I just received an email from Xcel that gave these details:
NEW SUMMER TIERED RATES BEGIN JUNE 1ST
Effective June 1st through September 30th, you will have a new tiered rate structure applied to your electricity bill. These rates are designed to promote efficient use of energy during periods when the demand for electricity is greatest and generation costs are highest.
What to Expect with Tiered Rates
Your electricity rate will change based upon the time of year and the amount of electricity you use. Your monthly bill will clearly indicate your use and the rate you are being charged.
TIER 1 RATE: This is the lower rate you will pay for the first 500 kilowatt hours of electricity you use in the billing period.
TIER 2 RATE: For energy use that exceeds 500 kilowatt hours of use in the billing period, you will be charged the higher tiered rate.
NON-SUMMER RATE: From October through May, you will be charged the lower rate for all of your electricity usage.
More here...
http://www.xcelenergy.com/Colorado/Residential/ManageMyAccount/UnderstandingMyBill/Pages/colorado-tiered-rates.aspx