How yield is calculated

The energy calculation turns one irradiance figure and one performance ratio into an annual generation number, then degrades that number across the plant lifetime and reports it at three exceedance probabilities. Every step is a single explicit relation, and every input to those relations is either a field you can see or a value fetched and displayed on screen. Nothing in the chain is hidden, which is what makes the output defensible.

This page is the whole chain in order. The sub-pages hold the detail behind each link in it.

What the calculation needs before it can run

Two things, and it will not produce a yield without either.

A generated layout. Plant capacity in kWp is the placed module count multiplied by the module wattage, so it does not exist until placement has run. See Your first layout. Everything else in the chain is a per-kWp figure that only becomes energy when it is multiplied by that capacity.

A site position. Latitude and longitude decide the solar geometry, the irradiance the weather source returns, the automatic tilt and the automatic row pitch. A Google Earth boundary file carries the position inside it. A CAD drawing and a raster image do not.

The controls

Three controls sit together in one block, and they become available once a layout exists.

The interval selector offers 1 / 10 / 15 / 30 / 60 min and ships at 15 min. TMY is a typical meteorological year — a single representative year rather than a specific calendar year. The export needs an hourly series to work from, which means either a loaded hourly file or an hourly fetch from the automatic service; an annual total on its own is not enough.

The chain, in order

1. In-plane irradiance

The calculation starts from GTI — global tilted irradiance, the annual irradiation arriving on the plane of the modules, in kWh/m²/yr. It is not the same quantity as GHI, global horizontal irradiance, which is what most weather records contain. GTI is the one that drives the yield.

Where GTI comes from depends on your weather source: the automatic service returns it directly for your position, tilt and orientation, and an hourly file either carries it or has it derived from the horizontal figure. See Weather data.

2. Performance ratio

PR, the performance ratio, is the fraction of the energy a plant would produce if every module ran at its nameplate efficiency that it actually delivers to the grid. It is a dimensionless number below 1, and it carries every loss between the light hitting the glass and the energy leaving the plant: inverter conversion, cable resistance, soiling, module temperature, mismatch, shading, availability, transformer and the rest.

You do not enter PR. You enter the components, and PR follows from them — the components are combined multiplicatively, as a product of the factors and of one minus each loss, not as one minus their sum. The full relation, component by component, is on The loss breakdown.

3. Specific yield

Specific yield = GTI (kWh/m²/yr) × PR

Specific yield is annual energy per unit of installed DC capacity, in kWh/kWp. It is the figure to compare two designs on the same site with, because it is independent of how big the plant is.

4. Year-one energy, before first-year degradation

Year 1 energy = capacity_kWp × specific yield

capacity_kWp is the plant's installed DC capacity — the DC(MWp) column of the summary table, in kWp. The product is in kWh; the summary reports MWh.

This figure is the plant before any degradation has been applied. It is not a number you will see reported on its own; it is the base the next two relations work from.

5. First year, after first-year degradation

Year 1 (actual) = Year 1 × (1 − first-year degradation %)

1st year degradation ships at 1.0 % and accepts 0–10 %. It stands for the loss a module takes in its first year of exposure that it never recovers — principally LID, light-induced degradation. It is separated from the annual figure because it is a one-off step, not a rate.

Year 1 (actual) is the year-one energy every reported figure is built from: the three probability columns, the capacity factor, and the lifetime total.

6. Every year after the first

Year n (n ≥ 2) = Year 1 (actual) × (1 − annual degradation %)^(n−1)

Annual degradation ships at 0.4 %/yr and accepts 0–5 %/yr. It compounds: the exponent is n − 1, so year 2 has taken one year of degradation and the final year of a 30-year plant has taken 29.

Plant lifetime ships at 30 years and accepts 1–50. It sets how far the series runs.

7. Capacity factor

CUF = Year 1 (actual) / (capacity_kWp × 8760) × 100 %

CUF, the capacity utilisation factor, is year-one energy expressed as a percentage of what the plant would generate if it ran at full DC nameplate output for every hour of the year. 8760 is the number of hours in a year.

Two things follow from the relation, and both matter when someone challenges the figure. It is computed on year one after degradation, not on the before-degradation figure, and it is computed against DC capacity, not AC. A capacity factor quoted against AC capacity on the same plant is a larger number and is not this number.

Degradation, worked from the shipped defaults

The degradation part of the chain can be worked end to end from defaults alone, so it is worth seeing once. The table below is the output multiplier relative to the before-degradation year-one energy, at 1.0 % first-year degradation and 0.4 %/yr annual degradation.

The last row is the one a lender's model usually wants: on the shipped defaults, the final year of a 30-year plant generates about 88.1 % of what year one would have generated before any degradation at all.

The first three steps of the chain cannot be worked the same way, and it is worth being clear about why rather than picking plausible numbers. GTI (in-plane) ships at 0.0 until a weather source fills it, and capacity comes from your own placed layout — so any absolute MWh example would be an invented site, not a default. Run the chain on your own site instead.

See Lifetime and P-values for how the same series is turned into the exceedance probabilities.

Monthly figures

Alongside the annual numbers the application produces a month-by-month table following IEC 61724-1, the standard for PV system performance monitoring. Three quantities per month:

Y_r  = H_i / G_STC          reference yield, in hours
Y_f  = E_AC / P_0           final yield, in kWh/kWp
PR_m = Y_f / Y_r            monthly performance ratio

• H_i is the month's in-plane irradiation and G_STC the standard test condition irradiance, so Y_r is the number of equivalent full-sun hours in the month.
• E_AC is the month's AC energy and P_0 the installed DC capacity, so Y_f is the month's specific yield.
• PR_m is their ratio — the same definition as the annual PR, evaluated on one month.

Monthly PR is not the annual PR repeated twelve times. It moves with module temperature, and module temperature moves with the month's ambient temperature and its own operating irradiance:

G_m = monthly GTI × 1000 / (days × 8 h/day)   W/m²

Monthly ambient temperature comes from a sinusoidal seasonal model whose amplitude scales with the absolute latitude of the site — or, when your weather file carries a temperature column, from the file itself. The module temperature for the month then follows the Sandia relation on Temperature loss. A hot low-latitude site therefore shows its lowest monthly PR in the hottest months, which is the behaviour a reviewer expects to see.

Where the results appear

Five columns at the right-hand end of the Layout & Energy Summary table carry the energy result, one row per plant plus a TOTAL row:

The three probability headers are built from your own settings. They read P50 / P75 / P90 because Exceedance prob. 1, 2 and 3 ship at 50.0, 75.0 and 90.0 %. Change any of them and the header changes with it. Two reports are therefore only comparable column for column if both were run with the same three probabilities — check the headers, not the position.

The two lower probabilities are derived from the first through the combined uncertainty:

P75 = P50 × (1 − 0.674 × σ)
P90 = P50 × (1 − 1.282 × σ)

σ is Combined uncertainty (1σ), which ships at 5.0 % and accepts 0.1–30.0 %. The multipliers are the standard normal deviates at those exceedance levels.

The full column list, including everything to the left of these five, is on Summary columns. The monthly IEC 61724-1 table and the multi-year forecast are written to the energy pages of the exported report — see PDF report.

What this model is and is not

State these plainly to anyone reviewing the output, because they are the questions a careful reviewer asks.

It is a yield estimate from a loss model, not an hourly plant simulation. The annual result is an annual irradiation total multiplied by a single performance ratio and a capacity. Hourly data is used — for the transposition from horizontal to in-plane irradiance, for the shading loss, and for the charts and the export — but the energy figure is not an hour-by-hour simulation of module, inverter and transformer behaviour, and it does not replace one.

Row-to-row shading is modelled; terrain and horizon shading are not. The near-shading loss is computed from your own row geometry, and tables falling inside the year-round shadow of a structure you place are cleared from the layout. Distant horizon profiles and shading from the site's own relief are not part of the model. See Shading loss.

The result is only as good as the weather data behind it. Every number on this page is downstream of one irradiance figure. A monthly climatology average and a purchased site-specific typical meteorological year will not give the same answer, and the difference between them is larger than most of the loss components you might argue about. Check which source produced your irradiance before quoting the yield — the application reports it on screen.

The energy pages

Where to go next