SolarExact

How we calculate

This page answers the question most calculators leave open: where do the numbers come from, and how reliable are they?

Every number has a provenance

Every value in our calculators belongs to exactly one of three classes. The class is shown with each calculation step.

exact
Mathematically or definitionally exact. For example: 1 Wh is 3600 J. There is no uncertainty here.
measured
Measured or published, with a named source and retrieval date. For example: the resistivity of copper per IEC 60228.
assumed
An assumption. It is visibly marked in the calculator, editable, and shown with a range. When an assumption dominates the result, the calculator says so.

The calculation path is fully disclosed

Every calculator page discloses the full calculation path: every formula, every assumption, every intermediate value with its provenance and source including retrieval date — plus cross-validation against independent references. You can re-trace every number with a pocket calculator. Verifiability is a stronger trust signal than any badge.

Every formula has limits

Each calculator states what its formula is valid for and what it explicitly does not model. A model without a stated validity range is not a statement but a claim.

Data has an expiry date

Formulas never change; data changes constantly. Every dataset carries a validity date. When it lapses our build fails and someone has to look at the value. Every change is logged publicly.

  • Irradiance and weather data: Photovoltaic Geographical Information System (PVGIS), © European Union, 2005–2023 — hourly SARAH3/ERA5 series, retrieved once and versioned; no live API calls.
  • Location database: GeoNames (geonames.org), licensed CC BY 4.0 — filtered to Germany, Austria, and Switzerland.
  • Grid emission factors: German Environment Agency (UBA), annual publication.

Changelog

Technical review

14 calculators are safety-relevant — an error in them can lead to overload, overvoltage or fire. They carry a visible review note with name, qualification and date. Where review is still outstanding, we say so rather than hiding it.

What we do not do

We do not reproduce standards tables, we do not dress up sales recommendations as results, and we do not guess. When a required input is missing, the calculator refuses and explains why. That is inconvenient and intentional.

Currently 62 of 62 planned calculators are published.