Vulcano Fossa's fumarole field (Italy) has been active for more than a century and has become a well-studied benchmark for fumarolic degassing, often being considered the "model" hydrothermal system. Satellite thermal monitoring is increasingly being used to monitor such systems, so we here use Vulcano to test a new method for assessing heat flux at such systems. Our methodology involves converting ground-based vent temperature measurements to heat fluxes emitted by the fumaroles, with the diffuse heat flux obtained from satellite-sensor (in our case Advanced Spaceborne Thermal Emission and Reflection Radiometer) data. While diffuse heat losses were typically 9 MW, vent heat losses were 1 MW. The average total flux of 10 MW over the 19-year period of study places Vulcano in the top 20 most active hydrothermal systems globally. This work highlights the value of high spatial resolution infrared satellite data in building thermal inventories for persistently active hydrothermal systems. Plain Language Summary Hydrothermal systems are the most ubiquitous and strongest emitters of heat by volcanoes worldwide. But these emissions are very poorly constrained; indeed, they are very difficult to measure so that we have data for only a few of the many active systems worldwide. We here present a method to obtain this value for all global hydrothermal systems. By way of case study, we apply the method to the Vulcano Fossa hydrothermal system in Italy so as to obtain an inventory over the last 20 years (with the last data point coming from May 2019) at this frequently visited tourist site. Our approach represents a globally applicable heat flux model that can be used to track the thermal evolution of hazardous volcanic systems on a time scales of tens of years.