DC/AC ratio (also called the inverter loading ratio) is the array's DC nameplate capacity divided by the inverter fleet's AC output capacity. A 125 MWp DC array paired with 100 MW of inverter AC capacity has a DC/AC ratio of 1.25 — the array can produce up to 25% more DC power than the inverters can convert to AC at any single instant.
Why oversize the DC side at all?
An inverter fleet sized exactly to the array's STC nameplate rarely runs at full output — irradiance is at its STC reference level (1000 W/m²) only briefly around solar noon on clear days. For most of the day, output is well below that level: mornings, evenings, and any cloud or haze. Oversizing the DC array relative to the inverters raises energy harvest during all of those lower-irradiance hours for a given inverter investment, because there's more DC capacity available to fill the inverter's headroom when irradiance is below peak.
What clipping actually is
When the array's instantaneous DC output exceeds what the inverter fleet can convert, the inverters cap their output at their rated AC capacity — the DC power above that ceiling isn't converted for that moment; it's lost, a phenomenon called clipping. It typically happens for limited periods around solar noon on clear, high-irradiance days, not continuously through the day.
The energy lost to clipping over a year is small relative to the extra energy harvested during lower-irradiance hours — that's the entire premise behind deliberately oversizing DC relative to AC. The exact trade-off point is a quantitative question, not a rule of thumb.
Why there is no single ideal ratio
The right DC/AC ratio for a given project depends on factors that vary project to project:
- Irradiance profile — a site with a flatter daily irradiance curve (more diffuse light, more cloud) benefits differently from oversizing than a site with a sharp, clear-sky peak.
- Array orientation and tracking — single-axis tracking spreads peak output across more hours of the day than fixed-tilt, changing how much clipping a given ratio produces.
- Inverter clipping behaviour — how gracefully (and safely) a given inverter model handles sustained operation at its power ceiling varies by manufacturer and model.
- Energy pricing and revenue structure — a flat feed-in tariff values every clipped kWh differently than a time-of-use or merchant price signal that pays more at certain hours.
- Curtailment exposure — if the grid connection already curtails output at times, additional DC oversizing interacts with that constraint rather than existing independently of it.
- Project economics — module cost relative to inverter/BOS cost shifts where the financial optimum sits, and that ratio has moved substantially over the years as module prices have fallen.
Because of this, quoting a single “ideal” DC/AC ratio without the underlying irradiance, orientation, tariff and cost assumptions is close to meaningless — two projects with different assumptions can both be correctly optimised at different ratios.
Confirming the right ratio for a specific project needs hourly production simulation against real site irradiance data and the project's actual revenue structure — well beyond what a DC/AC ratio calculator alone can tell you. Use the ratio and loading figures as an input to that analysis, not a conclusion from it.
Once a DC/AC ratio is chosen, checking each inverter's DC input loading against its own manufacturer limits is a separate, necessary step — covered by the DC/AC ratio calculator itself.