DC/AC Ratio
Also called: Inverter Loading Ratio · ILR · Oversizing ratio
In one line
The ratio of installed DC module capacity to inverter AC rating — deliberately set above 1.0 to keep inverters working near full output for more hours of the day.
What it means
Because arrays rarely reach nameplate DC output, matching inverter capacity one-to-one wastes inverter capability for most of the year. Designers therefore oversize the DC array: ratios of 1.2–1.35 are standard for fixed-tilt utility plants, 1.1–1.25 for trackers (whose output profile is flatter and peaks longer), and 1.15–1.30 for rooftops. The tradeoff is clipping — energy discarded when DC output exceeds what the inverter can convert. Optimal ratios have crept upward as module prices fell relative to inverter and interconnection costs, since discarding a few percent of cheap DC energy is now often cheaper than buying more inverter and grid capacity. Storage changes the calculus again, because clipped energy can be captured rather than lost.
Worth knowing
- Ratios above 1.0 are deliberate design, not oversight.
- Trackers justify lower ratios than fixed-tilt because their output profile is flatter.
- Falling module prices have pushed optimal ratios upward.
- Pairing with storage converts clipped energy into stored energy.
How it's calculated
DC/AC ratio = installed DC capacity (kWp) ÷ inverter AC rating (kW)- DC capacity — sum of module nameplate power at STC
- AC rating — inverter continuous output at site temperature, not its headline rating
Field notes
- 1Derate the inverter's AC rating for site ambient temperature before computing the ratio — nameplate is often quoted at 25°C.
- 2Where interconnection capacity is the binding constraint, a higher ratio raises annual energy within a fixed export limit.
- 3Model clipping hourly; annual averages hide the summer midday losses that matter.
Common mistakes
- Copying a ratio from another project without checking tracker type, latitude and export limit.
- Exceeding the inverter's maximum DC input current or voltage window while chasing a higher ratio.
Frequently asked
What is a good DC to AC ratio for solar?
Typically 1.20–1.35 for fixed-tilt utility plants, 1.10–1.25 for single-axis trackers and 1.15–1.30 for commercial rooftops. The optimum depends on latitude, module cost, inverter cost and any export limit.
Why do solar arrays oversize DC relative to AC?
Because arrays only briefly reach nameplate output. Oversizing keeps inverters near full load for more hours, raising annual energy and reducing cost per MWh, at the price of a small amount of clipped energy at midday peaks.
Sources
Related terms
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