Solar export limitation

Solar export limitation controls the net power a property sends to the distribution network. The limit is agreed with the DNO as part of the connection process. It may be lower than the combined output available from the solar, battery and other generation behind the meter.

Three quantities must be kept separate:

• Panel capacity is the array’s rated DC capacity in kWp.
• Registered Capacity describes the generating equipment declared under G98 or G99.
• Maximum Export Limit is the agreed maximum current at the connection point.

An export cap is not permission to disregard Registered Capacity, and it does not turn a larger generating installation into a G98 installation. The DNO assesses the equipment and the proposed site flow separately.

G98 is a capacity threshold, not a default export tariff limit

G98 applies to fully type-tested micro-generators up to and including 16A per phase. At 230V, 16A is 3.68kW on one phase. Larger or combined generating capacity follows G99, including the applicable Small Generation Installation procedure where its conditions are met.

This is often shortened to “the DNO only allows 3.68kW export”. That is not a safe general rule. A DNO may agree a higher or lower export figure after assessment. Equally, a 16A-per-phase G98 threshold does not mean that a larger inverter can be installed under G98 merely by setting its site export field to 16A.

Panel DC capacity is different again. A panel array can be larger in kWp than the inverter’s AC capacity, provided the complete electrical design remains within the equipment limits. The connection route is concerned with the AC generating equipment and its documented Registered Capacity, not a simple comparison with the sum of panel nameplates.

When G100 is used

Where the DNO cannot accept the requested unrestricted export, it may consider a lower Maximum Export Limit. A compliant Customer Limitation Scheme under G100 measures current and voltage at the connection point and controls specified generation, storage or loads.

The proposal still goes through the relevant G99 route. G100 defines the limitation scheme’s technical behaviour; it is not connection permission in its own right.

The DNO may assess:

• voltage rise and voltage variation
• thermal loading of network assets
• reverse power flow
• protection and fault-level effects
• power quality
• the temporary flow while the limiter responds or moves to fail-safe operation

This is why a zero or low export setting does not automatically make any amount of generation acceptable behind it.

Export is measured after household demand

The limiter acts on net flow, not simply the solar inverter output. If solar produces 6kW while the property uses 2kW, net export is 4kW. With an agreed 4kW limit, no reduction is needed. If demand falls to 1kW, the system needs to absorb or reduce 1kW to keep export at 4kW.

A larger inverter can sometimes remain useful behind a lower export limit because it can serve on-site demand at the same time as exporting up to the cap. That arrangement is valid only where the inverter, limitation scheme and Registered Capacity shown in the accepted DNO application match what is installed.

Do not assume that a large inverter is always preferable. Array voltage and current limits, part-load efficiency, clipping, phase balance, backup design and the likely time spent against the export cap all still matter.

Modelling the energy effect

There is no dependable percentage for energy lost to an export limit. It depends on the relationship between generation, demand, storage and the agreed limit at each point in time.

A useful estimate needs a time-series model that includes:

• expected solar generation at suitable intervals
• household demand during solar hours
• the inverter’s AC rating and conversion limits
• battery energy capacity, charge power and operating reserve
• controllable loads, if they are genuinely available
• the proposed Maximum Export Limit
• seasonal effects and periods when the battery is full

Annual generation alone is not enough. Two homes with the same annual use can have very different curtailment because one uses electricity during the day and the other does not.

The model should report at least:

• solar used directly in the property
• solar sent to storage
• exported energy
• energy reduced by the export constraint
• ordinary inverter clipping as a separate loss
• battery losses and any tariff-driven imports or exports

Keeping clipping and export curtailment separate prevents the same lost energy being counted twice.

Batteries and controllable loads

A battery can absorb surplus only while it has unused capacity and sufficient charge power. If it is full, too warm or cold, reserved for backup or already at its charge limit, the remaining surplus must be used, exported within the cap or reduced.

An immersion diverter or EV charger can also increase on-site use, but its availability should not be exaggerated. A car that is away or already charged, and a hot-water cylinder already at temperature, provide no useful headroom. The core export-limitation design must remain compliant when optional loads are unavailable.

All sources that can export at the same time need to be accounted for. An AC-coupled battery, a second solar inverter or another generator cannot be ignored simply because each device has its own control setting.

Equipment and commissioning

A compliant scheme normally needs a connection-point sensor, controller, secure control path and compatible generating or load equipment. G100 requires the design to address component, communication and power-supply failures. Commissioned National Grid schemes, for example, have settings protected from customer alteration and require written DNO agreement before those settings change.

The design and connection record identifies:

1. the Registered Capacity of every generating device
2. the proposed Maximum Export Limit and number of phases
3. the G98 or G99 connection route
4. the G100 system reference or other compliance evidence
5. which equipment the limiter controls
6. what happens when a battery or optional load is unavailable
7. the fail-safe response to sensor, controller and communication faults
8. the estimated annual curtailment and the assumptions behind it

The accepted application, schematic, protected settings and test record form part of the commissioning record. Raising the export setting later requires DNO agreement even if the equipment appears capable of more.

Related entries

• G100 export limitation
• G98 notification pathway
• G99 application pathway
• Curtailment and DNO export limitation
• Export limitation with batteries
• Inverter clipping