About the DC:AC Ratio
The DC:AC ratio (also called inverter loading ratio or ILR) is the ratio of the solar array's DC nameplate capacity to the inverter's AC output rating. A ratio above 1.0 means the array is 'oversized' relative to the inverter — a deliberate design choice to maximise energy yield at the cost of some clipping in peak hours.
Formula
DC:AC ratio = Array DC capacity (kWp) ÷ Inverter AC rating (kW)
- Array DC capacity (kWp)
- Sum of all panel nameplate ratings under STC
- Inverter AC rating (kW)
- Inverter's continuous AC output capacity at rated conditions
How to calculate it
- 1Select an inverter AC rating for the target system.
- 2Determine the array size (or select modules to fill the roof).
- 3Calculate the ratio: array kWp ÷ inverter kW.
- 4Check MNRE/CERC norms — most grid-connected systems accept ratios up to 1.2–1.3.
- 5Model clipping losses: above 1.2, clipping begins to exceed 1–2% of annual generation, reducing LCOE benefit of over-sizing.
- 6For bifacial modules with high rear-side gain, or for trackers, the effective DC:AC ratio during peak hours can be higher — account for bifacial gain in the clipping model.
Worked example
A 5 kWp array (12 × 420 W modules) on a 4 kW inverter: DC:AC = 5 ÷ 4 = 1.25. In peak summer hours the array may produce 4.8 kWp, but the inverter clips at 4 kW → 0.8 kW clipped. Annual clipping loss ≈ 0.8% of generation — acceptable and offset by higher generation in low-irradiance morning/evening hours.
What you can calculate
- Clipping loss %
- CUF impact
- Optimal ratio
- Inverter loading
Frequently asked questions
What is the recommended DC:AC ratio for Indian rooftop solar?
For grid-tied residential rooftop systems in India, a DC:AC ratio of 1.1–1.25 is standard. Going above 1.3 increases clipping losses without proportional benefit. Some utility-scale projects in high-irradiance states use up to 1.4 with a tracker to capture more energy in the shoulder hours.
Why not use a DC:AC ratio of 1.0 to avoid all clipping?
Solar panels rarely produce 100% of nameplate in real conditions (STC is rarely achieved; temperature, shading, and dust reduce output). A ratio of 1.0 means the inverter runs below its rated capacity most of the day. Inverters are more efficient at higher loading fractions (>70%), so a modest oversizing (1.1–1.2) improves overall system efficiency and energy yield.
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