Average toilet flush is 5 litres which weighs about 5kg. The amount of potential energy depends on the height difference between the source and the turbine. If it’s right in the toilet you maybe have a meter of height, so you could potentially generate 5 kg * 9.81m/s^2 * 1 m = 49 Joules of energy from a single toilet flush. The average house uses about 1000kWH of energy every month, which is 3.6 billion joules. If you could capture the energy with 100% efficiency you would need about 73.5 million toilet flushes to recuperate the amount of energy for one household in a month. If each toilet is used 10 times a day you would need 7.35 million of these devices. If they cost $1 each this would be a $7.35 million dollar project. If a kWh is 25cents, the average monthly power bill for a house is roughly $250, which means in order to see a return on this investment in terms of energy costs these devices would have to work without maintenance for about 294 thousand years. You can gain more energy with a larger height difference, so if you used a turbine further downstream, say 100m down, it would generate more energy… For 100m it would take 2940 years. This is not factoring in the costs to build and ship these devices, and naturally such devices would probably cost more than a dollar and break down and they would also not recover 100% of the energy (maybe 30% if you’re lucky!)