Work the solar year round and in the winter route all hot water through the solar heat storage tank prior to going through the coal DHW coil.
If you can raise water form the 40-60 degrees it typically comes into the house to say 90 degrees inw inter, you will use significantly less coal to heat it up the next 30-40 degrees for comnsumption.
1 BTU raises one pound of water one degree farenheit.
8 lbs of water to the gallon.
If you can use solar to raise a gallon of water 50 degrees prior to the coal boiler, you save 400 btu per gallon.
If you use 50 gallons of hot water per day... That is 20,000 btu saved. A little over a pound and a half of coal when you consider energy efficiency of coal boiler.