I use a cookie tin heater I built for less than $5. It's served me well through two winters. The one in the stores costs almost $50, and when it craps out, it's finished and you are out $50. If the cookie tin heater goes out, you just put in a new light bulb (I use a 40W) and you are back in business. See the link below, easy to build, it works, and it's CHEAP.
Jack
https://www.backyardchickens.com/t/107951/cookie-tin-water-heater
Jack
https://www.backyardchickens.com/t/107951/cookie-tin-water-heater