Everything I've read suggests the Visa network only processes ~5-10k transactions per second, which you could do on a single modest server. It is apparently built to handle up to 75k TPS[0]. Certainly it should be doable with less than 300 kW average.
Visa the company has over 20,000 employees. Presumably that network wouldn't function without them. Shouldn't the entire company's energy footprint be included in that 300kW?
[0] https://usa.visa.com/dam/VCOM/global/about-visa/documents/vi...