Duke Energy is giving its customers a coupon for free, energy-saving compact fluorescent light bulbs.

The coupon, good only at Walmart, gets you a six-pack of 13-watt CFLs. One 13-watt CFL is the equivalent of one 60-watt incandescent bulb.

According to the Charlotte utility, changing just one bulb will save customers $30 a year on their electric bills.

The coupons will be mailed this month, and expire May 16. If you have not received a coupon by April 16, call 800-244-5210.