Performance ratio from measured yield

How a metered year becomes a verdict on the whole installation

The performance ratio is the standard yardstick of PV system health — the quantity monitoring practice, IEC 61724-style, is built around. It compares delivered energy against a reference: what the same nameplate would have yielded from the same sun if conversion were perfect. Because that reference already contains the site’s solar resource, the ratio measures the SYSTEM, not the weather — a cloudy region and a desert can both host an installation that scores well.

Within this cluster it is the closing of a loop. The sizing page divided by an assumed ratio; the yield page multiplied by the same assumption; this page derives the real one from a meter reading, using algebra the estimate would recognise — the same product of nameplate, sun hours and days, now sitting under the measured energy instead of beside an assumed factor. Setting the derived value beside the assumed one is the single most informative comparison a system owner can make.

What the ratio cannot do is say WHY. Elevated cell temperature, soiling, a string down for a month, inverter losses, creeping degradation and plain shading all pull it down through the same arithmetic, so a disappointing value is a flag, not a diagnosis. Its job is to say “look closer” cheaply and defensibly; the pack’s derate calculators — temperature, shading, soiling — are where the looking happens.

Because the sun is divided out, the ratio travels. Systems of different sizes, on different roofs, in different countries can be ranked on it directly, which raw kilowatt-hours cannot do at all and even specific yield — energy per installed kilowatt — can only do within one region’s resource. That portability is why fleet operators track it as the first-order health metric across a portfolio.

When this calculation is used

• Commissioning: the first full year’s meter reading, turned into a ratio and set against the value the design assumed.
• Year-on-year tracking, where a slow slide in the ratio is how soiling or degradation announces itself before any fault does.
• Comparing installations across different sites or sizes on one scale, since the site’s resource is already normalised out.
• Interrogating the sun figure itself: a derived ratio that comes out implausibly high or low is often the peak-sun-hours estimate confessing, not the hardware.

Worked example

Let the cluster’s reference system face its reckoning: the 5 kWp array, at the site the design rated at 5.5 peak sun hours, with the generation meter showing 8,030 kWh over the first full year of operation.

The engine returns the performance ratio the installation actually achieved. Put it directly beside the value the sizing and yield pages assumed: matching it means the system is doing what the forecast claimed, landing below it means real losses the design did not concede, and landing above it usually means the sun figure was conservative rather than the hardware miraculous.

The verdict is only as honest as the reference. Overstate the peak sun hours and the derived ratio falls even though the system did nothing wrong; understate them and a mediocre installation grades itself excellent. When the derived value surprises, audit the sun input before blaming the roof.

What each input represents

Assumptions and limits

• The measurement spans a complete year. A partial year inherits whichever seasons it happened to contain, and no correction for that bias is applied here.
• The peak-sun-hours input represents the resource of the measured period. Using a long-term average during an unusually dull or bright year moves the ratio for reasons that have nothing to do with the system.
• The nameplate is taken at face value; module rating tolerance and early light-induced degradation fold silently into the derived ratio.
• One number aggregates every loss mechanism. Nothing here distinguishes soiling from shading from downtime — that separation belongs to the pack’s dedicated derate calculators.

What the guards protect against

• Peak sun hours below the physical band are refused — the pack ships that refusal as a declared test vector. A sub-unity figure is almost always an irradiance value in the wrong units, and dividing by it would manufacture a spectacular ratio out of a unit mistake.
• The measured energy and the array size must both be positive: a zero in either leaves nothing to audit, and the ratio is refused as a domain error rather than returned as zero or infinity.
• Upper bounds on the measured energy catch a lifetime totaliser reading entered where a single year belongs — an inflated ratio from that slip would read as a healthy system rather than as the data error it is.

Provenance

Performance ratio definition, PR = measured annual kWh / (kWp × peak sun hours × 365)

The measured annual energy is divided by the reference yield — nameplate array size times daily peak sun hours times 365 days — returning the dimensionless performance ratio the system achieved over the measured year.

Screening and reference material, to be checked against the governing standard and a qualified engineer; not a design determination. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.