Query electricity data (net generation, retail sales, prices) from the EIA using the API v2 route/facets/data hierarchy.

Verified steps

1. Register for a free API key at eia.gov/opendata/register.php; EIA emails the key automatically.
2. Browse the dataset tree by requesting a parent route without /data (e.g. GET https://api.eia.gov/v2/electricity?api_key=<API_KEY>) to discover child routes such as electricity/retail-sales.
3. Request the route itself (no /data) to see its metadata, including available facets (e.g. sectorid, stateid) and valid frequencies (e.g. monthly, quarterly, annual).
4. Query facet values directly, e.g. GET /v2/electricity/retail-sales/facet/sectorid/?api_key=<API_KEY>, to get valid facet codes (sample sectorid values: COM, RES, ALL, OTH, TRA, IND).
5. Pull data by appending /data to the route and specifying data[]=<column> (e.g. price, revenue, sales, customers) plus facets[...][]= and frequency= params, e.g. /v2/electricity/retail-sales/data?api_key=<API_KEY>&data[]=price&facets[sectorid][]=RES&facets[stateid][]=CO&frequency=monthly.
6. Use start/end, sort[0][column]/sort[0][direction], offset, and length params to page through results; response defaults to JSON (out=xml is also selectable).