Given the recent power-delivery problems in Denver, including an explosion at one of its substations, Xcel Energy employees likely have their hands full. But now, the company is contending with Colorado customers who are trying to figure out their new two-tiered bills—the ones that charge a higher price per kilowatt-hour after a 500-kwh threshold is reached.

The idea, according to some, will give consumers a simple incentive to lower their energy use, like maybe thinking twice about running the air conditioner too much this summer. But Xcel forgot to mention something in the ramp up to the new system: Most bills don’t run from the first to the last day of the month, making it difficult to judge the billing cycle, notes The Denver Post. And now, confused customers are asking questions, as their own calculations don’t add up to Xcel’s.

It turns out Xcel didn’t explain the program well enough, and many customers are left having to pay more than they anticipated.

“I think what we did was give [customers] a good idea of what was to be expected in the first full month,” says Xcel spokesman Tom Henley. “Let’s face it, explaining the proration in a system that’s already complex enough was probably one of those things we pushed to the back burner.”

Meanwhile, Xcel has filed a request with the Colorado Public Utilities Commission to reduce electricity and natural gas prices by four percent in coming weeks, reports Greeley’s Tribune.