Solid state nichrome heating element. Extremely economical to use. Heaters range from 4 to 24W and cost almost nothing to operate. Great heat source for your reptile. Ideal 24 hour primary or secondary heat source for tropical or temperate reptilian species. Zoo Meds Under Tank Heaters are UL/cUL approved (GS/TuV/CE in Europe). Under tank heaters permanently adhere to your terrarium forming a solid bond for optimum heat transfer. Inexpensive low wattage heaters. Excellent secondary heat source. Available in 4 x 5 6 x 8 8 x 12 and 8 x 18 sizes.