Electricity rates in Texas are not fixed. Your rate can vary greatly depending on your usage and your electric plan. Some plans have relatively flat rates, while others can be all over the place. This means that you could end up paying 7¢ for 999 kWhs and 8.5¢ for 1001 kWhs. That would be a 16% increase because you microwaved a few potatoes. Learn more on the different plan types here.
In Houston, 0% of people have switched to a plan that has some renewable energy component to it. Another 0% have switched to a plan that is partially renewable, while 0% have switched to a plan that powers homes completely by renewable electricity. This of course means that 100% of people have remained on a plan powered by traditional sources of electricity such as coal or nuclear power.