Off-grid switchover and circuit grouping

Off-grid switchover is the controlled move from a grid-connected installation to an island supplied by a battery inverter. Circuit grouping decides which loads remain connected to that island.

A grid-connected battery does not necessarily provide backup. Anti-islanding protection normally stops the inverter energising the network when the grid fails. Backup needs a separate output or gateway, switching that prevents backfeed, a valid island-mode earthing arrangement and circuits that the inverter can actually support.

Three practical levels of backup

Dedicated EPS outlet. One outlet is supplied from the inverter’s emergency output. It is simple but only helps appliances that can be safely moved and plugged into it. It should be clearly labelled and must never be used to backfeed the house through another socket.

Maintained-circuits board. Selected circuits are moved to a separate distribution board that the battery can supply in island mode. This is often the clearest way to keep refrigeration, lighting, communications and other priorities while leaving high loads disconnected.

Whole-home connection. The changeover supplies the normal consumer unit. “Whole-home” means every circuit is connected to the backup bus, not that every appliance can run together. Automatic load shedding or disciplined manual control may still be essential.

Software cannot create any of these wiring arrangements after the fact. An EPS toggle in an app is not a changeover device.

Choose circuits from needs, not labels

Start with the services the household needs during the outage and for how long. For each proposed maintained circuit, record:

• normal running power
• starting or inrush current
• whether it can restart unattended when power returns
• how often and how long it is likely to operate
• whether losing it creates a safety, health, security or property risk
• whether it has its own local UPS or stored energy

A fridge may use little energy over a day but have a short starting surge. A heat pump may be important but can exceed the inverter’s power or exhaust a modest battery quickly in cold weather. An EV charger, immersion heater, electric shower, cooker or resistance heater can consume the available backup power with little warning.

Circuit grouping should also consider shared neutrals, RCD layout, three-phase allocation and devices fed from more than one source. It may not be safe or practical to move one circuit without altering another.

Power, energy and fault current are different limits

Battery capacity, measured in kWh, determines roughly how long loads can run. Inverter power, measured in kW or kVA, limits what can run at once. Surge capability limits which motors and compressors can start.

There is a fourth constraint: inverter fault current is often far lower and behaves differently from the public supply. Protective devices that disconnect correctly on-grid must also do so in island mode. RCD arrangement, neutral referencing and the inverter’s fault response are therefore part of circuit selection.

Comparing daily energy use with battery kWh alone does not establish backup performance.

Automatic and manual transfer

A manual changeover lets the user decide when to use the reserve, but it provides no unattended backup. The switch must still be a correctly rated, interlocked break-before-make arrangement that prevents the grid and island source being connected together.

An automatic gateway detects loss of the grid and carries out the transfer. Its specified transfer time depends on the product, operating state, load and test conditions. “Seamless” is not a standard duration.

Ask for the guaranteed transfer performance in the technical documentation and decide which equipment can tolerate it. Computers, network storage, medical devices or security systems that genuinely require no interruption may need their own appropriate UPS even when the house has battery backup.

Load shedding

Whole-home systems often need to disconnect or limit non-essential loads in island mode. A robust scheme identifies the loads and their priorities in advance.

Possible methods include:

• physically leaving high-load circuits outside the maintained board
• hard-wired contactors controlled by the gateway
• manufacturer-supported smart relays or energy-management controls
• a current limit enforced by the inverter, with a documented overload response

Cloud scheduling alone is weak protection for an outage, when broadband or the manufacturer’s service may also be unavailable. Safety-critical shedding needs a local, defined fail-safe state.

The design should explain what happens if the shed-load contactor sticks, the CT reading is lost, the battery is nearly empty or the inverter is overloaded. It should also prevent an EV or heat pump from automatically restarting into an island that cannot support it.

Reserve and black start

Backup energy exists only if charge remains when the grid fails. The owner can normally set a reserve, but a larger reserve reduces the battery available for everyday tariff or solar optimisation.

Confirm whether the system can:

• enter island mode from its minimum state of charge
• restart after shutting down on low battery
• use solar generation to recover while the grid remains off
• form the island when the sun is present but the battery is very low
• recover automatically when the grid returns

These behaviours are product-specific and can depend on firmware, so they belong in the commissioning record rather than a generic promise.

Switching and earthing still decide safety

Every live conductor supplying the islanded part must be separated from the public network as required by the design. The island needs a controlled neutral-to-earth reference, a suitable consumer earth electrode and protective devices that operate with the inverter’s available fault current.

Most home EESS connected to a low-voltage public supply form a TN-S arrangement in island mode. PME conditions can still matter because the DNO earth remains physically connected even though it is not relied upon as the island reference.

Those functions are covered in the whole-home changeover article. They are required for a maintained-circuits board as well as for whole-home backup.

Commissioning the promised outcome

The installer should simulate loss and restoration of the grid using the approved method and prove:

• no energisation can reach the public network
• the intended circuits remain live and excluded circuits remain dead
• transfer and restoration occur in the specified sequence
• RCDs and fault protection work in both modes
• neutral-to-earth switching and the electrode are effective
• load shedding and overload behaviour are correct
• the battery reserve and restart settings match the handover
• alarms, indicators and manual isolation are understood by the user

Repeat the exercise periodically in accordance with the manufacturer and maintenance plan. A backup system first tested during a real outage is an uncommissioned system.

Related entries

• Whole-home backup changeover
• Battery location and ventilation
• Backup EPS output
• Island mode and anti-islanding
• State of charge control