This is a charge that electric utilities include in some of their rate plans. Typically, the utility measures the average demand of a home over a one hour or 30 minute period, and then charges the customer a multiplier of that demand.
For example, if a customer averages 8 kW of demand over the period measured, and the demand charge is $15 per kW, the customer will receive a $120 demand charge on their bill that month.
See APS Solar Rate Plans and SRP Solar Plans for information about demand charges in Arizona.