Solar surplus EV charging

How a chargepoint follows measured PV export while respecting vehicle minimum current, site load and other flexible devices.

Solar surplus EV charging varies the current offered to a plugged-in vehicle so charging uses electricity that would otherwise be exported. It needs a chargepoint or energy controller that can measure site import and export and communicate an allowable current to the vehicle.

It does not connect the panels directly to the car. The PV inverter supplies the home’s AC system, and the EV chargepoint remains an AC load subject to its circuit, vehicle and site limits.

Surplus is a measured balance

At any moment:

site surplus = PV generation − other household demand − other charging loads

The controller normally measures this at the grid connection using a meter or current transformer. It increases EV current when export rises and reduces it when the home would otherwise import.

Measurement errors can reverse the intended result. The sensor direction, phase assignment, location and communications should be checked during commissioning with known import and export conditions.

The vehicle has a minimum AC charging current

Mode 3 AC charging communicates an available current through the control pilot. The standards minimum is 6 A per active phase. Below that point, a controller cannot keep reducing smoothly towards zero.

When solar surplus is below the vehicle’s minimum, the chosen mode must do one of three things:

pause and wait for sufficient surplus
import the shortfall from the grid
combine solar with another permitted source

On a nominal single-phase UK supply, the standards minimum corresponds to roughly 1.4 kW. This is a technical threshold, not a promise that every car will start or remain stable at exactly that power. Three-phase minimum charging requires surplus on all active phases and therefore a higher total power.

Solar-only and blended modes

A strict solar-only mode minimises grid import but can pause frequently in variable cloud or when household loads switch on.

A blended mode maintains the minimum current using some grid energy and adds available solar. It can produce a steadier session and may be preferable when the car must gain energy by a deadline.

The interface should make the permitted grid contribution clear. A mode marketed as green or eco may still import by design.

Add delays and hysteresis

Without stabilisation, a passing cloud can make the car start and stop repeatedly. Controllers use a delay, import tolerance or hysteresis so a brief change does not immediately switch state.

The exact values are product settings. The aim is to balance low grid import against stable charging and the vehicle’s response. Excessive delay can export useful energy while the car waits; too little can create nuisance switching.

Coordinate the home battery

A battery and EV can both try to absorb the same surplus. If their controls are independent, the battery may discharge into the car or the two systems may oscillate around the grid meter.

Choose an explicit priority, such as:

preserve the battery backup reserve
fill the home battery for evening household use
give the EV enough energy for its next journey
send remaining surplus to the EV
export anything left

Another household may put the EV first. The correct order depends on energy needs, losses, tariff and backup value, not a universal rule.

Where the platform offers integrated control, confirm what it actually measures. A shared app is helpful but does not guarantee that every inverter, battery and vehicle follows one control loop.

Other diverters and flexible loads

Hot-water diversion, space heating and appliance schedules can also respond to PV. Two devices independently targeting zero export can hunt against each other.

Set priorities or distinct thresholds and delays. Keep safety, hot-water hygiene and electrical load management separate from the economic surplus decision.

Dynamic load management still applies

The chargepoint must stay within the circuit and site limits even when plenty of solar is present. Solar generation offsets net grid import, but cables and switchgear inside the installation still carry the EV current according to their arrangement.

Dynamic load balancing may reduce charging when household demand rises or if the generation sensor fails. Do not raise a configured site limit simply because the app shows PV production.

Export-limited sites also need coordinated control. The EV can absorb surplus, but it is not a guaranteed export-limitation device unless included in the approved scheme and fail-safe design.

Vehicle and phase compatibility

Check whether the vehicle charges on one or three phases, its minimum and maximum AC current, and whether it tolerates solar pauses. Some systems can switch phase mode; that transition must be specifically supported by the chargepoint and vehicle.

A three-phase chargepoint does not ensure three-phase charging. A single-phase vehicle uses one phase, and site metering must prevent an apparently balanced total from hiding an unsuitable phase condition.

Energy, value and carbon are separate aims

Solar charging can increase on-site use, but the most valuable schedule depends on the alternative export payment, off-peak import price, storage losses and when the car is present.

Tariff comparisons depend on current rates and eligibility. Actual kWh imported, exported and delivered to the car establish the result. A low-cost night charge can sometimes be preferable to waiting for uncertain daytime solar, especially before a required journey.

Commissioning checks

Test the system under changing conditions:

known grid import and export
household load switching
surplus above and below the vehicle minimum
loss of meter or communications data
home-battery charge and discharge
site maximum demand
vehicle unplug and reconnection

The fail-safe state should not exceed the electrical design or create uncontrolled import. Record sensor direction, mode, grid allowance, start and stop behaviour and priority with other devices.