Export limitation with batteries

How a battery, solar generation and site loads are coordinated behind a DNO-agreed export limit, including coupling, metering, fail-safe behaviour and commissioning.

A battery is generation equipment for connection purposes because it can discharge electricity into the installation and potentially onto the distribution network. Where a DNO agrees a site export limit, the control design must therefore account for battery discharge as well as solar generation.

The limit applies to net flow at the connection point. It is not simply a cap on one inverter. A system may have more inverter capacity behind the meter than it is allowed to export, provided the DNO accepts a compliant design that keeps the connection within the agreed Maximum Export Limit.

The scheme controls the site boundary

Under G100, a Customer Limitation Scheme measures current and voltage at or near the connection point. It then controls the generation, storage or loads included in the approved design so that net export remains within the agreed limit.

Consider a property with a 3kW export limit:

solar generation is 6kW
household demand is 2kW
the battery is charging at 1kW
net export is 3kW

If the battery stops charging, another 1kW must be absorbed or the solar output must be reduced. If the battery starts discharging instead, the controller needs to reduce battery discharge, solar output or both. The export limit does not disappear because a tariff schedule has requested a discharge.

The order in which a system serves the house, charges the battery, exports and reduces generation is not set universally by G100. It depends on the approved control design and the operating mode selected within that design. Generic claims such as “house first” or “battery first” do not define that behaviour.

Every source needs to be accounted for

A straightforward hybrid system may coordinate solar and storage within one inverter and controller. An AC-coupled installation can have a solar inverter, a separate battery inverter and perhaps other generation. The design must show how their combined effect is measured and controlled.

Two independent export settings are not automatically a site-level G100 scheme. If separate devices can export at the same time, the application and schematic need to show either how the approved controller coordinates them or why their combined output cannot breach the limit.

The connection-point sensor is critical. Its location, phase assignment, direction and communications path determine what the controller believes the site is importing or exporting. Commissioning should test real power flow, loss of communications and other fail-safe conditions, not merely confirm that a value appears in an app.

A battery reduces curtailment only when it has headroom

A battery can absorb solar that would otherwise exceed the export limit. This can improve self-consumption or move energy to a later export period, but only while the battery has:

unused energy capacity
enough charge power at that moment
a control mode that permits charging
temperature and state-of-charge conditions that allow it

Once the battery is full, surplus generation still has to be used by another load, exported within the limit or reduced. A small battery, a low charge rate or a battery deliberately held full for backup may provide little help during the sunniest part of the day.

The reverse is true during discharge. Battery power supplied to household loads does not count as export, but any amount left after meeting those loads does. Forced discharge and tariff automation must remain subordinate to the DNO-approved limit.

Zero export is still an engineered limitation scheme

A zero Maximum Export Limit is a special case, not a casual inverter preference. G100 provides for a Customer Limitation Scheme or DNO-agreed reverse-power protection where generation should not export.

The standard recognises normal operation, short excursions and fail-safe behaviour. “Zero export” should not therefore be sold as a promise that every app sample will read exactly 0W. The proposed equipment, response, temporary excursions and failure behaviour still need to be acceptable to the DNO.

A zero setting also does not guarantee that any size of installation will be approved. The DNO may still assess voltage, protection, fault level, thermal effects and power quality during the time the scheme takes to respond.

AC and DC coupling affect declared capacity

National Grid’s published connection guidance distinguishes the two common arrangements:

With a DC-coupled battery behind the same hybrid inverter as the solar array, the shared inverter capacity is counted for the AC generating capacity.
With an AC-coupled battery, the battery has its own inverter. Its capacity is counted alongside the solar inverter and any other generating equipment.

This affects the G98 or G99 connection route. It is separate from the export limit. A G100 cap at the connection point does not automatically reduce the Registered Capacity of the generating equipment behind it.

A generating device can sometimes have a documented Registered Capacity below its intrinsic design capacity, using a protected device-level limitation supported by compliance evidence. That limits the output of the device itself. G100 instead controls net site flow. The application must not treat those two mechanisms as interchangeable.

Connection route and evidence

Storage that can operate in parallel with the network must be included in the connection assessment. National Grid publishes one common single-phase fast-track example: G98-compliant solar up to 16A plus G98-compliant storage up to 16A, with G100 limiting total export to 16A per phase or less. The complete installation uses a G99 fast-track application rather than two unrelated G98 notifications.

The current G99 standard contains Small Generation Installation procedures, but the exact route depends on the aggregate Registered Capacity, equipment evidence, phases and proposed export limit. The relevant DNO must accept the installation before apply-to-connect work proceeds.

For an export-limited battery system, retain:

the DNO’s accepted Maximum Export Limit
the approved single-line schematic and equipment schedule
the Registered Capacity of every generating device
the G100 system reference or manufacturer compliance evidence
the installed sensor position and protected settings
the completed commissioning and fail-safe test record
the operating instructions for faults, lockouts and equipment replacement

National Grid states that commissioned limitation settings cannot be altered by the customer and may be changed only with written DNO agreement. A tariff, firmware update or replacement inverter must not bypass those protected settings. A material change to controlled equipment, sensors or communications may require review because it can change the scheme the DNO accepted.

Control and compatibility data

A working export-control scheme is defined by:

Registered Capacity and Maximum Export Limit
the device measuring flow at the connection point
the inverters, storage devices and loads under scheme control
operation when the battery is full or unavailable
fail-safe behaviour after loss of a sensor, controller, power supply or communication link
coordination of tariff-driven discharge with the export limit
G100 evidence and commissioning documents retained at handover

An “export limit” field on one inverter does not establish the behaviour of the complete scheme.