(1) 10' x 11" heat mat (1320 square inches), a temperature controller and probe. Add up to (4) 10'. Heat mats to this controller. The controller for each of these heat mat systems allows you to vary the temperature from 40 to 100 °F. (one degree increments), as exact temperatures are the key to good seed germination. Plug the 11" wide heat mats into the controller, place the copper probe in the soil, check the suggested soil starting temperature listed on each packet of Harris Seeds, and set the controller dial accordingly. Use a power strip when adding more than two heat mats per controller. Each kit is completely safe and waterproof, and properly grounded. This professional quality system lasts you for years!