Glossary
TMY (typical meteorological year)
A TMY is a synthetic twelve-month weather dataset assembled from many real years, chosen so each month is typical rather than extreme. Solar simulations run against a TMY to produce a representative annual yield instead of replaying one unusually good or bad year.
A TMY is stitched together month by month from a long historical record, so January might come from one year and February from another. The result is a year that never happened but represents the site well.
TMY files are a purchased input for most design tools, which is why weather and irradiance data is its own category in this dataset. PVsyst alone names seven sources — Meteonorm, NASA-Power, PVGIS, NLR/NSRDB, Solcast, SolarAnywhere and Solargis.
Resolution and history depth vary sharply between providers and tiers, and both affect the answer.
Why it matters
The weather source is an assumption inside every yield number you will be shown. Knowing which one is being used is part of reading the estimate.
Products where this is recorded
- Meteonorm — Weather, tariff & utility data
- Solcast — Weather, tariff & utility data
- SolarAnywhere — Weather, tariff & utility data
- PVGIS — Design & simulation