He seems to be mixing square feet with meters, you lose space probes that way!
So I think he has worked out the case for a 1500m2, which is huge and still not effective. Oh well
Edit:
OK, he is converting the water hitting the roof into litres, which makes sense, so he can calculate the potential energy.
1500sq ft is about 140sq m according to an online unit converter.
So 4m of rain across the roof is 4x140 = 560 m^3 of water. That is 560000 litres of water. That is 1kg per litre, so 560000kg of water.
Potential energy available is m.g.h so 560000 x 9.81 x 5 = 27468000J of energy, or 27468 kJ or 27.468MJ.
A Watt is a joule per second, so kJ = kWs. We want Wh to get normal domestic units.
1kWh is 3.6MJ, so we have 7.63kWh of power, which at the 11.5p I pay is nearly 90p a year
So his result is correct, he must be skipping some steps in the working?