small tubes or large tubes, the energy is proportional to the vertical drop. that, plus the quantity of water, are the only factors that apply.
If the area of your roof is, for example, 100 m² and it rains an inch an hour, which is a lot, in that hour you will accumulate 100m² x 0.025m = 2.5 m³
which is 2500 liters, which is 2500 kg.
The energy in that, for a 15 foot (5 meter) drop is E = mgh = 125000 J or 125 kJ
there are 3600 sec in an hour, and power = joules/sec
so that is an average power of 125000/3600 = 35 watts
think of it. In a very heavy rainstorm, you could generate 35 watts at best. Only while the rain lasted. What is that, an hour a month.?
35 watts for an hour is 35 watt-hour, or 0.035 kW-hour.
at 20 cents per kW-hour, that costs 0.7 cents.
so you are going to spend a lot of money to get a system that can save you 0.7 cents a month? I don't think so. Even if my estimates are off by a factor of 10, that's still only 7 cents a month.