plugged in from 10:00 today until 07:00 tomorrow. It needs 18 kWh measured at the charger, so any conversion from battery percentage has already allowed for car-charging losses. The plan allocates 6 kWh of AC input to home-battery charging; that is not 6 kWh of stored energy after losses.
The following six-hour PV window is invented to show the arithmetic. Each allocation assumes enough instantaneous surplus and device power to deliver it.
| Local time | PV production | Other home use | Home-battery charging | EV charging |
|---|---|---|---|---|
| 10:00–12:00 | 8 | 2 | 3 | 3 |
| 12:00–14:00 | 10 | 2 | 3 | 5 |
| 14:00–16:00 | 6 | 2 | 0 | 4 |
| Total | 24 | 6 | 6 | 12 |
The car receives 24 − 6 − 6 = 12 kWh from solar and still needs 6 kWh. A later permitted grid window can provide the rest. At a constant 3 kW, 6 kWh needs 2 hours; allow extra time if the car reduces power, household load limits intervene or charging pauses.
If PV falls to 18 kWh while the other allocations remain unchanged, only 6 kWh reaches the car and the remaining requirement becomes 12 kWh. Recheck actual charger energy before the final available window. Do not wait until departure to discover the forecast miss.
The home-battery allocation is a choice in this example. Your reserve requirement, car deadline and import/export tariff determine whether to change it. An expensive export window can also make exporting solar and charging later reasonable.
Why a charger may pause even when solar is available.
Solar surplus is power available now, not just energy over the day. myenergi documents a 6 A minimum for its AC charging operation: roughly 1.4 kW on a single 230 V phase, or 4.2 kW across three phases. Its ECO mode can use grid top-ups, while ECO+ can pause below the configured surplus threshold. Your own car, charger and phase settings may behave differently.
A forecast of 3 kWh over two hours averages 1.5 kW, but intermittent clouds can take the available power below the charging threshold. A smooth energy budget therefore needs checking against the actual power curve. Do not assume a three-phase charger automatically switches phases or follows every cloud.
Stop the EV from unintentionally draining the home battery.
Some systems treat the EV as ordinary house demand and discharge the battery to supply it. myenergi’s hybrid-battery guidance describes a model-specific export-margin approach for surplus charging. That is a configuration example, not a universal setting to copy to every inverter.
- Check the inverter and charger manuals for their supported battery-priority and surplus modes.
- During one charge session, compare PV, grid power and home-battery power. Verify that the observed flows match the intended priority.
- Check both sunny surplus charging and a scheduled grid-charge window. The battery may behave differently in each.
- Leave a departure-time fallback and keep the battery’s protected reserve intact.
GridPassport's product stance
GridPassport should make these tradeoffs visible enough to trust and automatic enough to forget. The point is not endless control. The point is a home that makes sensible decisions by default.
This is a practical example of Home Power Automation: one household plan coordinating devices that would otherwise optimize alone.
FAQ
Solar, battery and EV questions.
Can an EV charger drain a home battery?
Yes, depending on wiring, inverter settings and charger behavior. If the system sees the EV as ordinary home load, the battery may discharge into the car at a bad time.
Should solar surplus charge the EV or the home battery first?
It depends on the tariff, export value, battery reserve, car deadline and expected solar production. A fixed priority can be wrong on some days.
Is solar-only EV charging enough?
Solar-only charging can be useful, but it may fail when weather changes or when the car must be ready by a deadline. Good control mixes solar surplus, cheap grid windows and user needs.
Can I charge an EV with 1 kW of surplus solar?
Not necessarily using solar alone. Many AC charging setups need at least 6 A, about 1.4 kW on one 230 V phase. The charger may pause or supplement with grid power; check the car, charger, phase mode and settings.
Should I use the forecast total or the surplus for EV charging?
Use the surplus during the hours the car is plugged in. Subtract simultaneous household use and any planned battery charging from forecast PV. Allow for charging losses when translating the car’s stored-energy need into energy required at the charger.
Sources
References for this guide.
- myenergi: zappi charging modes
Manufacturer guidance on surplus tracking, minimum AC charging power and the difference between pausing and using grid top-ups.
- myenergi: hybrid PV and battery setup
Explains why hybrid batteries need careful configuration to avoid unintended discharge into an EV.
- IEA 4E EDNA: Residential HEMS and controllers
Defines HEMS as systems that connect residential energy devices and optimize generation, storage and consumption.
- SMA Sunny Home Manager 2.0
A mature HEMS reference for dynamic tariffs, EV charging, heat pumps and battery control.
- SPAN smart electrical panels
A premium infrastructure reference for circuit-level energy control and battery backup.
Make it personal
Forecast your own roof, then get tomorrow by email.
Use your location and system size instead of a generic benchmark. The result includes expected output, uncertainty and the strongest solar hours.