Inverter sizing in the UK has its own set of rules, shaped by grid connection standards, the country's relatively low peak irradiance, and the DNO (Distribution Network Operator) notification requirements that govern how much power your system can export.
What Is the DC-AC Ratio?
The DC-AC ratio (also called the clipping ratio or oversizing ratio) is the relationship between the total DC power output of your solar panels and the AC power rating of your inverter.
DC-AC ratio = Total panel Wp / Inverter AC output rating (W)
A ratio of 1.0 means a perfectly matched system. A ratio above 1.0 means the panels have been oversized relative to the inverter — a common and deliberate design choice.
Why UK Installers Often Oversize the DC Array
In the UK, irradiance is significantly lower than in southern Europe. The UK's peak sun hours average 2.5–3.5 per day (compared to 5–6 in Cyprus or Spain). This means:
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- Your panels will rarely, if ever, operate at their rated peak power in real conditions
- Oversizing the DC array compensates for low irradiance and generates more electricity in morning and evening shoulder periods
- The inverter's AC limit naturally caps output on the rare occasions when irradiance is high
A DC-AC ratio of 1.2 to 1.4 is typical and well-accepted in UK residential design. Going above 1.5 is possible but requires careful justification — particularly regarding clipping losses on bright summer days.
G98 and G99: The Grid Connection Rules
All UK solar installations must comply with Engineering Recommendation G98 or G99, depending on system size.
G98 — Systems Up to 3.68 kW (single phase)
G98 applies to the vast majority of UK home solar installations. Under G98:
- Your DNO must be notified before energising the system
- The notification process is straightforward (often handled by your installer)
- Export limiting may be required in some network areas
- Inverters must be G98-compliant (look for this in the inverter specification sheet)
G99 — Systems Above 3.68 kW (single phase) or Multi-Phase
G99 applies to larger or three-phase systems. The process is more involved:
- A formal application must be submitted to the DNO
- The DNO has up to 45 working days to respond
- Protective device requirements are more stringent
- Some DNOs require a G99 assessment fee (typically £100–300)
This is why many UK installers design residential systems to sit just under the G98 threshold — a 3.6 kW or 3.5 kW inverter avoids the G99 process entirely.
Inverter Sizing Comparison: Common UK Residential Scenarios
| Roof Size (Panels) | Panel Wp | Total DC (kWp) | Recommended Inverter | DC-AC Ratio | G98 or G99? |
|---|---|---|---|---|---|
| 6 panels | 430 W | 2.58 kWp | 2.5 kW | 1.03 | G98 |
| 8 panels | 430 W | 3.44 kWp | 2.5–3.0 kW | 1.15–1.38 | G98 |
| 10 panels | 430 W | 4.30 kWp | 3.68 kW | 1.17 | G98 (limited export) |
| 12 panels | 430 W | 5.16 kWp | 5.0 kW | 1.03 | G99 |
| 16 panels | 430 W | 6.88 kWp | 6.0 kW | 1.15 | G99 |
Inverter Brands and UK Compliance
Leading inverter brands available in the UK — including SolarEdge, Fronius, SMA, Sungrow, Huawei, and Deye — all produce G98/G99-compliant residential models. G98 compliance should be explicitly stated in the product's UK datasheet. When reviewing quotes, check that the specified model number carries G98 or G99 certification.
Export Limiting and DNO Constraints
Some DNOs impose export limits on networks where local grid capacity is constrained. Export limiting means your inverter will cap AC output to the agreed limit — typically 3.68 kW — even if the DC array could produce more. This is managed automatically by the inverter firmware and does not reduce self-consumption. If export limiting applies to your installation, it will be stated in the DNO's G99 approval response.
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