Estimate solar energy production for a site using the NLR (formerly NREL) PVWatts API

Steps

1. Obtain a free API key by registering at developer.nlr.gov; the key is passed as the 'api_key' query parameter. (The former developer.nrel.gov domain was retired May 29, 2026 with no redirect — only the hostname changed, existing keys continue to work.)
2. Make a GET request to https://developer.nlr.gov/api/pvwatts/v8.json with the required parameters: api_key, system_capacity (kW DC), module_type, losses, array_type, tilt, azimuth, and either lat/lon or a file_id identifying a specific climate data file — none of these have a default value, all must be explicitly supplied.
3. Review the 'outputs' object in the JSON response; key fields include 'ac_annual' (annual AC energy in kWh), 'ac_monthly' (array of 12 monthly values), and 'solrad_annual' (average daily solar radiation).
4. Set 'losses' (system losses, percent) to reflect site-specific factors like soiling, shading, or wiring losses; the API has no built-in default for this value, so you must pass an explicit number — this parameter significantly affects modeled output.
5. For time-series hourly output, add the 'timeframe=hourly' parameter; the response will include an 'ac' array of 8760 hourly AC power values in watts, along with matching hourly poa, dc, dn, df, tamb, tcell, wspd, and alb arrays.