AC/DC Ratio (Inverter Loading Ratio)

Quick Facts

What is AC/DC Ratio?

Formal definition

The inverter loading ratio is the quotient of installed DC power (kW_DC at STC) divided by the inverter AC continuous power rating (kW_AC).

Engineering definition

ILR controls the tradeoff between morning/evening yield gain (DC oversizing) and midday clipping loss (inverter saturation).

Industry definition

Commonly called “DC:AC ratio” by EPCs. Specified as 1.20:1, 1.30:1, etc. The higher the ratio, the more DC modules per inverter watt.

Permitting definition

Not a permit metric, but appears on the SLD as the kW_DC and kW_AC nameplates of the system.

Why ILR > 1.0 Makes Sense

A 100 W STC module rarely produces 100 W in real life:

• High cell temperature reduces Pmax 8–12% midday.
• Soiling reduces 2–5%.
• Mismatch + DC losses reduce 1–2%.
• Wiring + connection losses reduce 0.5–1.0%.

Real-world midday Pmax ≈ 80–87% of STC nameplate. So a 100 kW_DC array delivers ~80–87 kW to the inverter, even at peak sun. Sizing inverter to 100 kW_AC wastes capacity 99% of the year. Right-sizing inverter to ~75–85 kW_AC captures the same midday peak and saves cost.

Engineering Deep Dive — ILR Optimization

Step 1: Identify clipping threshold

Inverter saturates when DC power = AC power × η_inverter. For a 75 kW_AC inverter at 98% efficiency, DC threshold ≈ 76.5 kW_DC.

Step 2: Map DC power vs. time

Use PVsyst hourly simulation to compute 8,760-hr DC power profile.

Step 3: Calculate clipped energy

For each hour where DC > 76.5 kW, clipped energy = (DC − 76.5) × η_inverter.

Step 4: Compute baseline gain

Without inverter saturation, would the additional DC capacity produce energy?

• Yes, in morning/evening shoulder hours: morning ramp, evening decline.
• The shoulder gain is typically 8–15% of nameplate AC, depending on latitude.

Step 5: Optimize for project economics

LCOE-optimal ILR: where marginal cost of additional DC = marginal value of additional AC energy.

Worked example — 1 MW Las Vegas plant

• Module: 545 W bifacial.
• Inverter options: 750 kW or 850 kW.
• Module count: 2,200 → 1,200 kW_DC.

LCOE-optimal: ILR ≈ 1.35–1.45 for Las Vegas conditions. Higher ILR reduces $/W_AC but loses too much to clipping.

Site-dependent ILR optimums

ILR vs. Storage Strategy

DC-coupled batteries can absorb clipped DC energy:

• Without storage: midday clipping = lost energy.
• With DC-coupled storage: clipped DC charges battery, dispatched later at higher value.
• Effect on optimal ILR: pushes optimum up to 1.30–1.45 in NEM 3.0 environments.

Design Considerations

• String sizing must accommodate the higher DC current per MPPT.
• Cable sizing based on the higher DC current.
• Combiner box OCPD ratings must match the DC current.
• Inverter selection should support overcurrent during clipping (most modern inverters do).
• Thermal management at high ILR — inverters run hotter during sustained clipping.

Permitting Implications

The SLD must show both kW_DC and kW_AC. Some AHJs and utilities flag ILR > 1.4 for additional review. NEC 705 interconnection limits apply to the AC capacity, not DC.

Common Mistakes

1. Sizing inverter to match nameplate kW_DC — wastes inverter capacity.
2. Targeting zero clipping — over-sized inverter, higher LCOE.
3. Ignoring site-specific weather; copying ILR from textbook.
4. Forgetting battery storage in NEM 3.0 — DC-coupled storage shifts ILR optimum.
5. Not validating with PVsyst clipping loss diagram.

Best Practices

• Run PVsyst sweep across ILR 1.10, 1.20, 1.30, 1.40 for the specific site.
• Choose ILR at LCOE minimum, not just maximum production.
• Document ILR rationale in design notes.
• Validate inverter datasheet supports continuous operation at sustained clipping.

Comparison Tables

ILR Recommendations by Project Type (2024)

Standards & Certifications

No direct certification. PVsyst, SAM, and Helioscope all support ILR modeling per inverter datasheet specifications.

Key Takeaways

• AC/DC Ratio (ILR) is the ratio of installed DC array kW to inverter AC kW; typical 1.15–1.40.
• Above 1.0 captures real-world DC degradation; saves inverter cost; gains morning/evening yield.
• Clipping is the cost: midday energy lost when DC > inverter AC capacity.
• Site-dependent optimum balances clipping loss vs. inverter cost — sunny utility 1.30–1.40, cloudy 1.10–1.25.
• DC-coupled storage shifts optimal ILR higher; NEM 3.0 (low export value) shifts it lower.