WattOff-grid and solar EV charging
Charging a car from your own generation is the cheapest mile there is. It's also the one people most often build wrong, usually by assuming panels equal power.
Grid-tied solar does not work in an outage
A standard grid-tied inverter is required to shut down when the grid goes away — anti-islanding protection, so linemen aren't working on a circuit your roof is energising. If your plan for a blackout is "I have solar," the plan doesn't work. You need a hybrid inverter with islanding capability and usually storage. This is the single most expensive misunderstanding in this whole subject.
Three architectures
1. Solar surplus charging (grid-tied)
The most common and least expensive to add. Current transformers on your service measure generation and household load; the charger throttles to consume only the excess that would otherwise export. On a sunny day the car soaks up surplus for free. When a cloud passes, charging pauses or falls back to grid.
Key constraint people miss: the minimum charge current is about 6A, roughly 1.4 kW. Below that surplus, nothing happens at all. A modest array on a mediocre day may never clear the threshold, which is why surplus charging shines in summer and largely disappears in a northern winter.
2. Hybrid inverter with storage
Solar charges a battery bank; the bank runs the house and the car, grid optional. This is what actually delivers charging during an outage, and it decouples charging from the weather at that moment. Expect a hybrid inverter sized well above your charging load — charging at even 24A is a 5.8 kW continuous draw on top of household demand.
3. DC-coupled charging
The efficiency purist's option: send DC from panels or battery straight toward the vehicle, skipping DC→AC→DC conversions and, in some designs, the car's onboard charger entirely. Bidirectional DC-coupled units exist and enable vehicle-to-home. Expensive, ecosystem-locked, and worth understanding before you commit to a platform.
Sizing reality check
| Array size | Good-day surplus | Realistic charge rate | Miles/day added |
|---|---|---|---|
| 4 kW | 1.5–2.5 kW | 6–10A | 15–25 |
| 8 kW | 3–5 kW | 12–20A | 35–60 |
| 12 kW+ | 5–8 kW | 20–32A | 60–100 |
Halve all of it for winter at northern latitudes, then halve the miles again because cold cuts efficiency too. Solar EV charging is a summer sport where I live.
Chargers with real solar integration
Most Level 2 units are dumb switches — fine, but they can't follow your generation. The ones that can fall into two camps:
- Ecosystem-native. Chargers that talk directly to a specific inverter platform. Cleanest experience, but you're marrying the platform.
- Monitor-paired. A charger plus a separate energy monitor with CT clamps on the service. More wiring, less lock-in, usually cheaper.
Charger selection criteria and the solar-capable options →
The open-source route
If you'd rather own your stack than rent a cloud service, two projects matter:
- evcc — an open-source EV charge controller and home energy management system that runs locally on hardware you own, with the explicit goal of avoiding cloud dependence. It talks to a very long list of chargers, inverters, and vehicles, and does solar surplus charging natively. It's MIT licensed with a sponsorship model for some components.
- OpenEVSE — open hardware and firmware for the charging station itself. The ESP32 gateway accepts real-time solar and grid data and adjusts the pilot current accordingly, so it charges from surplus. MQTT throughout, so it drops into Home Assistant without a fight.
The two combine: OpenEVSE as the hardware, evcc as the brain. There are documented community builds pairing evcc with off-grid-capable inverters and a plain Tesla Mobile Connector, which is a notably cheap way into surplus charging if you're comfortable with the configuration work.
Where a generator fits in an off-grid stack
It's the layer under everything else. Solar covers most days, storage covers the night, and the generator covers the week in December when neither is enough. Sized correctly it runs rarely — which is exactly the point. Sizing →