If you have ground for your coop floor or in your run you can use this method to prevent water from freezing without the use of electricity.
Use a bucket that is appropriately sized for your flock although a bigger bucket retains heat better than a small bucket. I use either a 3 or 5 gallon flat back tapered bucket. The use of a tapered bucket makes it easier to remove the bucket from the hole for cleaning or adding more manure to the hole.
5 gallon flat back tapered bucket
Dig a hole that is 6" - 8" deeper and at least 9" wider in diameter than the bucket.
Illustration of size of hole compared to size of bucket
Put enough fresh manure (not composted) in the bottom of the hole so that the top of the bucket will sit 2" to 4" above the ground level.
Set the bucket in the hole and finish filling the hole with fresh manure.
Fill the bucket with water before tamping the manure in place.
Overflow the bucket to add water to the manure to get it started composting.
As the manure composts, it will release heat and keep the water from freezing.
If you live in an area where the temperature does not get below zero degrees F., your water should never freeze. After the manure has composted, it will need to be replaced. I normally replace the manure annually.
I prefer to use horse manure but since I no longer have a horse, I use the chicken droppings that I collect from under their roosts.
At -20°F I usually get a very thin layer of ice that the poultry have no trouble pecking through to get at the water. At -30°F I may have 1/2" to 1" of ice that I remove first thing in the morning and the water in the bucket stays open until it freezes the next morning.
As the manure composts and compacts, I pull the bucket out and add additional fresh manure to get the top of the bucket back up to where it was when I started. The whole procedure needs to be repeated annually. Once the manure is done composting, it no longer creates heat.
For those that do not have a ground floor, you could build an insulated box around a bucket and fill it with manure.