You would need at minimum 4 kw just to stay in the air and for people like me who live at high elevations and like to climb mountains, I say that 10 kw would be the absolute minimum consistent power that I would need with 15 kw peak for a few minutes at a time.
Here are the spec sheets from Intelligent Energy that I am basing my response on:
To get over 10kw power that I would need from the Intelligent Energy fuel cells, It would take five of the 2.4 kw fuel cells coupled with two 9 liter hydrogen tanks which would push the 5 gallon legal fuel limit for us in the USA. The total weight of the ESC, motor, hydrogen, and fuel cell would be a hair under 80 pounds. So for a unit that fulfills my needs, the total weight would be upwards of 100 pounds and only have 6.2 kwh energy for about 1.5 hour flight times. The really cool part is that you only need .8 pounds or 372 grams of hydrogen to get a 1.5 hour flight, but it takes 18.9 pounds in tanks to store the hydrogen, between 19.4 and 48.6 lbs depending on power requirements to turn the hydrogen into electricity, and another 10 pounds to turn the electricity into usable mechanical power. To top it all off, I am almost willing to bet that the hydrogen fuel cells and tanks will cost over $10k even if you only have a 4.8 kw setup.
My consensus is this: I do believe that we will someday see fuel cell powered paramotors with far better performance than ICE powered paramotors, But the technology is not developed to where it needs to be and to get over 1.5 hour flights in the USA, we will need rule changes to allow for more than 5 gallons of fuel.
Methanol may be a decent alternative to hydrogen as it is far more dense so we wouldn´t legally be nearly as limited by the energy of fuel we can carry, But if they are anything close to hydrogen fuel cells in performance, they would still need more development.