15.3 G100 Export Limitation and Why Add Storage

Quick Answer: Handbook v1.3 outcome 6.1.3 places G100 with G98 or G99. Export-limiting arrangements divert power when export is being limited, or because total site generation is larger than site export capacity. G100 details tests a DNO expects during commissioning of an export limitation system to prove correct function. Outcome 6.2.1 gives three reasons for batteries: store generated electricity to use later, increasing self-consumption; store electricity to export at peak demand when export rates are higher; and provide island-mode capability if the grid fails. The battery inverter AC rating counts toward 16 A. 2923-34 is the separate EESS award; 2922-34 still examines these LO6 ideas. This OpenExamPrep section is independent study material for 2922-34; it is not a City & Guilds publication.

G100 sits with G98 or G99 — it is not a size band

Design chapter 9 already split the ENA recommendations: G98 where total generation is 16 A or less per phase, G99 in all other cases, G100 where the DNO or DSO requires export limitation. Outcome 6.1.3 re-enters that split from the prosumer side. A PEI often wants more AC conversion than the feeder will accept as unconstrained export. The network may still allow the generation equipment if a Customer Export Limitation scheme holds the boundary power to an agreed kilowatt figure.

G100 does not delete G99. If aggregated AC generation is above 16 A per phase, you apply and wait under G99, and you fit and test the limiter if export must be capped. G100 does not create G98. Setting the limiter to 3.68 kW on a 5 kW hybrid does not turn the job into connect-and-notify. The 16 A test uses generation / conversion capability at the connection, including the battery inverter, not the export slider.

Independent OpenExamPrep teaching repeats that trap because LO4 and LO6 will both punish it. OpenExamPrep is not the DNO and does not issue G100 certificates.

Divert when limited, or because generation exceeds export capacity

Handbook 6.1.3 names two reasons export-limiting arrangements divert power.

When export is being limited. The scheme is actively holding the point-of-connection export at or below the agreed value. Surplus that would have gone through the meter must go somewhere else or not be generated. The honest somewhere else on an LO6 job is usually the battery (soak), then house loads, then inverter throttle / clip.

Because total site generation is larger than site export capacity. That is the design fact that made G100 necessary. A 6 kWp array into a 5 kW hybrid on a feeder that will only accept 3.68 kW export is already in this bucket before the sun comes out. The limiter exists because capability > allowance. Divert is how you use the extra capability on site instead of throwing it away as clip, and how you stop it appearing as illegal export.

Divert is not a third Engineering Recommendation. It is what the limitation does with energy that must not cross the boundary. A limiter that can only trip the AC isolator every time the cap is hit is a crude fail; a scheme that throttles and soaks is what 6.1.3 is describing.

DNO commissioning tests — qualitative pass and fail

G100 is not a sales menu item. It details tests the DNO expects at commissioning to prove the export limitation system functions. You do not need to recite millisecond tables on 2922. You do need the qualitative picture: the scheme must hold the agreed export, it must fail safe if a sensor or controller is lost, and those facts must be demonstrated and recorded.

Typical pass observations: with generation larger than the cap, measured export stays at or below the agreed kilowatts; surplus is diverted into the battery (or another designed sink) rather than pushing the meter; unplugging or failing the export measurement drives the scheme to a safe reduced export (often toward zero); loss of power to the controller does not leave inverters at unconstrained output; the agreed set-point and the test results sit in the commissioning record the DNO asked for.

Typical fail observations: export climbs above the cap once the battery is full and nothing throttles PV; removing the current transformer leaves full PV plus battery export (fail-unsafe); an inverter app slider with no fail-safe is offered as the G100 scheme; no tests are done because the hybrid has a setting; the installer tells the DNO it is still G98 because the export number is 16 A while the inverter is 5 kW.

If the DNO witnesses the test, treat that as their evidence requirement, not as a City & Guilds exam-fee event. Missing the tests is a connection failure.

Why add storage — handbook 6.2.1

Outcome 6.2.1 gives three reasons. They map onto 15.1 terms on purpose.

Store generated electricity to use later, increasing self-consumption. Midday PV charges the battery; the house uses it after sunset instead of exporting cheap kWh and importing expensive kWh. This is also time shifting of generated energy. On a G100 site it is the soak path that makes a limiter useful rather than just a clipper.

Store electricity to export at peak demand when export rates are higher. That is arbitrage aimed at outgoing value, and it is the storage half of 6.1.2 peak grid support. You may store PV, or cheap-rate grid energy, then discharge when the regional peak pays. The discharge still must not break G100.

Provide island-mode capability if the grid fails. Storage (house battery and, if designed, V2H) is what keeps isolated backup circuits alive when PV is dark. This reason only counts if island-mode is a designed changeover arrangement. A battery that cannot isolate from the feeder is not backup; anti-islanding will shut it down, which is correct.

Do not invent a fourth 6.2.1 reason such as hiding generation from the 16 A test. Storage adds conversion capability when the battery inverter can present AC.

Combined generation — the battery still counts

16 A × 230 V ≈ 3.68 kW on single-phase. Every converting device that can present generation counts: PV inverters, hybrid AC ratings, AC-coupled battery inverters, bidirectional vehicle chargers. MCS installation practice already used 3 kW PV plus 3 kW battery on the same single-phase supply as a G99 example. LO6 repeats the arithmetic in prosumer clothing.

A G100 cap of 3.68 kW export on a 5 kW hybrid is a 5 kW G99 machine with a 3.68 kW boundary limit. Write 21.7 A generation, 3.68 kW export, not 16 A generation.

2923-34 is a different award

2923-34 is City & Guilds small EESS — a separate Ofqual pathway for storage installation and maintenance. 2922-34 remains the small PV award. LO6 still spends 10 percent of the PV knowledge test on prosumer installations, EEMS and these storage reasons. Passing 2922 does not issue 2923. Skipping LO6 because you will do batteries later drops three questions on the PV paper you are sitting.

G100 commissioning: qualitative fail versus pass

Scenario: 6 kWp PV, DNO offers 3.68 kW export, G100 limiter plus battery soak

A customer wants 6 kWp of modules. The DNO will not accept unconstrained export above 3.68 kW on this single-phase feeder, but will accept the generation equipment with G100. You specify a 5 kW hybrid (about 21.7 A) and a battery.

Connection route: aggregated AC is 5 kW, so G99 permission before connection. G100 holds export at 3.68 kW. Do not file G98 because the export offer equals 16 A.

Midday, battery has headroom. Inverter output 5.0 kW, house load 0.5 kW. Unconstrained export would be 4.5 kW. The limiter must hold export at 3.68 kW, so about 0.82 kW is diverted into the battery (soak). Self-consumption rises. The DNO sees ≤ 3.68 kW.

Midday, battery full. There is no soak left. The EEMS must throttle PV toward 3.68 kW + 0.5 kW = 4.18 kW generation so export stays at the cap. If it does not, commissioning fails the hold-the-cap test.

Evening peak, outgoing rate high. Discharge for 6.2.1 peak export and 6.1.2 grid support, still ≤ 3.68 kW at the meter. The 5 kW nameplate is not the export target.

Commissioning: demonstrate the hold, the soak or throttle, and fail-safe if the export sensor is lost. Record it. Island-mode, if sold, is a separate changeover proof, not a G100 slider.

The 2922 answer on this site is G99 + G100 + battery as the divert path + 16 A counted on 5 kW, not on 3.68 kW. The incompetent answer is 6 kWp therefore G98, or limiter instead of tests.