Ocean energetics is a useful framework for understanding El Niño–Southern Oscillation; however, its key element, available potential energy (APE), requires accurate ocean subsurface data that are hard to measure. Here, we describe a sea surface height‐based index, SSHI, that accurately captures APE variations and can be easily computed from satellite observations. Using SSHI, we obtain an observation‐based estimate of the APE damping timescale of approximately 1.7 years, slightly longer than previous ocean reanalysis‐based estimates. Furthermore, SSHI serves as an indicator for El Niño “flavors” while recording the relative strength of the thermocline feedback. SSHI captures a decadal shift in El Niño–Southern Oscillation properties in early 2000s, with a more tilted mean thermocline and weaker thermocline slope variations indicative of the “Central Pacific” El Niño type dominating the past two decades, whereas SSH averaged over the tropical Pacific (a proxy for upper‐ocean heat content) shows a significant rising trend over this time.