G99 rejections delay solar installations by weeks. These are the most common causes of failed applications - and what to fix before you submit.
Matt Franklin
The gap between a customer signing your proposal and their system going live is largely determined by one thing: how cleanly your G99 application goes through. A first-time approval keeps that gap under six weeks. A rejection adds two to eight weeks. Two rejections and you are looking at three to five months on some networks.
For an installer doing 30+ solar installations a month, that is not a compliance problem. It is a cash flow problem and a capacity planning problem. You have panels ordered, an engineer scheduled, and a customer who has already told their neighbour the system would be running by now.
Managing G99 applications at scale?
Payaca tracks G99 and G98 applications within the project workflow - from original application through to commissioning notification. See how it works.
G98 covers smaller systems - those exporting no more than 16A per phase at the connection point. For a typical single-phase domestic install, that is up to around 3.68kW of inverter capacity. G98 works on a notify-and-install basis: you submit the notification, wait 20 working days for the DNO to raise an objection, and if there is none you can proceed. In practice, most G98 notifications go through without a hitch.
G99 covers everything else: larger domestic systems, commercial installations, multi-phase connections, and any system where the inverter is capable of exporting above the G98 threshold - including battery storage installations where combined inverter capacity pushes past the limit. G99 requires formal application, technical assessment, and a connection agreement before you install. It is the process where rejections happen and timescales slip.
DNOs assess G99 applications against technical standards and their own network capacity models. The vast majority of rejections are not about network capacity - they are about the quality of the application itself.
Wrong inverter model or outdated specifications
The DNO checks your inverter against its own database and the MCS approved product list. If the model number in your application does not match exactly - a different variant suffix, a firmware-specific designation, or a model updated since you last submitted - the application comes back. This is the most common rejection reason and the most avoidable. Check the exact model designation against the current MCS approved product list, not against the manufacturer datasheet or a previous G99 approval for a similar unit.
Incorrect export limitation configuration
If you are applying for a system with active export limitation - via a power diverter, a zero-export relay, or inverter-level export management - the application must specify the exact limitation method, the target export limit, and the protection settings. Vague entries like "export managed" or "zero export" without the technical configuration will be rejected. The DNO needs to know exactly how the limitation is enforced before it will accept the application.
Battery storage: capacity vs power
Applications for systems with battery storage frequently include the battery's storage capacity in kWh but omit the battery inverter's power rating in kW, or conflate the two numbers. The DNO needs the power rating to assess network impact. A 10kWh battery with a 5kW inverter has a very different effect on the network than a 10kWh battery with a 10kW inverter.
Missing or incorrect protection settings
G99 systems require specific protection relay settings for loss-of-mains detection and anti-islanding. The application must state the protection settings that will be applied, not the defaults the inverter ships with. Installers working across multiple inverter brands sometimes carry protection settings from one manufacturer's application to another - which will be wrong for every model where it does not match.
Connection point details
The DNO assesses your application against the actual connection point: your specific meter, supply cable, and local network topology. Applications that use a postcode or street address without the MPAN (Meter Point Administration Number) get bounced regularly. The MPAN is not the same as the meter serial number. If you are collecting it from a customer by email rather than from the meter on site, verify it before you submit.
The published target for G99 assessment is 45 working days. In practice, timescales vary significantly by network:
The clock only starts when the DNO deems your application complete. A rejection and resubmission does not pause the clock - it resets it. An application rejected at week three and resubmitted at week five is sitting at the back of the assessment queue again.
The fastest path to approval is a complete, technically accurate application in a single submission. That means:
For installers working across multiple inverter brands, a model-specific reference document for protection settings is worth the one-off effort. The copy-paste error - carrying settings from a Solis application into a Growatt application - is responsible for a substantial share of rejection letters.
For an installer doing 10-15 solar installations a month, G99 applications are manageable with a spreadsheet and a calendar reminder. At 30-50 installs a month, the volume creates a different kind of problem: tracking which applications are at which stage across multiple DNO territories, ensuring commissioning notifications go back to the right DNO on time, and catching applications that are approaching the 12-month approval window before they expire.
The commissioning notification - the step that formally tells the DNO the system has been installed and matches the approved specifications - is where the process most often breaks down for growing installers. It is easy to track the original G99 application because there is a firm pending outcome to watch. Commissioning notifications are less time-critical in the short term, get deprioritised against active jobs, and then three months later a customer rings asking why their export tariff is not set up.
Keeping G99 applications, commissioning notifications, and the corresponding electrical certificates in the same record as the customer, proposal, and scheduled engineer visit removes the gap where things are forgotten. If the DNO workflow lives in a separate spreadsheet from the job data, things will fall through the gap. If it is part of the job itself, your office team does not need to remember to check it - the status is visible every time someone opens the project. That is what Payaca's DNO workflow is built around.
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