Following the blackout on April 28 that cut nearly 60 percent of Spain’s electricity in five seconds, TRG Datacenters ranked the countries best prepared to survive a major electricity outage — and which might be left in the dark.

The result of this study is the Energy Independence and Resilience Index, which analyzed 79 countries using five key energy metrics. The index gives the highest weight to three core factors: Electricity production from hydroelectric sources (because hydro stations can return to full output within five minutes after a blackout, faster than any other power source), high reliance on alternative and nuclear energy, and low fossil fuel energy consumption. Other indicators such as energy efficiency and domestic production versus imports also were considered but weighted less heavily in this context of blackout survivability. Here are the 10 countries that are most prepared to handle a large-scale outage and why, according to the index.