Ah. This writeup https://gigazine.net/gsc_news/en/20240604-voyager-1-photons-... says that the 70m dish collects 240,000 photons per second, which is about 60 per second per square meter. The 1/sec figure was from the recent outage when Voyager's transmitter glitched and fell back to S-band. Received energy per bit is 4.5e-21 Joule across the whole dish, or 1e-24J per square meter.
