The G99 Fast Track is a streamlined route within the wider G99 framework, designed for smaller or less complex generation and storage installations that shouldn't materially affect network stability. Instead of the full G99 application process, Fast Track sits in three tiers — SGI-1, SGI-2 and SGI-3 — each with its own thresholds, timings and paperwork.
Understanding which tier your project falls under matters, because it decides whether you can install first and inform the DNO afterwards, or whether you need approval before a single panel goes on the roof.
The eligibility rules that apply to all three tiers
Before looking at the tiers individually, every Fast Track application has to meet these baseline conditions:
- All generation and/or storage equipment is located in a single installation.
- All new and existing generation/storage equipment is type-tested to G83, G59, G98 or G99.
- Each individual device has a basic design capacity of 32A or less.
On top of that, the total aggregate rating and the export limit determine which tier you're in.
SGI-1 — fit and inform
SGI-1 is the simplest route. It applies where every generation/storage device is rated at no more than 16A per phase, and the total of all ratings is also no more than 16A per phase.
Because the aggregate is within the standard G98 threshold, this is a fit-and-inform process. Provided the eligibility conditions are met, an installer can proceed with the installation and then notify the DNO using an A3-3 form.
The practical difference from G98 is that under SGI-1 the rating of a device can be limited by the manufacturer to something lower than its basic design capacity — that isn't allowed under G98.
SGI-2 — apply, then install
SGI-2 covers installations where each device is rated at no more than 16A but the total aggregate rating is up to 32A per phase, provided a G100-compliant export limitation scheme caps export to the network at no more than 16A.
This is an application-before-installation process:
- Submit an A1-2 form along with the supporting information.
- The DNO has 10 working days to confirm acceptance to proceed.
- Once accepted, you have up to 3 months to install and commission.
- After commissioning, submit Form A3-2 within 28 days.
SGI-2 is the tier most commonly used for larger domestic solar-plus-battery systems where the combined inverter rating exceeds a standard G98 threshold but the export can be capped in software or by a G100 device.
SGI-3 — larger installations, network study required
SGI-3 pushes the envelope further. Each device can be rated up to 32A, the total aggregate rating can be up to 60A per phase, and a G100 scheme must limit export to no more than 32A.
The process is application-before-installation, but with one important extra step: the DNO carries out a network study to check that the earth loop impedance at the property is within the allowable limit for the installation to proceed.
- If the network study passes, acceptance is issued within 10 working days and the installer has up to 3 months to install and commission (then A3-2 within 28 days, as with SGI-2).
- If the study fails — typically because the earth loop impedance is above the allowable limit — the application is bumped out of Fast Track and processed as a standard G99 enquiry with the usual timescales.
Quick reference
| Tier | Per-device limit | Total aggregate | G100 export limit | Process |
|---|---|---|---|---|
| SGI-1 | ≤16A | ≤16A/phase | Not required | Fit and inform (A3-3) |
| SGI-2 | ≤16A | ≤32A/phase | ≤16A | Apply first (A1-2), then A3-2 |
| SGI-3 | ≤32A | ≤60A/phase | ≤32A | Apply first + network study, then A3-2 |
Which tier applies to your project?
A good rule of thumb:
- Small single-inverter installs near the 16A/phase threshold → usually SGI-1.
- Solar plus battery where inverters add up beyond 16A but export can be capped → usually SGI-2.
- Larger systems approaching a 60A aggregate with an export limitation scheme → SGI-3, but be prepared for the earth loop study to redirect you into full G99.
If you're not sure, submit through us — we'll classify the application against the current Fast Track rules for your DNO and use the fastest legitimate route available.
Common pitfalls
- Missing the A3-2 within 28 days of commissioning. SGI-2 and SGI-3 both require it — installations get flagged if this step is skipped.
- Assuming SGI-3 acceptance means you're through. Even after acceptance, a subsequent earth-loop failure can still redirect the project into standard G99.
- Treating manufacturer-limited outputs as G98-eligible. Rating limits below basic design capacity are only accepted under G99 SGI-1, not under G98.