 This study shows that annual average carbon accounting can overestimate or underestimate carbon inventories by up to 35% in certain settings and result in effectively no bias in others, with greater bias in regions with high variation in carbon intensity and for end users with high variation in electricity consumption. The authors recommend adopting hourly accounting as the best practice for emissions inventories of consumed electricity. This article was offered by Gregory J. Miller, Kevin Novan, and Alan Jem.