Quick Facts
What DC:AC ratio means
DC:AC ratio (also called Inverter Loading Ratio or ILR) is the ratio of installed solar DC capacity in kWp to the inverter’s AC output rating in kW. A 6.5 kWp DC array paired with a 5 kW AC inverter has a DC:AC ratio of 1.30 (or 1.30x).
The ratio is a key solar plant design parameter that affects:
Capital efficiency: How effectively the inverter capacity is used.
Annual energy capture: How much energy the plant produces over a year.
Inverter clipping: How much energy is lost when DC output exceeds inverter capacity.
Operating temperature: How hard the inverter works on average.
Project economics: LCOE, IRR, and payback period.
Modern Indian solar designs typically use DC:AC ratios of 1.10 to 1.40, depending on the application. The ratio has trended higher over the past decade as module costs have fallen relative to inverter costs.
How DC:AC ratio works
To understand why higher DC:AC ratios produce better economics, consider the daily operating profile.
A solar inverter sees its rated AC output only briefly each day, around solar noon on clear sky conditions. The rest of the day, the inverter operates well below its rated capacity. Mornings and evenings, the inverter sees only 20% to 50% of its capacity.
Adding more DC capacity (raising the DC:AC ratio) fills more of the inverter’s operating curve at off-peak hours, capturing additional energy. The trade-off is brief midday clipping when DC output briefly exceeds the inverter’s AC rating.
For a typical Indian fixed-tilt installation:
At DC:AC ratio 1.0: Inverter operates well below capacity most of the day. No clipping. Limited energy capture from extended hours.
At DC:AC ratio 1.2: Inverter operates near capacity for more hours. Minor midday clipping (under 1% annually). Significantly more energy captured at off-peak hours.
At DC:AC ratio 1.3: Inverter operates at capacity for several hours daily. Moderate midday clipping (1% to 2% annually). Maximum energy capture from off-peak hours.
At DC:AC ratio 1.4: Inverter approaches maximum input capacity. Significant clipping (2% to 3.5% annually). Diminishing returns from additional DC capacity.
At DC:AC ratio above 1.5: Heavy clipping starts to outweigh energy gains. Inverter input limits may be approached.
The optimal ratio depends on cost economics (relative cost of DC and AC capacity), location (irradiance profile), and technology (module type, inverter capabilities).
Optimal DC:AC ratios by application
| Application | Typical DC:AC Ratio | Notes |
|---|---|---|
| Conservative residential | 1.05 to 1.15 | Minimal clipping, simple design |
| Standard residential | 1.15 to 1.25 | Common in PM Surya Ghar installations |
| Aggressive residential | 1.25 to 1.35 | More energy capture at marginal cost |
| Standard commercial | 1.15 to 1.30 | Common for C&I rooftop |
| Aggressive commercial | 1.30 to 1.40 | Premium installations |
| Utility-scale fixed-tilt | 1.30 to 1.40 | Standard for large ground-mount |
| Utility-scale tracker | 1.35 to 1.45 | Higher with trackers due to extended profile |
| Bifacial utility | 1.25 to 1.40 | Reduced slightly to manage bifacial gain |
For Indian projects, DC:AC ratios in the 1.20 to 1.35 range are most common. Specific values are optimised through detailed financial modelling for each site.
Inverter clipping at high DC:AC ratios
Clipping occurs when DC output exceeds the inverter’s AC rating. The inverter operates at its rated output and the excess DC power is “clipped” (not captured).
Typical annual clipping loss:
DC:AC ratio 1.10: under 0.5% annual clipping.
DC:AC ratio 1.20: 0.5% to 1% annual clipping.
DC:AC ratio 1.30: 1% to 2% annual clipping.
DC:AC ratio 1.40: 2% to 3.5% annual clipping.
DC:AC ratio 1.50: 3.5% to 6% annual clipping.
The clipping loss is concentrated in midday hours of clear summer days. Most of the year, the inverter is below its AC rating and no clipping occurs.
For most Indian designs, clipping loss of 1% to 2% is acceptable in exchange for the additional energy capture at off-peak hours.
DC:AC ratio and inverter selection
When selecting an inverter for a planned solar capacity, the DC:AC ratio is determined by:
Available inverter sizes in the market.
Inverter’s maximum DC input limits.
Inverter’s MPPT input current limits.
Project’s CAPEX budget.
Inverter manufacturer specifications typically state:
Maximum input voltage (Voc must stay below this).
Maximum input current per MPPT.
Maximum recommended DC kWp.
Recommended DC:AC ratio range.
Designs that stay within manufacturer specifications carry full warranty. Designs exceeding specifications may need consultation with the manufacturer.
Common DC:AC ratio mistakes
Treating the ratio as fixed. The optimal value depends on site, module type, and economics.
Pursuing very high ratios without modelling annual clipping. Heavy clipping eliminates the gains.
Ignoring inverter MPPT input current limits. Even if total DC kWp is within limits, individual MPPT currents may exceed.
Mismatching ratio with module type. Bifacial designs need lower ratios than monofacial for the same clipping target.
Using historical norms without re-evaluation. The optimal ratio has evolved as costs and technology change.
Skipping the financial analysis. The optimal ratio is project-specific.
Best practices
For new designs, model the DC:AC ratio’s impact on annual energy and LCOE using PVsyst or similar software.
For commercial rooftops in India, target 1.20 to 1.30 ratio as a baseline.
For utility-scale projects, target 1.30 to 1.40 with tracker bifacial designs.
Verify that inverter input limits accommodate the planned ratio.
For new module types (especially bifacial and TOPCon), adjust ratios based on technology characteristics.
For multiple-MPPT inverters, distribute strings to balance the load across MPPTs.
Standards and references
DC:AC ratio is not directly regulated but must respect inverter manufacturer specifications. Inverters comply with IEC 62109. PVsyst and SAM software model DC:AC ratio impacts on annual energy.
Related glossary terms
- DC Oversizing
- String Inverter
- Inverter Clipping
- MPPT
- What is kWp
- Performance Ratio
- Capacity Utilisation Factor
- Temperature Coefficient
Key takeaways
DC:AC ratio (Inverter Loading Ratio, ILR) is the ratio of installed solar DC capacity in kWp to the inverter’s AC output rating in kW. Modern Indian solar designs typically use ratios of 1.10 to 1.40, with utility-scale tracker plants using higher ratios. The optimal ratio balances annual energy capture against midday inverter clipping. Mild clipping (1% to 2% annually) is generally accepted for the energy gains at off-peak hours. The optimal ratio depends on site irradiance, module type, inverter limits, and project economics, and is determined through detailed financial modelling for each project.