Internet Exchange Points (IXPs) operate from colocation facilities that draw power from regional grids and store limited backup fuel. In April 2025, the Iberian blackout kept the Spanish grid down for 16 hours, beyond the 12 hours of fuel required by the industry standard, while traffic at major Iberian IXPs fell by up to 70%. Yet operators cannot tell which facilities a grid event cuts off, whether it outlasts their backup, or where traffic from a failed IXP goes. We present SPARC, which maps power-grid and Internet dependencies from substations to IXPs and their traffic. Using open data validated against transmission operators, SPARC links 552 facilities hosting 275 IXPs to the substations that likely feed them. We spread grid events over this map and measure failover through controlled BGP withdrawals on the PEERING testbed. Disabling the most-shared 16% of facility-feeding substations cuts power to half of all IXP presences (an IXP at one facility), under half of what randomized maps require. Losing the 5% most central substations cuts power to 0.1% of IXP presences directly but to 9-52% once the grid splits into disconnected islands. Traffic often fails over to nearby sites that share the failing grid: a cascade affecting four metropolitan areas sends up to 40% of one area's monitored addresses to another of the four. We introduce SPARC-Shield to select substations to harden or facilities whose backup to extend. Against an attacker disabling ten substations, hardening the ten selected by SPARC-Shield reduces offline IXP presence by 72%, compared with 52% for the best standard ranking.

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