Smart EV charging: schedules, solar power and shared capacity

Smart charging adjusts when an electric vehicle charges and how much power it draws. It can use cheaper hours or surplus solar generation, share a limited supply between vehicles, and respond to requests from the electricity system.

How parking time creates flexibility

When your car can stay plugged in longer than it needs to charge, the controller has room to move charging within that window. A fixed schedule sets permitted charging hours; adaptive control can revise the schedule as prices, available power or the car's needs change. The U.S. Department of Energy's fleet guidance notes that manually programmed schedules need updating when operating hours or tariff periods change. U.S. Department of Energy — Managed EV charging for federal fleets

Consider a hypothetical car parked for ten hours that needs 24 kWh delivered at its charging connection. If it can continuously accept 6 kW and that power remains available:

Charging time = 24 kWh ÷ 6 kW = 4 hours.

The ten-hour window leaves six hours in which charging can be deferred or interrupted. If the available power falls, charging takes longer and that flexibility shrinks. This example uses energy at the connection, not energy added to the battery; conversion losses and vehicle auxiliary consumption reduce the amount stored.

To plan for departure, the controller needs to know when the car will leave and how much energy it needs by then. These inputs may come from the driver's settings or from vehicle data and fleet route plans. The controller also needs the available charging capacity to work out a feasible schedule. U.S. Department of Energy — Smart charge management implementation

Choose the charging window using the applicable bill, including any demand or capacity charge. A cheap energy period can still increase the bill if charging alongside an oven or heat pump raises the peak used for billing. DOE distinguishes charges for energy consumed from charges based on peak demand. U.S. Department of Energy — Evaluating utility rate options In Norway, NVE recommends spreading consumption to reduce network charges and lists overnight EV charging as one option. NVE/RME — Household electricity bills and network charges

Sharing the available electrical capacity

Fixed power sharing divides a configured charging allowance among vehicles. Dynamic load management adjusts that allowance as other building loads change. A meter measures demand so the controller can update charging limits. Tesla's U.S. Wall Connector guidance gives a familiar example: charging slows while a dryer and water heater are running, then speeds up as household demand falls. Its dynamic mode requires a Tesla-approved meter. Tesla — Wall Connector power management

For an illustrative site with a 20 kW total limit and 8 kW of other demand, up to 12 kW remains for charging. If other demand rises to 14 kW, the remaining allowance falls to 6 kW. These are simplified power-budget calculations; an actual installation must also respect circuit and per-phase current limits.

How that allowance is divided is a separate decision. With a fixed 12 kW allowance shared equally, one car drawing 12 kW drops to 6 kW when a second joins, provided both can use their allocation. Departure-based control can instead prioritise an early-departing vehicle over one with a longer stay. DOE describes allocation based on available capacity, state of charge and parking time. U.S. Department of Energy — Smart charge management implementation

On an AC connection, the charging equipment tells the car the maximum current it may draw. The car may take less, so increasing the allowance does not guarantee a matching increase in consumption. Argonne's tests of 25 plug-in vehicle models found that most drew less current than offered. Argonne National Laboratory — PEV charging response characterization, October 2025

A limit chosen to avoid a billing peak serves a different purpose from the installation's maximum permitted load. The installer must configure electrical limits to suit the supply, circuits and protection. For installation and handover checks, see Home EV charging: power, installation and daily use.

Following solar surplus

Solar-surplus control follows the generation left after other household loads. As clouds pass or appliances switch on, the controller adjusts the car's charging allowance to reduce electricity imported from or exported to the grid. Myenergi describes this operation for its zappi charger. Myenergi — Zappi charging modes

For the AC control used by zappi, myenergi specifies a minimum charging-current setting of 6 A per phase. At a nominal 230 V line-to-neutral, that corresponds to roughly 1.4 kW on one phase or 4.2 kW on three phases. Switching to one phase therefore allows charging from a smaller surplus. Supported three-phase zappi units with V5 firmware can switch automatically as generation changes. Myenergi — ECO/ECO+ charge rates and phase switching

With 1 kW of surplus on one phase, zappi's ECO mode draws the shortfall from the grid. ECO+ at a 100% minimum green level pauses after its configured delay; a lower setting permits some grid support. Start and stop timers prevent rapid switching as generation fluctuates. Myenergi — Zappi charging modes

As departure approaches, decide how much grid electricity you will accept to finish charging. A zappi boost can temporarily prioritise charging in ECO or ECO+ mode. Myenergi — Zappi charging modes It still respects the device and grid limits configured for the installation. Myenergi — Zappi operation and installation manual, March 2023

Price signals and grid requests

A utility can request lower demand through a participating charging service. A company that pools many vehicles or sites to coordinate their response is called an aggregator. DOE describes these companies as intermediaries between fleets, charging infrastructure and the grid. U.S. Department of Energy — Smart charge management implementation

For example, a participating site could reduce charging during a requested period, then reschedule the deferred energy before departure. The remaining time and power determine whether it can catch up. Buying electricity in a cheaper tariff period is a different arrangement from providing a contracted response: check the programme's eligibility, payment terms, metering requirements and opt-out conditions before counting on income.

Starting every vehicle at the beginning of a cheap period may create a large site peak. A controller can instead spread or cap charging within that period, using the available capacity and the vehicles' energy needs. DOE identifies both avoiding expensive hours and reducing coincident demand as managed-charging objectives. U.S. Department of Energy — Managed EV charging for federal fleets

Managed one-way charging, often called V1G, provides flexibility by changing demand while energy flows into the vehicle. Bidirectional charging also allows energy to leave the vehicle. U.S. Department of Energy — Managed and bidirectional charging Reverse power flow and isolated backup introduce additional equipment, protection and approval requirements beyond ordinary charging; see Bidirectional EV charging: powering appliances, homes and the grid for those arrangements.

Control, connection loss and restarts

The control decision may run in the vehicle, the charging equipment, a local building controller or a remote service. OCPP carries charging limits and smart-charging profiles between a station and its management system, while ISO 15118 covers communication between a compatible vehicle and charging equipment. Open Charge Alliance — Open Charge Point Protocol ISO 15118-20 — EV-to-EVSE communication and bidirectional power transfer For the interfaces needed to make these systems work together, see EV Charging Protocols and Services.

Internet access and local measurement are separate dependencies. Tesla's documented Wall Connector power-management functions work without an internet connection, although dynamic management requires a compatible meter. Tesla — Wall Connector power management Losing a cloud service and losing the measurements used to enforce a site limit therefore need separate checks. The fallback must keep charging within the installation's electrical limits; the installer should confirm how it behaves.

Vehicle restarts also need checking. In Argonne's October 2025 laboratory wake-up tests, two of the 24 vehicles tested did not resume charging on the standard SAE J1772 wake-up signal. Some vehicles needed unplugging and replugging between tests. These findings describe the tested vehicles, but give a concrete reason to try scheduled starts and restarts with your own car. Argonne National Laboratory — PEV charging response characterization, October 2025

Before choosing equipment or a service

Ask for confirmation of:

• The inputs it uses: fixed times, live prices, household demand, solar surplus or vehicle energy requirements.
• How departure targets and immediate charging interact with site limits and other vehicles' priorities.
• What continues locally without internet, and the separate fallback when measurement or control equipment fails.
• The compatible vehicle, charger and service combination, including any required meter, subscription or programme enrolment.

Once the system is running, check that your car starts on schedule, resumes after a planned pause, and receives the intended energy before departure.