the electricity was going to be consumed regardless whether you ask chatGPT or not.
It would have been either idle, or serving other users' requests.
so the incremental kWh consumption is zero, since costs are fixed and sunk.
as a rule of thumb you can lookup the power consumption of the latest nVidia chip, multiply by factor of two or three (to account for cpu/storage/cooling/network/infra)
OpenAI GPU wont be idle for long because they have all other requests to serve. Over time there will be a certain % of idle GPUs, amortized across all hundreds of millions of requests they receive.