Datacenters have always kept reciprocating engines on site as emergency standby, kicking in when the grid drops and running a handful of hours a year. That role is changing, as recips are increasingly being repurposed to prime power, energizing datacenters around the clock. We estimate that recip OEMs (e.g., Caterpillar, INNIO, Cummins) have been contracted to supply ~1 GW of BTM power this year, and 4+ GW in each of 2027 and 2028. Our SemiAnalysis Energy Model tracks these BTM contracts OEM-by-OEM, project-by-project. Yet, that is dwarfed by the opportunity to come. (1/3)🧵