Two approaches to renewable energy accounting. Annual matching means purchasing renewable energy certificates (RECs) equal to total annual consumption—but the renewable generation may occur at different times than consumption. Hourly (or 24/7) matching requires renewable generation to actually occur during each hour of consumption. Annual matching is more common but criticized as "greenwashing" because it doesn't guarantee carbon-free operations in real-time.
Discussed in Chapter 6 of This Is Server Country