The performance ratio of your solar plant, and every loss behind it
The performance ratio compares the energy a plant delivers with what its modules would produce, at their rated efficiency, from all the light they receive. pvSpline computes it on every run, per IEC 61724-1, and shows the loss cascade that explains it, stage by stage.
The formula
PR = EAC / (Pnom × HPOA / GSTC): the AC energy delivered, divided by what the installed peak power would give if it converted all the irradiation received in the plane of the modules at its rated efficiency. It has no unit, and it does not depend on the exceedance percentile: a P90 year and a P50 year have the same PR.
The figures beside it
- Specific yield, the final yield Yf: the net AC energy per installed kWp, the headline figure for a bank.
- Reference yield, Yr: the year’s irradiation in the plane of the modules, as equivalent peak-sun hours.
- Capacity factor: the net AC energy against the nameplate power running all year.
- Temperature-corrected PR, on the results.
The loss cascade behind the PR
Every loss is a line in one cascade, in a fixed order: transposition, far and near shading, the trees, angle of incidence, soiling, snow and spectrum, then the module’s conversion, low light and temperature, the rear-side gain of bifacial modules, mismatch, the DC wiring, the inverter and its clipping, the AC wiring, the transformer, availability and, over the years, ageing. A term with no energy is left out.
Where each figure comes from
Each line carries its provenance: attested by PVGIS, measured on a horizon profile or a skyline sampled in 3D, computed from the weather for soiling and snow, integrated from your cable runs for the ohmic losses or from the inverter’s efficiency curve hour by hour, or a constant you set. A reviewer can trace each loss to its source.
The loss diagram
The loss diagram draws the cascade as a staircase of bars, from the irradiance on the modules at the top to the energy at the meter at the bottom, each stage drawing only the slice it removes. The losses from PVGIS, the losses you set and the gains each have their colour, and the report prints the diagram.
Measured against simulated
Once the plant runs, load its metered production, or fetch it from the inverter’s cloud, and the twelve simulated months sit beside the real ones, with the difference month by month.
Read a project’s PR in four steps
- Run the simulation from the Production tab.
- Read Performance ratio (%) and Specific yield (kWh/kWp) at the head of the Loss cascade card.
- Open Results and read the loss diagram from top to bottom.
- Adjust the losses you control on the Loss cascade card, run again and compare.
Questions
What is a good performance ratio?
It depends on the climate and the design: heat lowers it, so the same plant reads a lower PR in a hot climate. Compare the PR of several designs for one site, or the temperature-corrected PR from one climate to another.
Does the PR change between P50 and P90?
No. The percentile applies to the energy: the specific yield and the annual production carry it in their caption, and the PR, which sets the energy against the irradiation received, stays the same.
Why do the card and the diagram show different percentages?
They describe the same cascade. A percentage on the card is a loss of the energy remaining at that stage, so an early 2% and a late 2% remove different amounts of energy, and the diagram shows each in kWh.
The fastest way to judge a simulation tool is to run it on a project you already know.