A dedicated power meter that shows instantaneous power draw to the nearest .1W. I ran the bed and nozzle tests separately and did not see the bed heat draw much more than 250W. You’re right that the printer heats the bed first and then the nozzle. As mentioned in this thread, the bed is heated directly by the AC input and thus doesn’t tax the power supply.
I’ll repeat the test tomorrow and see what I can find. I have three different power meters, so it will be interesting to see how much they agree.