The Export Limit You Didn't Choose: How G100 Shapes a Home Battery in Practice

A home battery is usually sold on two numbers: usable capacity in kilowatt-hours and inverter power in kilowatts. There is a third figure, however, that can matter just as much and is often buried in the connection paperwork. That figure is the agreed export capacity, and on many domestic installations it is enforced by a G100 export limitation scheme.

The result can be a system that is technically compliant, safely connected and apparently working, yet unable to export as much as its hardware could produce. For a homeowner trying to judge whether a battery is earning its keep, the export limit is not a technical footnote. It is part of the operating envelope.

The condition behind the connection

G100 is an Energy Networks Association engineering recommendation for export limitation schemes used with small-scale embedded generation. In practice, it gives distribution network operators a way to accept a generation or storage connection while capping the amount of power that can flow from the property to the grid.

That cap may be imposed because the local network has limited headroom, because the property has a single-phase supply, or because the applicant has asked for a smaller connection than the inverter's full rating. The DNO approves a maximum export figure. The installer then configures a limiting device, often integrated into the battery inverter or an energy management system, to keep export at or below that figure.

For a domestic battery, this is not unusual. A 5 kW battery inverter might be connected under an approved export limit of 3.68 kW, particularly on a single-phase supply. The battery can still discharge at full rate into the house. What it cannot do is send the full surplus to the grid.

Consent is not the same as configuration

The first distinction to grasp is between the connection that has been consented and the limit that has been configured.

The consented figure appears in the DNO's approval or connection offer. It is the legal and technical ceiling for export at the point of connection. The configured figure is the setting held in the inverter, gateway or control system. It should match the consented figure, or sit below it where the designer has chosen a more conservative setting.

Problems arise at both ends of that relationship. A system configured above its consented capacity is non-compliant and may be ordered to be changed. A system configured well below its consented capacity may be unnecessarily restricted, reducing export earnings and leaving stored energy unused. Neither outcome is visible from a glossy handover folder unless the paperwork records the actual setting.

It is also possible for a property to have several controlled devices. If a battery and a separate solar inverter share one connection, the limiter must control aggregate export, not just one unit. That requires correct current transformer placement and a control scheme that understands the whole site.

Where the limit appears in monitoring

Homeowners often look at battery state of charge and solar generation. The export limit appears in the relationship between those figures and the grid meter.

In a monitoring portal, look for fields labelled export limit, grid setpoint, site export or curtailment. Some systems show a live limit in kilowatts; others show only the resulting power flow. If the portal offers half-hourly or five-minute data, compare grid export with battery discharge and house load.

A flat-topped export curve is the classic signature. The battery may be discharging at 4 kW, the house may be drawing 0.5 kW, and grid export may sit exactly at 3.5 kW for an extended period. If the approved figure is 3.68 kW, that is consistent with a limiter doing its job. If the approved figure is 5 kW, it may indicate a setting that is tighter than necessary.

The limit is harder to see when the house is using most of the battery's output. A battery discharging into a heat pump or an evening cooking load may never approach the export ceiling. Absence of clipping in the chart does not prove that no limit exists; it may simply mean the limit was not reached.

Commissioning evidence to demand

A G100-compliant device is not the same as a commissioned G100 scheme. The device may carry a type test certificate, but the installation still needs correct settings, correct sensing and a documented test.

Ask for the DNO approval showing the agreed export capacity. Ask for the export limitation commissioning certificate or equivalent record, signed by the installer. It should state the configured limit, the device used, the firmware version and the location of the current transformer. A single-line diagram should show where export is measured and which equipment is controlled.

The handover pack should also record the fail-safe behaviour. If the limiter loses communications or a sensor fails, what happens? A properly designed scheme should default to a safe state rather than allowing uncontrolled export. The commissioning record should show that this was tested, not merely asserted.

Finally, ask how the setting can be checked in future. A homeowner does not need to adjust it, but they should know where the figure is recorded and what evidence exists that it remains correct after firmware updates or later additions such as a second inverter or an electric vehicle charger.

Design choices that decide the economics

An export limit changes the economics of a battery, but it does not automatically ruin them. The important question is where the value comes from.

If the battery is designed mainly to store rooftop solar for evening use, a modest export cap may have little effect. The battery absorbs surplus generation that would otherwise be exported, then discharges it into the house when demand rises. The limiter matters only when the battery is full and solar generation still exceeds the site's needs.

If the business case depends on exporting large volumes at a premium tariff, a tight cap is far more consequential. A battery that charges overnight and discharges straight to the grid may be constrained for much of its operating life. Similarly, a large solar array paired with a small battery and a low export limit will spend more time curtailed than a better-matched system.

Good design therefore starts with the site's consumption pattern, the agreed export capacity and the tariff. The battery should be sized to serve the house first. Export should be treated as a residual benefit, not the foundation of the payback calculation, unless the connection genuinely supports it.

The questions worth asking before signing

Before accepting a battery quotation, ask what export capacity the DNO has approved and what limit will be configured. Ask whether the limiter controls all generation at the site. Ask for the commissioning evidence and a simple explanation of how curtailment will appear in the monitoring app. Ask what happens if the homeowner later adds more solar, a second battery or a charger.

A G100 limit is invisible in daily use until it is not. It is better to understand it at the quotation stage than to discover a flat-topped export curve months later and wonder why a well-specified battery seems to stop short.