G98 vs G99: the core difference
Both G98 and G99 are Engineering Recommendations published by the Energy Networks Association that define how solar generators must connect to and interact with the UK's distribution grid. They replaced the older G83 and G59 standards in 2019. The fundamental difference is about size, process, and timing.
G98: the notification procedure for smaller systems
G98 replaced Engineering Recommendation G83 in 2019. It covers single-phase micro-generators up to 3.68 kW, and three-phase generators up to 11.04 kW (three times the single-phase limit). The vast majority of pre-2020 domestic solar installations were registered under G83, and those registrations remain valid — no retrospective change is required.
Under G98, the installer (the "competent person" — typically an MCS or NAPIT-registered electrician) files a notification with the DNO after installation, ideally before the system is commissioned but at most within 28 days. The homeowner does not file this themselves. You should receive a copy of the notification confirmation letter from either your installer or the DNO. If you never received one, there is a strong chance the notification was either not filed or was filed without being passed on to you.
Almost nothing. G98 is a notification procedure, not an approval one — the DNO is being informed, not consulted. The DNO logs the notification, acknowledges receipt, and adds the generation unit to their network records. No network study is carried out. The DNO cannot reject a G98 notification; they can only acknowledge it. This is why G98 is fast and straightforward compared to G99.
The DNO uses G98 data for network planning — knowing how many small generators are connected to each feeder helps them manage voltage and capacity. This is also why, in areas with high solar density, the DNO may eventually require G99 even for small new systems — the cumulative effect of many G98-registered systems starts to affect the network.
G98 sets the technical standards the inverter must meet to be connected: voltage limits (must disconnect if grid voltage goes outside ±10% of 230 V), frequency limits (must disconnect if frequency goes outside 47–52 Hz), reconnection delay (minimum 20 seconds after grid returns to normal range before the inverter reconnects), and anti-islanding protection (must disconnect from the grid within 2 seconds of a power cut).
All CE/UKCA-marked solar inverters sold in the UK are pre-configured to comply with G98. Your installer should verify the settings at commissioning and document them in the commissioning report.
G98 does not inherently impose an export limit — it is a connection notification, not an export consent. However, if your DNO's network is congested, they may require a zero-export configuration even for G98-registered systems. This is communicated separately, usually as a condition when the installer checks with the DNO before installation. If your system exports freely and the export causes no problems, you will not receive such a restriction. See our export limitation guide for full detail on how DNO limits work in practice.
G99: the approval process for larger systems
G99 replaced Engineering Recommendations G59 (for larger generators) and parts of G83 in 2019. It applies to any generator above the G98 thresholds, or in areas where the DNO requires formal assessment regardless of size. Unlike G98, G99 requires prior written approval from the DNO — you cannot legally connect a G99-scope system without the approval letter in hand.
G99 is required when: your system's maximum export capacity exceeds 3.68 kW on a single-phase connection (or 11.04 kW on three-phase); you are in a DNO-designated constrained network area where G99 is required for all new generators regardless of size; you are upgrading an existing G98 system in a way that increases export capacity above the G98 threshold; or your system type requires additional technical assessment (three-phase large battery, virtual power plant participation, advanced reactive power control).
A G99 application is submitted through the DNO's connections portal. Required information includes: the installation address and MPAN; the type and capacity of the generating unit (inverter make, model, rated export power); the type of connection (single-phase or three-phase); any storage component (battery capacity and max charge/discharge rate); proposed installation date; and the installer's MCS registration number and contact details.
Some DNOs also require a single-line diagram of the proposed installation and a copy of the inverter's G99 compliance certificate. Your installer should handle the application as part of the installation package — it should not fall to the homeowner to navigate the DNO portal.
For standard (non-Fast-Track) G99, the DNO carries out a network capacity study — they model the effect of your proposed generator on the local feeder voltage, fault level, and thermal loading. If the study identifies a concern, the DNO may impose conditions: a hard export limit (e.g., 3.68 kW), a requirement for the inverter to support reactive power absorption (to help manage voltage rise), or a requirement to install a remote disconnect device allowing the DNO to curtail export in emergencies.
The G99 approval letter is a legally binding document that specifies how the system may be connected. Conditions may include: a maximum export limit (must be configured in the inverter settings — not doing so is a compliance breach); a power factor or reactive power requirement; a requirement to notify the DNO before commissioning; a requirement for a G99 completion form to be returned after installation; and in some cases, a requirement for annual maintenance records. Keep the approval letter permanently — it is needed for property sales, insurance, and any future upgrade applications.
Fast Track G99: the four-week route for most domestic systems
Fast Track G99 was introduced to avoid making homeowners wait three months for a network study when their system is small enough that the DNO can assess it without detailed modelling. For the majority of domestic solar-plus-battery systems that exceed the G98 threshold, Fast Track is the applicable route. Understanding how it works — and its exact timeline rules — is important for planning an installation.
Fast Track is a Stage 1 application within the G99 framework. It is available for generating installations that meet a set of standard parameters: single-phase inverters exporting up to 50 kW, or three-phase up to 50 kW total, using standard MCS-certified equipment and connecting in areas where the DNO does not have an immediate objection on network capacity grounds. The key distinction from standard G99 is the timeline: under Fast Track, the DNO has just 20 business days (approximately four weeks) to respond. Under standard G99, the timeline is 65 business days (approximately 13 weeks).
This is the most important aspect of Fast Track: if the DNO does not respond within 20 business days, the connection is deemed approved. The applicant can proceed with installation as if written approval had been received. This is a legal provision within Engineering Recommendation G99 designed to prevent DNO administrative delays from holding up compliant installations indefinitely.
Fast Track is available when: the total export capacity is 50 kW or less; the equipment is standard (listed or type-tested); and the DNO's pre-application check does not flag the connection point as a network constraint area. Fast Track is not available — and standard G99 applies — when: the system is above 50 kW; the DNO has flagged the area as having capacity issues that require a network study; the connection involves non-standard equipment or configuration; or the application is for a variation to an existing G99 approval where the change increases export significantly.
A Fast Track G99 approval letter typically specifies: the maximum export limit (often set at the inverter's rated output, e.g., 5 kW, or reduced to 3.68 kW in certain areas); a requirement to configure the export limit in the inverter before commissioning; a G99 Stage 2 (commissioning) form to be returned by the installer after installation is complete; and contact details for DNO notification before the system goes live.
The export limit set in the Fast Track approval must be configured in the inverter settings. This is a legal compliance requirement — not just a recommendation. Your installer should verify and document this at commissioning. If the export limit was not configured at installation, it can usually be added retrospectively through the inverter portal, but you should confirm the correct value from your approval letter before making changes. See our export limitation guide for how inverter export limiting works in practice.
Understanding the timelines helps with installation planning — particularly for jobs where the customer wants a confirmed date.
Which standard applies to my system?
The determining factor is your system's maximum export capacity — not the panel array size, battery capacity, or inverter generation rating. Export capacity is what the inverter can push onto the grid simultaneously at maximum output.
G98 applies. Your installer should have filed a G98 notification (or G83 under the old standard). You should have a DNO acknowledgement letter. If you do not have one, contact your installer or DNO to check the record. Retrospective filing is possible.
G99 (likely Fast Track) applies. This covers most modern hybrid installations: GivEnergy 5 kW, Solis 6 kW, Growatt 6 kW, Fox ESS H3 6 kW, Sunsynk 5 kW. Your installer must have submitted and received G99 approval before commissioning. Check your documentation — you should have a G99 approval letter, not just an MCS certificate.
Depends on the new inverter's export capacity. If you are replacing a 3.5 kW string inverter with a 3.68 kW hybrid inverter (same export capacity), a G98 variation notification may suffice. If you are replacing it with a 5 kW hybrid inverter, G99 Fast Track is required before work begins. Confirm with your installer before booking the installation — not after.
Standard G99 (network study required). Allow 13 weeks minimum from application submission. A network study fee is typically charged by the DNO. The approval may include complex conditions including reactive power management, remote disconnect, and ongoing monitoring requirements.
Check your conveyancing documents for a DNO letter. If one is not present, request the DNO check your property's MPAN for existing G98/G99 records. Our bought house with solar section covers the full process for understanding an inherited system, including how to check DNO registration status without the original installer's involvement.
A remote diagnostic includes a documentation review — we confirm your G98/G99 status, identify any compliance gaps, and advise on the correct path to resolve them.
Changing an existing approval: DNO variation applications
Once a G98 notification has been filed or G99 approval granted, changes to the system that affect grid connection parameters require a variation. The variation process is similar to the original application but usually faster, since the DNO already has your installation on record.
You need a variation to your existing registration when: you replace the inverter with a model of different capacity; you add a battery that increases export capacity; you add additional solar panels that change the generation unit's characteristics; you change the export limit settings beyond what the current approval allows; or your system changes from single-phase to three-phase. Minor changes — replacing like-for-like components with identical specifications — typically do not require a formal variation, but your installer should confirm this with the DNO before proceeding.
If your current G99 approval includes an export limit that is lower than your inverter's capability, you can request a variation to increase it. Submit a variation application through the DNO's connections portal, explaining the proposed change. The DNO carries out a network assessment and may approve, approve with revised conditions, or decline if the network cannot support additional export. There is usually an administrative fee, and the timeline is typically 20 business days for smaller variations.
When an existing G98-registered system is upgraded in a way that increases export capacity above 3.68 kW, you do not file a variation to the G98 registration — instead, you file a new G99 application (typically Fast Track). The G99 approval supersedes the G98 notification for the upgraded system. Your installer must submit the G99 application and receive approval before the new inverter or battery is commissioned, even if the physical solar panels are unchanged.
What to do if G98 or G99 paperwork is missing
Many homeowners discover that their solar system was never registered with the DNO — either because the installer never filed the paperwork, the installer has since gone out of business, or the documentation was not passed on at point of sale. This is fixable in almost all cases.
Many installers filed the notification correctly but never gave the homeowner a copy. Contact your DNO with your property's MPAN number (found on any electricity bill) and your installation address. Ask them to check whether a G98 notification or G99 application has been recorded for your property. If the DNO has a record, request a copy of the acknowledgement or approval letter.
If the DNO has no record and your system is G98-scope (≤3.68 kW), retrospective notification is straightforward. An MCS or NAPIT-registered electrician can file the notification on your behalf using the system's MCS certificate details. Most DNOs accept retrospective G98 notifications without issue, as long as the installer can demonstrate the system meets G98 technical requirements. Cost is typically £50–£200 and takes 1–3 weeks.
If your system is G99-scope and was never approved, the DNO may require a full retrospective G99 application — essentially treating it as a new application. The DNO will carry out their standard assessment and may impose conditions including an export limit. If the system has been operating without approval and export has been causing network problems, the DNO may require temporary curtailment while the study is carried out. This is rare but does occur.
Missing DNO registration causes practical problems beyond the technical compliance issue: Smart Export Guarantee suppliers require DNO registration evidence before activating export payments — missing paperwork means you are not getting paid for exported power; home insurance policies often require grid-connected generation equipment to be correctly registered; and a property buyer's solicitor will flag missing DNO documentation as a legal issue during conveyance, potentially causing delays or reducing the sale price. Resolving registration proactively is much easier than dealing with it at exchange of contracts under time pressure.
Related guides and pages
How DNO export limits work in practice, how your inverter enforces them, and what to do if yours is set too low.
What to do when your installer has gone bust and you have no DNO registration documents.
System not exporting as expected — diagnosis for export limits set in the inverter vs imposed by the DNO.
We handle G98 notifications, G99 approvals, Fast Track submissions, and retrospective filings. From £65.
Inverter tripping due to high grid voltage — a G98/G99 protection function, not a system fault.
How to check what DNO documentation was passed on and what to do if registration records are missing.
Anti-islanding, grid synchronisation, and why inverters must disconnect during power cuts — the technical side of G98/G99.