The Ender 3 came in at 322W when starting up & 60W when printing PLA with a 50C bed & 230C nozzle. The power supply was only rated for 300W. When heating the bed & nozzle simultaneously, it dumps the full output & probably browns out.
This brings up the question of how practical a 300x300mm bed is. Even if the print is 10x10mm, the entire bed has to be heated. So if you're paying for electricity instead of using solar panels, you'd need a smaller printer for smaller prints & a bigger printer for bigger prints. If electricity is unlimited, you could use a bigger printer full time.
Printing TPU with a 260C nozzle & no bed heating burns 65W. 10 hours of printing burned 0.650kW hours.
The idle printer burns 7W. With the motors on, it burns 22W. Motors + 260C nozzle heating burns 65W. Motors + 260C nozzle + 65C bed heating burns 330W as the worst case PETG setting. Once at steady state, it oscillates between 65W & 300W with the worst case PETG temperature settings. 300W seems to happen when the bed is heating. The nozzle heating seems to be proportional while the bed heating is binary & sucks most of the power.