Xcel Energy Inc., Colorado's largest utility, connected its 1,000th small-solar customer to the grid on Monday.

Xcel's "Solar*Rewards" program, which gives customers rebates to help cover the cost of buying and installing solar panels on their homes and small businesses to generate electricity, has grown rapidly in the last 18 months.

Xcel (NYSE: XEL) is based in Minneapolis. It has 1.3 million electricity customers in Colorado.

To date, Xcel has handed out $19.5 million in rebates and payments for "Renewable Energy Credits" for customers offering solar power for use in their homes, businesses and to the utility.

The money comes from a special charge on every Xcel customers' bill in Colorado. Currently, customers pay an extra 0.6 percent, or roughly 36 cents on a $60 monthly electricity bill. The charge currently brings Xcel $13 million a year, a spokeswoman said.