Weather is the biggest reason for fluctuations in your bill. On average, 55 - 60% of your electric bill is made up of heating and cooling costs. Throughout the summer, one month's bill can differ greatly from another, as the summer weather in this area can be unpredictable.
When the temperature is 65 degrees Fahrenheit outside, your system doesn't have to work very hard to maintain a comfortable temperature. When the average temperature outside rises above 65 degrees Fahrenheit, your system works harder to cool your house. That’s why hot weather can affect your electric bill, even if you leave your thermostat on a constant setting. As you move into the summer, the temperature remains hotter for a longer period of time.
Below are some examples of how weather affects your bill, even if you keep your thermostat steady: