Smoke: The Solar Yield Risk the Industry Has Not Priced

Smoke dims the sky, not the glass

Smoke is not soiling. Part 05 of this series covered what settles on the module; smoke attenuates sunlight in the atmosphere before it arrives, which is why its losses appear and vanish with the plume rather than accumulating. The mechanism prices almost linearly: New South Wales rooftop fleet data puts the loss at about 13% of PV energy per 100 micrograms per cubic metre of PM2.5, consistent with the 12.5% insolation loss per 100 micrograms found across urban-haze cities globally. At the concentrations major fire events now produce (Sydney's Black Summer peaked near 350, New York's 2023 Canadian-smoke day at 185, California's 2020 statewide peak at 659), that arithmetic turns severe quickly.

The measured record, restricted to energy and output figures so unlike quantities are never averaged:

From the 18-event ledger in the public dataset. Each impact applies over its own window and must not be read as an annual loss. Irradiance-basis observations (Athens 2021: 10-20% daily GHI on fire days; southern Brazil 2024: 10-20% clear-sky irradiance) and blank-by-rule rows (NYISO published megawatts, not a percentage; no confirmed Jakarta yield figure) are recorded in the dataset with their bases flagged.

The pattern across the ledger is consistent: single days lose a quarter to nearly half of output, fortnights lose around an eighth, and even the archetypal sustained event, two months of Black Summer smoke across Australia's grid, cost 9.5% of grid-scale solar. Large, but short-window. A 160-system NSW study put the whole 71-day episode at 4.2% of energy, roughly 175 GWh state-wide.

Two opposite trends, and the one that matters

The global fire record runs in two directions at once. Satellite burned area fell roughly 24 to 27% over two decades, and that decline is real; it is driven by savanna-to-cropland conversion in Africa and South America, grass fires with low smoke plumes far from any PV fleet. Meanwhile forest-fire carbon emissions rose 60% between 2001 and 2023, human fire exposure rose 40% even as area burned shrank, and Canada's 2023 season alone emitted about five times its 20-year average. Forest and boreal fires are the ones that loft dense smoke over cities and grids.

For solar, the relevant trend is the rising one, and it is regionally specific. The measured 20-year record supports calling smoke a recurring seasonal yield factor in exactly three regions: the North American West (California burned area up fivefold since the 1970s, critical fire-weather days up from 14 to 34 a year, the 2020 insolation deficit more than twice any year back to 2001), south-east Australia (forest burned area up 350% between the periods 1988-2001 and 2002-2018 before Black Summer, with AEMO now modelling smoke explicitly in its solar forecasts), and Mediterranean Europe (significant fire-weather trends across most of Iberia's burnable area, and 2023 producing the largest fire ever recorded in the EU). Southeast Asia's haze is recurring but El Nino-linked and trending down in severity; East Asia's chronic attenuation is anthropogenic aerosol, a different ledger; and for smoke-importing regions like the US Northeast, transported plumes set air-quality records without touching the long-run resource, which stays within about 5% of baseline.

What a buyer should do with this

Smoke changes three planning numbers, and leaves one alone. It does not change the resource: in every documented market the long-run mean irradiance loss from smoke is small against the year-to-year weather noise, so it is not a reason to skip solar anywhere in our baseline. It does change bad-year planning: in the three structural regions, a P90 yield estimate should carry a fire-season term, because a Black Summer costs a summer 6 to 13% of grid-scale output and a 2020-scale September cost California an eighth of a fortnight's energy. It changes storage sizing at the margin: a battery specified for multi-day autonomy earns its keep in smoke weeks, when output can run 20 to 40% under forecast for days. And it changes what to expect from forecasts: day-ahead solar forecasts overestimated Californian output by an average 27% on heavy-smoke days, and grid operators on two continents now model smoke because of it. Owners in smoke regions should also rinse panels after ash-fall events; ash that settles is soiling, and part 05's rules apply.

Method and limits

The event ledger holds 18 market-events across 12 of the 40 baseline markets, each with its window, measurement basis (grid operator, fleet, satellite model or study), confidence grade and source; ranges are recorded at midpoints and flagged. Three boundaries matter. This is an event ledger, not a climatology: annualising it requires the regional frequency evidence, and that conversion belongs to modelling, not the dataset. Irradiance-basis rows are flagged and never averaged with energy-basis rows. And absence is honest: no credible smoke-on-PV quantification exists yet for the desert-dust and Sahel markets in the baseline, whose particulate story belongs to part 05, and blank cells in the ledger state their reason rather than carrying a proxy.