Maintained-load boundary
The maintained-load schedule identifies the circuits needed during an outage, their expected simultaneous demand and the interruption each load can tolerate.
Common maintained loads include:
- lighting
- fridge and freezer
- router and communications
- heating controls and pumps
- medical or accessibility equipment
- alarm and security
- selected sockets
- water or drainage pumps
The list is property-specific. A gas boiler may still need electricity for controls and circulation. A heat pump can present a much larger demand. A private water supply or sewage pump can have a high starting current.
High-demand loads commonly kept outside the island include:
- EV chargepoint
- electric shower
- immersion heater
- cooker
- large heat pump
- workshop equipment
The actual boundary depends on the system rating and load-management design. A generic list does not define the installed system.
Essential circuits and whole-home backup
An essential-circuit arrangement supplies a separate backed-up consumer unit or selected final circuits. The design boundary is clear and high-demand loads can remain on the grid-only side.
A whole-home arrangement connects the main household distribution to the backup source through a suitable changeover. It can be convenient, but the inverter still has a power and energy limit.
Whole-home backup remains limited by the inverter output, usable energy and any per-phase or load-management limits. The system may need:
- owner load discipline
- automatic load shedding
- contactors that disable selected equipment
- staged restoration
- per-phase limits
The circuit schedule marks every backed-up and excluded circuit. A socket circuit label alone may not show all fixed loads connected to it.
A three-phase property may have backup on one phase or all phases, with different load distribution in each case. A three-phase grid connection does not establish three-phase island output.
Stored energy and reserve
Battery energy is stated in kWh. The usable energy available during an outage can be lower than the product’s usable capacity because of:
- normal minimum SoC (state of charge)
- protected backup reserve
- conversion loss
- battery temperature
- state of health
- power required by the inverter and controls
- reserve held below the displayed limit
The app value is a BMS estimate inside the protected operating window.
Build a load schedule rather than dividing capacity by one appliance rating. Add the energy required by each backed-up load over the intended period and include the possibility that several operate together.
The backup reserve is the energy protected from normal tariff or solar-self-use discharge. A larger reserve provides more available energy at the start of an outage but leaves less for daily operation.
The control record also defines what happens if an outage begins below the reserve because the system was in a manual mode, supplier event or commissioning state.
Continuous and surge power
Battery power is stated in kW. Keep these ratings separate:
- continuous island output
- short-duration surge output
- motor-start capability
- grid-connected discharge power
- per-phase output
- battery-module limit
- inverter limit
The normal on-grid discharge rating may differ from the EPS rating. The complete battery, BMS, inverter, wiring and configured limit determine what can be supplied.
Motors and compressors can draw a starting current above their normal running power. A fridge, pump or heat-pump compressor therefore needs the product’s starting evidence and an installed test, not only the steady nameplate value.
An overload may trip the backup output, shed a circuit or attempt a restart. The product record and handover instructions define the exact behaviour and safe reset method.
EPS equipment and changeover
EPS can describe a dedicated inverter output or a complete backup function. An unused terminal on an inverter does not mean the home is wired for outage power.
The installed arrangement needs:
- compatible inverter or gateway
- source separation
- changeover switching
- controlled neutral treatment
- island-mode earthing
- protective devices suitable in both modes
- backed-up distribution
- labels and isolation
The changeover must prevent unintended parallel operation with the network. It must switch the conductors required by the design and be suitable for the full duty.
A manual bypass for maintenance also needs to preserve source and neutral separation. The handover includes an operating diagram rather than an unexplained switch position.
Island-mode earthing and fault protection
In connected mode, the installation receives the network supply and grid earthing arrangement. During island mode, the backed-up circuits are supplied by the inverter after separation from the grid.
The island needs an earthing and neutral arrangement that allows protective devices to operate. The IET guidance describes a coordinated island-mode isolator, neutral-to-earth bond and consumer earth electrode within the design.
This is not an owner wiring task. Commissioning verifies:
- separation from the grid
- timing and position of the neutral-to-earth bond
- earth electrode and protective conductors
- automatic disconnection under island fault current
- RCD operation where applicable
- protection after bypass or restoration
Inverter fault current can be lower and more tightly controlled than grid fault current. On-grid loop measurements cannot simply be assumed to prove island-mode disconnection.
PME (protective multiple earthing) conditions can remain relevant because the DNO earth may stay physically connected even when live supply conductors are separated. The design must follow the current installation standard and actual earthing arrangement.
Anti-islanding and network protection
Grid-connected PV and battery inverters have loss-of-mains protection so they stop energising the public network after supply is lost. G98 or G99 type-test evidence covers the applicable parallel-generation behaviour.
Intentional home backup uses a switched island after separation. It does not disable anti-islanding and leave the street energised.
The handover record includes:
- inverter type-test reference
- G98 or G99 record
- gateway or changeover information
- island-mode commissioning results
Type testing of an inverter does not prove the wiring, gateway isolation, earthing or circuit protection of the installed backup system.
Transfer and interruption
Transfer time is the interruption between grid loss and the backup supply becoming available. It depends on the exact inverter, gateway, settings, load and installation.
Some household equipment will ride through a brief interruption. Other devices reset, reboot or reject the transferred supply. Commissioning tests the actual interruption-sensitive equipment within the agreed scope.
UPS (uninterruptible power supply) equipment has different product purposes and test requirements from a domestic battery system. An EPS marketing description does not establish UPS continuity or supply quality.
Medical, safety-critical, data or other interruption-sensitive equipment needs an explicit assessment. It may still require a dedicated UPS even where the home has battery backup.
The commissioned transfer record covers clocks, alarms, routers, heating controls and relevant motor loads.
PV operation during an outage
PV continuation is product and architecture specific.
A DC-coupled PV array may be able to charge the battery through the hybrid inverter in island mode. An AC-coupled PV inverter needs a compatible island AC supply, power control and protection. Some systems stop solar generation during an outage.
The island-mode specification records:
- whether PV can operate in island mode
- maximum PV and battery power in that mode
- response when battery SoC is high
- response when household load is low
- supported solar inverter or optimiser
- restart after a night-time or empty-battery outage
Black start means forming the island supply without the grid after the system has shut down. It should not be assumed from PV continuation or backup capability.
The supported recovery sequence forms part of commissioning. An outage starting at night and continuing into the next day can behave differently from a daytime grid-loss test with a charged battery.
EV chargers, heat pumps and other large loads
EV chargepoints and heat pumps can affect both the island load and the changeover design.
An EV charger should not continue normal charging on an undersized backup supply. A load-control contact or excluded circuit may be required. Vehicle-to-home is a separate bidirectional design and should not be inferred from an ordinary chargepoint.
A heat pump may need more output than the backup inverter can supply, particularly with supplementary or immersion heat. A smaller backup may maintain heating controls and pumps without running the compressor.
For each large load, record:
- included or excluded
- continuous and starting power
- automatic disconnection
- manual override
- restoration behaviour
Do not rely on the owner remembering to switch every load off during a sudden outage where automatic control is needed to protect the source.
DNO connection
The grid-connected battery inverter still follows the applicable DNO connection route in normal operation. In Great Britain this is G98 or G99, with G100 where an import or export limitation scheme applies.
The DNO record and type-test evidence do not replace the island-mode design. Keep both parts of the handover.
Changes to backup equipment, inverter capacity or the grid-connected operating mode may require a revised DNO submission.
Northern Ireland uses the separate G98/NI and G99/NI framework through NIE Networks.
Grid-loss and restoration tests
Commissioning should use the final circuits, settings and owner controls. It should cover as applicable:
- normal connected operation
- simulated loss of grid supply
- separation and island formation
- voltage and frequency in island mode
- protective-device operation
- important loads starting
- overload and load shedding
- low-battery shutdown
- PV continuation
- manual bypass
- grid restoration
- repeated transfer
The test should not backfeed the network or expose occupants to an unsafe condition. It belongs to the competent installer.
Record the starting SoC, loads present and observed behaviour. A successful test with almost no load does not demonstrate the agreed whole-home or essential-circuit schedule.
Give the owner the safe reset, isolation and bypass instructions. Protected installer settings should remain documented without being presented as routine owner adjustments.
Operating the battery during an outage
During an outage:
- confirm the system has entered the expected backup mode
- follow the agreed load plan
- avoid excluded high-demand equipment
- monitor SoC and available power
- keep ventilation and equipment clearances unobstructed
- use the documented reset method after an overload
Do not connect a portable generator or another source to the island unless the system was designed for it. Multiple sources need compatible switching, earthing and control.
After grid return, confirm that normal supply, the documented connected-mode indication and the expected battery schedule have resumed. Refer an unexpected indication or failure to reconnect to the installer.
Retesting backup after electrical changes
Backup should be retested at the interval specified by the design, manufacturer and maintenance plan. Keep:
- test date and result
- battery SoC and state of health
- circuit changes
- firmware updates
- inverter, gateway or battery replacement
- earth and protective-device test records
- DNO changes
Adding a circuit, EV charger, heat pump or larger appliance can invalidate the original load schedule. A consumer-unit replacement can also alter the changeover or protection boundary.
Review the backup design before adding or moving an electrical circuit or large load.