[dojomouse]

It wouldn’t help with kinetic energy harvesting from the raindrops as that would go into the funnel as heat.

It might provide a way to harvest the remaining gravitational potential energy of the rain (possible funnel being your roof and guttering) but the only upside is that you could concentrate the energy with something that’s already there (and hence harvest over a smaller area). The amount of energy (and hence value) available would be even lower - unless you had a really high roof.

This is also the reason I abandoned my high school scheme of hydro turbines at the bottom of downpipes.

As the comments below say - you need to be working at the scale of a few major geographic features as a funnel before it starts to get really interesting.