Charging an EV from solar: what really happens at the meter
"Charging the car from solar" sounds like one thing and is really three: the panels, the home battery and the grid, all feeding one large load at once. By the end you will know which of them is actually filling the car, why the home battery usually shouldn't be, and how to keep the two batteries from fighting.
The car is a load behind the meter
An electric car plugged in at home is a large appliance, nothing more. A typical 7 kW charger draws more than an oven, a hob and a kettle together, for hours at a time. Your meter doesn't know it is a car; it sees the house drawing more than it makes and bills the difference.
Solar, the home battery and the grid all feed the same wiring, and the car takes what it needs from whatever is available. Where the energy really comes from is decided by physics and the settings on each device, not by the label on the charger.
A 7 kW charger against a 4 kWp roof
A 4 kWp array is rated at 4 kW under ideal test conditions. On a UK roof a good summer midday is usually around 3 kW, and the house is using some of it. A 7 kW charger pulls its full 7 kW whenever the car will accept it, which most cars will until nearly full.
So on the sunniest day of the year the panels supply well under half of what the charger asks for. Solar alone charges a car only when the charger is told to slow down to match the sun, which comes later.
Where the rest comes from
When the car asks for more than the panels make, the shortfall comes first from the home battery, if it is allowed to discharge, and then from the grid. A Powerwall in Self-Powered mode sees a 7 kW draw as the house needing power and discharges to meet it, up to its own limit: 5 kW continuous for a Powerwall 2, the full 7 kW and more for a Powerwall 3.
On a sunny afternoon with a 7 kW charger, 3 kW of solar and a Powerwall 2:
- 3 kW from the panels
- up to 5 kW from the Powerwall, until it reaches the backup reserve
- the remainder from the grid, rising to 4 kW or more once the battery is empty
Within about three hours the home battery is flat, the car is perhaps a fifth fuller, and the house faces the evening with nothing in store.
Why the home battery is a poor way to fill a car
Feeding a 60 kWh car from a 13.5 kWh home battery is a bad trade, for three reasons.
The scale is wrong
A full Powerwall is roughly a fifth of a large car battery. Emptying it buys the car perhaps 50 miles and leaves the house importing at the evening rate, the dearest of the day.
The losses stack
Energy that goes solar to Powerwall to car passes through two batteries and several conversions. The Powerwall loses around 10 % on a round trip; the car's own onboard charger loses a similar share turning AC back into DC for its pack. Roughly a fifth of the solar never reaches the wheels. Sending the same solar straight to the car, or to the grid for export credit, wastes far less. See where the missing ten percent goes.
The battery was bought for the evening
A home battery earns its money by covering the hours when import is dear. If it is empty at 6 pm because the car took its charge at 3 pm, it has done nothing that day. The car, unlike the house, can wait for the cheap window.
There are exceptions: far more solar than the house can use, an export limit that leaves midday surplus with nowhere to go, or a tariff with no cheap window at all. For most homes, though, the battery should be kept out of the car.
Solar-following charging
The way to genuinely charge from solar is to throttle the charger to the surplus. Most home chargers can vary their current, and several have modes built for this.
- Zappi-style eco modes. A myenergi Zappi in its Eco modes measures export at the meter and adjusts the charging current so the car takes only what would otherwise leave the house. One mode tops up from the grid to keep the car above its minimum current; the other stops rather than import.
- Tesla Wall Connector. Paired with a Powerwall, the Tesla app offers a similar option that charges the car on solar surplus, using the Powerwall's own meter readings.
- Other chargers. Easee, Zaptec, Ohme and others offer solar or "green" modes of varying sophistication. Check what yours measures.
Two things to know. Cars have a minimum charging current, around 6 A or about 1.4 kW, below which they won't charge at all, so a solar-following mode does nothing on a dull day. And the charger can't tell whether the surplus it sees is solar or the Powerwall discharging. Unless the Powerwall is told to hold back, the eco mode will happily drain it.
The cheap window for the car, the sun for the house
For a home with a battery, an EV and a cheap overnight window, the pattern that works for most people is simple:
- The car charges in the cheap window, at full speed, from the grid. It is the largest load and the most patient one.
- The home battery fills from solar during the day and covers the evening, or fills in the same cheap window in winter.
- On sunny days with real surplus, a solar-following mode tops the car up with what the house and the battery don't need.
Intelligent Octopus Go extends the idea by scheduling the car's charging and adding cheap slots when it does. See Go and Intelligent Go with a battery.
Telling the battery the car is plugged in
The remaining problem is that the home battery's schedule doesn't know about the car. Plug in at 3 pm in Self-Powered mode and the Powerwall discharges into it regardless. The usual answer is a rule: when the car starts charging, raise the backup reserve to 100 %, which stops the Powerwall discharging at all, and put it back when the car stops. If your charger can't report its state, a rule on house power rising above 6 kW for ten minutes is a rough stand-in, though an electric shower will trip it now and then.
Export limits and clipping while the car charges
Many homes are limited to 3.68 kW of export under G98, or to a figure the network operator set under G99; see the 3.68 kW export limit. On a bright day a larger array can make more than the house uses plus the limit, and the inverter throttles the panels. That is clipping: energy you never get.
A car charging is the cheapest fix there is. Any load inside the house reduces export, so a car taking 3 kW at midday turns clipped solar into miles. The trade-off cuts the other way too: while the car charges there is nothing left to export, and a solar-following charger set to keep export near zero will, by design, take everything that would have earned export credit. Whether that is a good deal depends on the gap between what a kWh earns on export and what it costs to put in the car overnight. On most tariffs the overnight price is the lower of the two, which is one more reason to charge the car at night.
In SunHarvest
Once a charger is linked under Settings › Site, or a Tesla Wall Connector is already paired to the Powerwall, the rule editor gains two signals: EV charging (charging or not) and EV charge power in kW. The rule most owners write is the one above: when EV charging changes to charging, set backup reserve 100 %, with a second rule that puts it back. The change log on Control › Powerwall shows each time it fired. The AI planner sees the same reading and treats the car as the one large load the home's learned load profile can't predict. SunHarvest only reads a charger; starting, stopping and solar-following stay with the charger's own app.
General information, not financial or electrical advice. Tariff terms, prices, warranties and connection rules change and differ by supplier, region and installation; check the current documents for yours. SunHarvest is not affiliated with Tesla or Octopus Energy.