tldr: If you are using Google Chrome or Mozilla Firefox, you are contributing to the world’s wasteful use of energy that’s equivalent to the annual power usage of an entire country of Hungary, Ecuador, or Azerbaijan. Switch to Microsoft Edge (Windows) or Apple Safari (Mac) to reduce the world’s carbon footprint.
Based on this report https://wearesocial.com/special-reports/digital-in-2017-global-overview there are almost 3.8 billion of internet users, with around 45% of them are computers. That means there are 1.7 billions of computers online in the world
Based on this report https://www.netmarketshare.com/browser-market-share.aspx, 59% of those are using Google Chrome to browse the internet, followed by 13% on Firefox. Therefore, there are 1 billion people using Chrome and 221 million people using Firefox on their computer.
Based on this testing https://microsoftedge.github.io/videotest/2017-12/, it is proven that Google Chrome uses 30% more energy to finish the same task than Microsoft Edge, while Firefox manages a whooping 70% in the same scenario.
The computer that was used in testing was a Microsoft Surface Book, which based on this specification sheet https://www.notebookcheck.com/Test-Microsoft-Surface-Book-Core-i5-940M-Convertible.153423.0.html, has 78 Wh battery. From the previous report, it was reported that Microsoft Edge lasted 16:41, Google Chrome lasted 12:54, and Mozilla Firefox lasted 9:51 on the very same computer. That means, Edge uses 4.6 W on average, Chrome uses 6.1 W, and Firefox uses 7.9 W for doing the same controlled task. There is a gap of 1.5 W between Edge and Chrome, and 3.3 W between Firefox and Edge.
Important to understand that those numbers were all acquired using the efficient low-voltage mobile chip (Intel Core i5-6300U) with 15 W rated TDP used in their reference laptop. The effect is much more apparent on older and/or higher-voltage chips. The average desktop computer chip like the Core i5-7500 is rated at 65W TDP, more than 4 times as much, and the previous generation of Core i5-2400 (2011) is rated at 95 W, or 6 times as much. For my calculation, I would assume that all computers have the efficient 15 W chip as the Surface Book. https://ark.intel.com/products/97123/Intel-Core-i5-7500-Processor-6M-Cache-up-to-3_80-GHz https://ark.intel.com/products/52207/Intel-Core-i5-2400-Processor-6M-Cache-up-to-3_40-GHz
Therefore, if all Chrome users switch to Edge, the world would save 1500 Megawatts per hour, and if all Firefox users would switch to Edge, the world would save an additional 730 Megawatts per hour. They would total to 2230 Megawatts per hour, or 19.534.800 Megawatts per year. Based on this data, it is roughly the same amount of power used to power the entire country of Hungary, Ecuador, or Azerbaijan https://en.m.wikipedia.org/wiki/List_of_countries_by_electricity_consumption
Note that this calculation was done on computers based on Microsoft Windows operating system, and the Edge browser is also only available on, and built-in, Windows. However, there is a similar trend in macOS-based computers in regards to the built-in Apple Safari browser as seen on this report here https://www.theverge.com/2015/4/10/8381447/chrome-macbook-battery-life