Since National Grid (US) recently changed their API endpoints and no longer works with the core Opower integration, I decided to figure out how the API endpoints work and create both an API library (aionatgrid) and a custom integration.
This is still very early and is designed to work with AMI/smart meters. For electric meters it should capture the 15-minute interval usage periods every hour. For gas meters it will capture the hourly usage periods with up to a 72 hour delay.