solar-spa

NREL Solar Position Algorithm compiled to WebAssembly. Calculates solar zenith, azimuth, incidence angle, sunrise, sunset, solar noon, and equation of time for any location and date. The WASM binary is inlined as base64, so there is no external .wasm file to locate. It works in Node.js, browsers, Webpack, Vite, Next.js, and web workers without configuration.

Installation

npm install solar-spa

Quick Start

import { spa } from 'solar-spa';

const result = await spa(
  new Date(2025, 5, 21, 12, 0, 0), // June 21, 2025 at noon
  40.7128,                           // latitude (NYC)
  -74.0060,                          // longitude
  { timezone: -4, elevation: 10 }    // EDT (UTC-4), 10m elevation
);

console.log(result.zenith);   // ~27   (degrees from vertical)
console.log(result.sunrise);  // ~5.4  (fractional hours)
console.log(result.sunset);   // ~20.5 (fractional hours)

CommonJS also works:

const { spa } = require('solar-spa');

TypeScript

import { spa, SPA_ALL } from 'solar-spa';
import type { SpaResult, SpaOptions } from 'solar-spa';

Documentation

See the Wiki for the full API reference, architecture notes, build flags, bundler compatibility, and validation benchmarks.

Related

• nrel-spa: Pure JavaScript port of the same algorithm. Synchronous, no WASM dependency.
• pray-calc: Islamic prayer times built on nrel-spa.

Acknowledgments

This package includes the Solar Position Algorithm (SPA) developed at the National Renewable Energy Laboratory (NREL) by Ibrahim Reda and Afshin Andreas. The C source files spa.c and spa.h are copyright Alliance for Sustainable Energy, LLC (2008-2011).

Reda, I., Andreas, A. (2004). "Solar Position Algorithm for Solar Radiation Applications." Solar Energy, 76(5), 577-589.

License

MIT (wrapper, TypeScript source, and build tooling). The NREL SPA C source (src/spa.c, src/spa.h) is subject to its own terms; see the notice in those files.