In my experience it depends somewhat on the sites/web apps one uses (some are not well optimized for WebKit), but Safari definitely tends to be easier on the battery. It seems to try harder to get to an idle state and keep CPU usage down where Chrome and Firefox are happy to keep the CPU spun up, perhaps due to a “speed and bells and whistles at all costs” mentality in development (traditionally browsers have been marketed on speed and features rather than efficiency).
And I hope Apple doesn't manipulate the "power usage" data that always claims Safari is the lowest-power best-battery-saver browser on their platform.
I run Safari on MacOS for only that reason, that it allegedly gives me an extra hour or so out of my laptop vs. Chrome or Firefox. Of course, I should actually benchmark myself and find out.