One number, and no year that delivers it

Every solar proposal rests on an annual yield figure, and it is almost always quoted as a single number: so many kilowatt-hours per kilowatt, per year. The atmosphere does not work that way. The same site, the same panels, the same tilt will produce several per cent more in one year and several per cent less in the next, and the difference has a driver: the ocean and atmosphere cycling through modes that redistribute cloud and rainfall across entire hemispheres. The three this report tracks are the El Nino-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM). This report is about the swing they produce, and about the one number on a proposal that is designed to price it: P90.

The engine: El Nino and La Nina

ENSO dominates the year-to-year variability in this record. Measured by the Oceanic Nino Index, it swings between warm El Nino and cool La Nina phases every few years, and the twenty-one-year record shows just how uneven those swings are:

Selected years from the 21-year climate-signals record (2005-2025); full annual table in the public dataset. ENSO peak is the strongest three-month ONI of the season; IOD peak is the September-November DMI; SAM is the annual mean. Across the record, ENSO peak correlates with global temperature (r = 0.42) and with the global precipitation total (r = 0.49): warm phase runs warmer and, on the global mean, wetter.

The strong El Nino years cluster at the warm, globally wet end (2015-16, 2023-24); the sustained La Nina of 2020-22 sits at the dry end, with the global precipitation total at 1007 mm in 2021 against the record's wettest year, 1059 mm in 2024. But the global mean hides the real story, because these modes do not add rain everywhere. They move it. An El Nino that floods the coast of Peru is the same El Nino that dries out eastern Australia and Indonesia, and a positive Indian Ocean Dipole, at its 2019 record, drives drought and the catastrophic fire season that followed into south-east Australia while soaking east Africa.

The planet is calm; your site is not

Averaged over the whole planet, the annual precipitation total varies by only about 1.3 per cent year to year, which makes the global climate look almost steady. Any individual site is far less steady, because the oscillations move cloud around: where one hemisphere clears, another clouds over. For solar, what matters is the variability of annual irradiance at a given address, and the published solar-resource literature measures it as a coefficient of variation that depends strongly on climate type:

Representative interannual variability of annual GHI from the published solar-resource literature (SolarGIS, Vignola, Meteonorm), not computed from the per-site baseline, which is a multi-year climatology rather than a per-site time series. P90 as a share of P50 uses the normal-distribution approximation P90 = P50 x (1 - 1.28 x CoV).

P90, and what it protects

A proper resource assessment does not quote one number; it quotes a distribution. P50 is the median expectation: half of years come in above it, half below. P90 is the level that nine years in ten will exceed, and it is the number a lender or a careful buyer plans against, because it answers the question a single figure cannot: how bad can a normal bad year be? The arithmetic follows directly from the variability above. In hyper-arid Atacama, where annual irradiance varies by about 2.5 per cent, P90 sits near 97 per cent of P50 and a bad year is barely felt. In monsoon Jodhpur, where it varies by about 8 per cent, P90 falls to roughly 90 per cent of P50, so a one-in-ten year delivers a tenth less energy than the headline figure, every projection built on the median overstates the hard years, and a battery or a loan sized to P50 is undersized for the year it matters. The chain is short: proposals almost always quote one number, that number is P50 unless the seller can say otherwise, and in the swingiest climates P90 sits roughly 10 per cent below it. The standard single-number proposal overstates the one-in-ten-year output by up to 10 per cent, and everything sized from it inherits the gap.

For buyers, the checks are simple. Ask whether the yield figure on a proposal is P50 or P90; if the salesperson does not know, it is P50, and in a variable-climate market that runs optimistic by several per cent in exactly the years the margin exists for. The margin scales with the climate type: a desert site can be planned close to its median, while a monsoon or maritime site needs a P90 buffer several times larger. A weak year reads against its driver before it reads against the equipment, because a La Nina or a positive-dipole year can take a real bite out of a perfectly healthy system. And these are swings around a stable mean, not a trend: unlike the brightening and dimming of part 02, ENSO and its siblings oscillate, so a run of strong years is not a new normal and a run of weak ones is not a failing array.

Twenty-one years of three climate drivers

The driver record is twenty-one years (2005-2025) of published indices: NOAA's Oceanic Nino Index for ENSO, the Bureau of Meteorology's Dipole Mode Index for the IOD, and the Marshall Southern Annular Mode index, joined to NASA GISTEMP global temperature and GPCP global precipitation by year. The correlations here are global-mean relationships (ENSO to temperature and to total precipitation); the solar-relevant effect is regional and teleconnected, and a market-by-market ENSO-sensitivity map is a larger piece of work than this pillar contains. The P90 coefficients of variation are representative literature bands by climate type rather than figures computed from the per-site series, because the yield baseline is a climatology; they are the right order of magnitude for planning, and a site-specific assessment should use a site-specific number. And twenty-one years spans only a handful of full ENSO cycles, enough to characterise the oscillation but not to pin down its rarest extremes. Every index is public and named to its source.