Sorry, with your Northern Michigan location, I would not bother with solar panels, since your sunshine is not powerful enough to be much help. Not like California or even the Great Plains States with high powered sunshine, more lumens in it. Michigan has sunshine, just not a lot of lumens according to my MSU Gardening Experts when I tried certain plants. Plants died because of ‘lack of sunshine’, not suitable to the area. Actually Michigan is in the sixth cloudiest state in the USA, not a good place for solar powered stuff.
My Grampa used a Propane tank heater years ago, I don’t think they had electric heaters then. It worked quite well for him and the baby pigs he was raising. The horses got led to the tank inside the barn several times a day for water, no tank in their pasture.
We are in mid Michigan, use insulated boxes over the tanks with covers, which still won’t prevent freezing in the wind and low temps of teens or single digits. We run the heaters at night (cheaper electric too), tanks totally covered as horses are in the barn at night. Heaters cost plenty to run, so are not plugged in during daylight hours, water stays open for the horses even with heaters off.
I don’t know if battery would work well or for very long, since an electric heater draws a LOT of electricity to keep water unfrozen. I hate watching the meter dials spin!!
No other real ideas besides the propane heater for you. You will want to protect the propane tank from horses, so they can’t rub or knock it over and set off an explosion with leaking gas.
Any chance of putting in a power pole with meter, outlet, so you could plug in a tank heater, fencer, on that pasture location? Not sure what it costs these days. Having power over there could be handy for other farm uses later on.