G99 Inverter Calculator
Calculate total inverter capacity and effective export limits to determine your UK DNO connection route under Engineering Recommendation G99.
G99 Inverter Calculator
How to Use G99 Inverter Calculator
Verifying grid eligibility routes is mandatory in the UK for clean energy assets. Our calculator handles individual ratings, multiple units, and export limit schemes to check whether you qualify for G98 micro-generator terms or need a formal DNO application. Follow these practical steps:
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1Select supply type. Choose Single Phase (typical for domestic properties) or Three Phase (typical for larger commercial developments and agricultural connections).
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2Enter inverter quantity. Input the number of independent grid-tied inverters in the system (e.g. 2 inverters for solar and battery storage systems).
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3Enter individual rating. Input the continuous maximum active AC power rating (in kW) for a single inverter.
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4Choose export limitation. Indicate if you are using an ENA-approved export limiting scheme (G100 standard) to cap the export current.
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5Enter export limit value. If using limitation, input the programmed threshold in kW (for example, a standard 3.68 kW single-phase limit).
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6Click Calculate. Review your G98/G99 compliance outcome, connection pathway, and the specific application form required.
How to Calculate G99 Inverter Capacity
Under Engineering Recommendation G99, DNOs regulate the active generation power entering the network. The capacity limits are calculated based on the maximum export capability of the entire system. Below are the equations used to size your connection capacity.
G99 Mathematical Formulas
To determine the installed capacity, effective export capacity, and per-phase current values, the following math is applied:
Total Installed Capacity (kW):
Effective Export Capacity (kW):
Three-Phase Capacity Per Phase (kW):
Worked Example: Single Phase Solar PV System
Consider a standard residential system with multiple generators where export limitation is not implemented:
- Supply configuration: Single Phase
- Number of Inverters: 2
- Inverter AC Rating: 3.00 kW each
- Export Limitation: No
Step-by-step capacity evaluation:
Result: Since the Effective Export Capacity is 6.00 kW, which is greater than the standard G98 threshold of 3.68 kW (6.00 kW > 3.68 kW), a G99 Prior Approval Application is Required before the system is connected to the grid.
In the UK, DNO approval is necessary to ensure local grid assets (transformers, cabling, voltage regulation equipment) can tolerate the extra power without causing voltage spikes or system failures.
G99 Inverter Capacity Chart
This reference chart details ENA connection pathways, application requirements, and standard system routes across single and three-phase supplies. Values assume a standard nominal phase-to-neutral voltage of 230 V (400 V line-to-line) and unity power factor.
| System Type | Export Capacity | Typical Connection Route | Application Requirement | Example System |
|---|---|---|---|---|
| Single Phase | Up to 3.68 kW | G98 | Notification | 3.68 kW Solar PV |
| Single Phase | Above 3.68 kW | G99 | Prior Approval | 5 kW Solar PV |
| Three Phase | Up to 11.04 kW Total | Typically within 3.68 kW per phase | Assessment Required | Commercial Solar |
| Three Phase | Above 11.04 kW | G99 | Prior Approval | Large Solar Installation |
| Three Phase | 50 kW+ | Enhanced Assessment | DNO Review | Commercial Generation |
Note: Actual approval requirements depend on DNO assessment, export limitation arrangements, and site-specific network constraints.
G99 Inverter Frequently Asked Questions
Engineering Recommendation G99 (EREC G99) is the UK grid connection standard issued by the Energy Networks Association (ENA) for connecting generation equipment (like solar PV and battery storage) to the distribution network. It replaced G59 in 2019 and applies to systems exceeding 16 Amperes per phase.
Any inverter system in the UK that has a continuous AC output capacity exceeding 16 Amperes per phase (which corresponds to 3.68 kW on a single-phase 230V supply, or 11.04 kW on a three-phase 400V supply) falls under G99 requirements and requires prior approval.
The G98 limit is 16 A per phase. Under ENA G98, generators of 16A or less per phase can be connected using a simplified "notify after installation" route within 28 days of commissioning. Exceeding this limit requires a G99 application.
Yes, an ENA-approved export limitation scheme can cap export capacity at G98 limits (e.g., 3.68 kW). However, because the physical inverter rating is larger, you must still submit a G99 application (often Fast Track Form A1-1) to the DNO before installing the system.
G99 capacity is calculated as the sum of all grid-connected inverter AC output ratings. If export limitation is applied, the effective export capacity is the lower of the total inverter capacity and the export limitation value set on the G100-compliant limiter.
Yes, battery energy storage systems (BESS) are treated as generators under ENA rules. If the combined rating of the battery inverter and the solar PV inverter exceeds 16A per phase, a G99 application is required even if the battery is programmed for self-consumption.
A standard G99 application (Type A or B) typically takes between 45 to 65 working days for the DNO to assess and approve, depending on grid constraints. G99 Fast Track applications (Form A1-1) are processed faster, typically within 10 to 20 working days.
Yes, compliance with EREC G99 is legally mandatory under the UK Distribution Code for all generating units connected to the distribution network that do not qualify under the G98 notification scheme. Connecting without approval is a breach of grid terms.