I work for an electric utility, so I can provide a little bit of insight as to how the billing works for most US people. Some of y'all may be aware of this already, but some may not.
1. There are two components to your bill. First there is the base rate, which utilities charge to cover all their costs except fuel, plus a "reasonable rate of return" that is generally set by the state Public Service Commission. A notable exception to this is TVA (and I think, WPA in Washington state), which set their own base rates. The second component is the fuel adjustment charge, which is a straight pass-through of whatever the utility pays for fuel... coal, uranium, natural gas, etc.
2. Most variations you see in the total rate are due to the fuel costs. Gas costs peak at the height of winter, with a smaller peak in the summer. The peak is bigger in winter because a lot of the country heats with gas. As electric production has shifted more to natural gas over the last 15-20 years, people have seen even more seasonal volatility in their electric bills for this reason. Expect it to get worse as we burn less coal and "cheap" natural gas becomes an even more dominant factor in your electric costs.
There might be other factors in play, such as tiered billing (i.e., charging more per kWh above a certain level of use) or time of use billing (i.e., charging more during peak demand periods each day). But as I tell people around here all the time, when your bill goes up, it's either because you used more electricity or because the utility had to pay more for fuel. And the utility doesn't make a dime off that fuel, so they are just as interested in minimizing that cost as you are.