WhatsApp web interface in Safari reloads because it is using "significant energy" how to fix?
I believe the easiest way to fix this is to use a different browser. Both Firefox and Chrome shouldn't reload web.whatsapp.com or any other website without asking.
I do not have the latest Safari, but I dug through the latest Safari user guide and it seems there are no options in Safari's preferences to disable reloads due to significant energy consumption.
If you still really want to use Safari you might be able to find an extension that will stop these reload events, or you might be able to file a bug with WhatsApp and hopefully they will fix it soon. I do not recommend manually finding and blocking the guilty scripts from web.whatsapp.com. From my web development experience I can assure you that this will not be an easy task.