Develop solar option for heating water troughs in winter

We have 10 horse corrals and each has a water trough with electric heaters in the winter. Temps in Montana average below freezing in the winter getting as a low as -10 at times. We want to purchase solar powered heaters to reduce our utility costs which can exceed $1500 per month in Jan/Feb. Using solar to power or augment will allow us to use our funds for feed and medical services for the equines in our care and be able to care for more equines.