In 2012, the average American home consumed 10,837 kilowatt-hours per year. This works out to an average use of 0.34 watt being used per second, which means that one megawatt of energy generated could furnish power to approximately 2,941,176 homes per second.
That entire example is largely dependent on no change in power use per household per second and represents the absolute maximum number of houses that can be powered by one megawatt. The realistic number of houses that can be powered is significantly lower. Peak demand hours of electricity during the daylight hours tend to skew power demand to middays. This is why the largest power plants in the world have generating capacities that run into the tens of thousands of megawatts. The Three Gorges Dam hydroelectric power plant in China, for example, has a peak generating capacity of 18,460 megawatts.