I am not sure which fuel cell he has (did not listen to this one yet), but probably a direct methanol fuel cell such as Efoy. They run with a mixture of ethanol and distilled water. Due to the much higher electrical efficiency of fuel cells compared to marine diesel engine (running on low partial load when charging), the needed fuel quantity is relatively small. According to Efoy specification, you get 1 kWh (around 77 Ah at 13 V) out of 0.9 litres of fuel (methanol-water mix). So with for example 50 litres of fuel onboard in addition to usual solar arrays, you can run in silent mode for a very long time... And the good thing is that the fuel is not flammable as it's a mix of water and methanol so you can store it in a similar fashion as you store your sundowners - although separately as methanol is poisonous if ingested!https://www.my-efoy.com/wp-content/uploads/sites/8/datasheet-efoy-80-150-en.pdf