9.2 G98, G99, G100 and Combined Generation
Key Takeaways
- The 2922 handbook (4.1.3) places G98 where total generation is 16 A or less per phase, G99 in all other cases, and G100 where the DNO or DSO requires export limitation, plus notification timeframe requirements.
- 16 A times 230 V is about 3.68 kW single-phase, and 16 A per phase on balanced three-phase is about 11.04 kW; the threshold is aggregated AC generation or export capability at the connection, not DC kWp.
- The note to MCS MIS 3002 clause 5.1.9 states that 3 kW PV plus 3 kW battery on the same single-phase supply exceeds 16 A, so G99 applies even though each machine looks small on its own.
- G98 is connect-and-notify for fully type-tested micro-generators; DNOs use a 28-day commissioning-confirmation window, while handbook v1.3 names notification timeframe requirements without printing 28. G99 needs permission before connection.
- A G99 relay is a monitoring device that controls a contactor or other isolating means, watches grid voltage and frequency, and disconnects the inverter from the grid on a voltage or frequency fault; it is typically required only on larger systems over 16 A per phase.
Quick Answer: Handbook outcome 4.1.3 uses Energy Networks Association (ENA) Engineering Recommendations: G98 where total generation is 16 A or less per phase, G99 in all other cases, and G100 where export limitation is required by the Distribution Network Operator (DNO) or Distribution System Operator (DSO). The same outcome requires you to know notification timeframe requirements. 16 A × 230 V ≈ 3.68 kW on single-phase; 16 A per phase on balanced three-phase is about 11.04 kW. The line is aggregated AC capability at the connection, not DC kWp. Outcome 4.4.4 describes a G99 relay: a monitoring device that controls a contactor or isolating means, watches grid voltage and frequency, and disconnects the inverter on a voltage or frequency fault, typically only on systems over 16 A per phase.
The handbook split: G98, G99, G100
Independent OpenExamPrep teaching for 2922 uses the handbook outcomes together with the published ENA recommendations that those outcomes name. OpenExamPrep is not a City & Guilds, MCS, or ENA partner and does not set connection policy.
G98 is the Engineering Recommendation for fully type-tested micro-generators connected in parallel with a public low-voltage network, up to and including 16 A per phase. In installer language it is the connect-and-notify route: you install and commission a qualifying, type-tested unit, then notify the DNO/DSO. You do not wait for a connection offer before you fit a genuine G98 system.
G99 is the Engineering Recommendation for generation equipment in parallel with public distribution networks in all other cases — anything above 16 A per phase, anything that is not a qualifying fully type-tested G98 micro-generator, and any job where the DNO/DSO’s G99 process applies. The order of work reverses: apply and receive permission before connection (before you energise the generator onto the network).
G100 is the Engineering Recommendation for customer export limitation (an export limitation scheme). It applies where the DNO/DSO requires export limitation, not as a third size band between G98 and G99. A site can be G99 and G100: the network agrees a connection, and a limiter keeps export at or below the agreed cap. G100 does not delete G99.
Notification timeframe requirements
Outcome 4.1.3 includes notification timeframe requirements for the DNO/DSO. Handbook v1.3 names that duty without printing the number 28. In EREC G98 and in DNO commissioning practice, the G98 installation document is submitted as a commissioning-confirmation within 28 days of commissioning. Treat 28 days as that G98 commissioning-confirmation window used by DNOs, not as a figure the 2922 handbook v1.3 prints. Missing the window is a connection-agreement failure, not a paperwork courtesy.
G99 uses a different clock: the application and permission sit before connection. After permission, commissioning still produces G99 forms, but you must not treat “we will notify in 28 days” as a substitute for a missing G99 approval.
G100 adds commissioning tests of the export limiter: evidence that the scheme actually holds export to the agreed value, including fail-safe behaviour if a sensor or controller is lost. Those tests belong at commissioning, not as a sales slide.
16 A is AC at the connection, not DC kWp
16 A × 230 V = 3 680 W ≈ 3.68 kW on a nominal single-phase supply. On three-phase, 16 A per phase is 3 × 3.68 kW ≈ 11.04 kW if the generation is balanced. The handbook wording is per phase. A 10 kW three-phase inverter that is genuinely 16 A or less on each phase can still be in G98 territory if it is fully type-tested and the total stays within 16 A per phase; a 5 kW single-phase inverter is 5 000 / 230 ≈ 21.7 A and is not G98.
DC kWp is not the threshold. A 4.6 kWp array feeding a 3.6 kW fully type-tested inverter may still be a G98 connection if the AC generation/export capability at the connection is 16 A or less per phase and no other generator sits in parallel. A 3.0 kWp array feeding a 5 kW hybrid inverter is judged on 5 kW AC, not on 3.0 kWp.
Aggregated means every converting device that can export or present generation onto that connection: PV inverters, AC-coupled battery inverters, hybrid inverters, small wind, CHP, vehicle-to-grid. The DNO looks at the connection, not at the newest invoice.
MCS MIS 3002 combined-generation note
MCS installation practice sets the route in MIS 3002 clause 5.1.8 (G98 at or below 16 A per phase, G99 above it), and the note to clause 5.1.9 states that the 16 A per phase threshold is the total aggregated AC output of all generators, giving the example that 3 kW PV plus 3 kW electrical energy (battery) storage on the same single-phase supply produces a combined maximum theoretical output greater than 16 A, so EREC G99 applies.
Check: 3 kW / 230 V ≈ 13.0 A. Two such machines in parallel ≈ 26.1 A. Each brochure looks like a “small domestic system.” Together they are G99. That note is MCS practice a 2922 designer should know when combining PV and storage; it is the same arithmetic as handbook 4.1.3’s “total generation.”
Existing generation counts
Combined existing generation counts. If the house already has a notified G98 battery, a new PV inverter is added to that AC capability. You cannot start a second G98 clock as if the incoming service were empty. Read the existing MCS certificate, G98 installation document, or inverter rating plate. If the installer of the battery has left, the rating is still on the equipment and on the DNO’s record.
ENA G99 also makes a related plant point: new generation that pushes the installation through 16 A per phase must follow G99; existing G98/G83 plant is not automatically rebuilt to G99 settings just because something new was added. The new connection route is still G99. Do not energise the new inverter on a connect-and-notify story.
G98 vs G99 vs G100
| Topic | G98 | G99 | G100 |
|---|---|---|---|
| Handbook 4.1.3 trigger | Total generation 16 A or less per phase | All other cases | Export limitation required by DNO/DSO |
| Typical AC size | Up to ≈ 3.68 kW single-phase; ≈ 11.04 kW balanced three-phase | Anything above that, or non-qualifying plant | Not a size band; a limiter on an agreed export cap |
| What is summed | Aggregated AC capability at the connection, including batteries | Same aggregation | Limits export, not the existence of generation |
| Equipment | Fully type-tested micro-generators | Type-tested or not, as the G99 process requires | Export limitation scheme to G100 |
| Order of work | Connect, commission, notify | Apply and receive permission before connection | Install the agreed limiter; test it at commissioning |
| Timeframe to remember | DNO 28-day G98 commissioning-confirmation window; handbook v1.3 says notification timeframe requirements without printing 28 | Permission before energising; then G99 commissioning documents | Commissioning tests of the limiter, including fail-safe |
| 4.4.4 G99 relay | Not the typical G98 micro-generator package | Typically required on these larger systems | Does not replace interface protection |
G99 relay (handbook 4.4.4)
Outcome 4.4.4 describes the G99 relay as a monitoring device controlling a contactor or other isolating means. It monitors grid voltage and frequency. On a voltage or frequency fault it disconnects the inverter from the grid. It is typically only required on larger systems over 16 A per phase.
That is interface protection: the network must not be kept energised by a generator during a fault, an island, or an out-of-range voltage or frequency condition. The relay is the brain; the contactor / isolating means is the muscle. Settings live in G99; the 2922 point is the function and when it appears.
Fully type-tested inverters often contain this protection in firmware and internal switching. A discrete G99 relay and separate contactor is what you should expect when the DNO requires external interface protection, or when the plant is not relying on a single type-tested inverter package. Either way, the handbook description is the same chain: monitor V and f → open the isolating means → inverter off the grid. Do not describe a generation meter as a G99 relay. Do not describe a G100 export limiter as a G99 relay. The limiter caps power; the relay trips on network voltage and frequency (and related loss-of-mains functions in the G99 settings).
On G98 micro-generation the type-test already includes the protection the EREC requires. That is why 4.4.4 ties the discrete relay picture to larger systems over 16 A per phase.
Worked example: 3.6 kW PV inverter + 5 kW hybrid battery
A single-phase house is quoted with a 3.6 kW PV inverter and a 5 kW hybrid/battery inverter, AC-coupled so both can present power at the consumer unit.
- PV alone: 3.6 kW / 230 V ≈ 15.7 A — under 16 A, so G98 could apply if the inverter is fully type-tested and nothing else generates.
- Battery inverter alone: 5 kW / 230 V ≈ 21.7 A — already over 16 A, so the battery on its own is G99.
- Combined AC capability: 3.6 + 5.0 = 8.6 kW ≈ 37.4 A. G99 applies. Permission is required before connection. A G99 interface arrangement (integrated type-test and/or a discrete G99 relay and contactor as 4.4.4 describes) belongs on this larger system. If the DNO also caps export, G100 commissioning tests sit on top; they do not turn the job back into G98.
If instead the 5 kW hybrid is the only grid-tied inverter and the 3.6 kW of PV is DC-coupled into it, the AC capability is the 5 kW inverter: 21.7 A. Still G99. You cannot hide behind “the panels are only 3.6 kW.”
The MIS 3002 clause 5.1.9 note’s 3 kW + 3 kW example is the same lesson at smaller numbers: two sub-16 A machines in parallel are not two G98 notifications.
Scenario: adding PV to a house that already has a G98 battery
The property already has a G98 battery, notified when it was commissioned. The owner now wants roof PV.
- Read the battery’s AC rating, not the sales name. A 3.6 kW battery inverter is already near the 16 A ceiling.
- Add the proposed PV inverter AC rating. A further 3.6 kW PV inverter makes about 31 A combined on 230 V single-phase.
- G99 applies to the new total. Apply and wait for permission before connecting the PV (and before changing the agreed export if the DNO says so).
- Do not commission the PV on a G98 form because “the battery was G98.” The empty-service G98 route is used up.
- If the DNO will only accept the extra generation with a reduced export, that is G100 on a G99 connection: limiter tests at commissioning, plus the G99 interface protection 4.4.4 describes for the larger aggregated system.
- Tell the customer the delay sits before scaffolding, not after. Connecting first and arguing later is the failure mode this outcome exists to catch.
According to handbook outcome 4.1.3, which statement correctly places G98, G99 and G100?
A single-phase job has a 3.6 kW PV inverter and a 5 kW hybrid battery inverter that can both present AC at the connection. Which connection route follows from the aggregated AC capability?
Which statement matches handbook outcome 4.4.4 on a G99 relay?