EV charging, solar, storage and backup power

Houses are no longer only consumers of electricity. Electric vehicles add the largest new load most homes have ever seen; rooftop solar panels generate power that flows back to the grid; home batteries store it; and standby generators and portable generators supply the house during outages. Each must connect safely to a service designed for one-way power flow, without overloading it and without ever sending power back onto utility lines being repaired. This lesson covers EV charging equipment and circuits, energy management systems, solar photovoltaic (PV) systems and their interconnection, battery storage, generators and transfer equipment, and planning these together.

Electric vehicle charging

Charging levels are defined by the supply:

Home charging is almost always Level 2. The charger (electric vehicle supply equipment, EVSE) is a continuous load, so its circuit is rated at 125% of its output: a 32 A charger needs a 40 A circuit, a 48 A charger a 60 A circuit. It may be hard-wired or plugged into a 240 V receptacle (commonly a NEMA 14-50R or 6-50R), in which case the receptacle needs ground-fault protection in many editions and should be a heavy-duty type rated for repeated insertion and continuous load.

In the US, NEC Article 625 covers EV charging: each charger needs an individual branch circuit (625.40) sized at 125% of its rating, receptacles for EV charging need GFCI protection (625.54), and an energy management system may limit the load used in calculations (625.42 and 220.70). The service calculation counts EVSE at the larger of 7,200 W or nameplate (220.57). Many jurisdictions require EV-ready new homes.

Energy management and load control

An EV, a heat pump with backup heat, an induction range, a dryer and a water heater can together demand more than a 100 A or even 200 A service. Rather than upgrading the service, which can cost thousands and may require utility work, homeowners can manage loads:

• EV energy management systems measure the service current and reduce or pause EV charging when other loads are high.
• Load shedding relays switch off a non-essential load (such as a water heater) while another large load (such as a dryer or backup heat) runs.
• Smart panels and smart breakers monitor every circuit and can switch or limit loads automatically, with a smartphone interface.
• Circuit sharing devices allow two 240 V loads (for example a dryer and an EV charger) to share one circuit, giving priority to one.

Load management devices must be listed for the purpose, and the code allows the service calculation to reflect the managed (lower) load only when the device is approved for that use.

Solar photovoltaic systems

A residential PV system has solar modules on the roof or ground, an inverter (or microinverters on each module) that converts their DC output to AC synchronized with the grid, disconnects, and an interconnection to the house's electrical system. Grid-tied inverters shut down automatically when the grid fails (anti-islanding), so they cannot energize utility lines during an outage, which also means a basic grid-tied system provides no backup power.

Key electrical requirements:

• DC circuits: modules in series produce high DC voltages (up to 600 V in houses in many jurisdictions); conductors, connectors and disconnects must be rated for DC, and rapid shutdown equipment reduces voltages on roof conductors when firefighters shut the system down.
• Interconnection: the inverter output connects to the house either on the load side of the service (usually a backfed breaker in the panel) or on the supply side (a tap ahead of the main breaker).
• Busbar limit: a backfed breaker adds current to the panel's busbar from a second source. The codes limit the total, commonly through the 120% rule.

In the US, NEC Article 690 covers PV systems (including rapid shutdown, 690.12), and Article 705 covers interconnected sources, including the 120% busbar rule (705.12); the utility also requires an interconnection agreement before the system may operate.

Utilities require an interconnection application and, in many places, a visible, lockable disconnect accessible to utility workers, and they install a bidirectional (net) meter that records energy both imported and exported.

Battery storage

Home batteries (typically lithium-ion, 10 kWh to 20 kWh per unit) store solar energy for evening use, shift consumption to cheaper times, and provide backup during outages. For backup, the system includes a transfer function that disconnects the house (or a critical-loads panel) from the grid during an outage and supplies it from the battery and inverter, and reconnects when the grid returns.

Batteries are installed according to their listing and the fire and electrical codes, which limit locations (for example, not in habitable rooms, with clearances and fire separations in garages), capacity per area, and require disconnects and signage. Most systems are pre-engineered, and the installer follows the manufacturer's requirements closely.

Generators and transfer equipment

Generators are the traditional backup source. The essential rule is that a generator must never be connected so that it can feed the utility lines: backfeed can kill line workers repairing a "dead" line, damage the generator when power returns, and start fires. Connections are therefore made only through transfer equipment:

• A manual transfer switch or a listed interlock kit on the main panel, which mechanically prevents the main breaker and the generator breaker from being on at the same time.
• An automatic transfer switch (ATS) with a permanently installed standby generator, which starts the generator when utility power fails and transfers the load, then transfers back when power returns.
• A generator inlet (a flanged receptacle outside the house) for a portable generator, wired to the transfer switch or interlocked breaker.

Plugging a generator into a house receptacle with a male-to-male cord ("suicide cord") is dangerous and illegal.

Standby generators need a fuel supply, exhaust clearances from windows and doors (carbon monoxide kills), a pad, and a dedicated circuit for their battery charger and block heater. Portable generators must be run outdoors, well away from openings.

Planning it together

Many homeowners now combine these systems: solar panels, a battery, an EV charger, a heat pump, and sometimes a generator. Their interactions need planning:

• Service and busbar capacity, with load management to avoid upgrades where possible.
• Which loads are backed up, and whether the battery or generator can start them.
• Bidirectional EV charging (vehicle-to-home), now offered by some vehicles and chargers, which turns the car into a home battery; it requires transfer equipment just like any other backup source.
• Utility programs that pay for managed charging or battery dispatch.
• Clear labeling of every source and disconnect, so that firefighters and utility workers can de-energize the house.