According to the thread about this on reddit (r/programming), the electrical costs are for the servers.
This is one of those essential costs that’s required when developing free/open source software that is meant for servers or for particular hardware configurations. This is also required for mobile web app development, to ensure that the interface works on all sorts of smartphones. Donating to a free/open source project helps a lot with the essential costs. Donations may not be able to pay for more full-time development work but they can at least keep the electricity flowing to the hardware and keep the servers running!