By my rough, first season burning, calculations I burn a ton every 800 degree days. My previous 185 gallon +/- oil delivery came every 900 degree days. The oil furnace is, allegedly, 85% efficient. This would seem to indicated the coal boiler is less efficient, HOWEVER, I use the boiler for all DHW. Makes the calculation more difficult but it appears I'm somewhere in the neighborhood of 85% efficiency. That's also the efficiency Keystoker states between gross input and net output. Whatever the case,.....I'm saving tons of $$$
Edit: Of course, I keep the house 2-4 degrees warmer with the coal than I ever did with the oil. Further muddies the calculation puddle
How efficient is your oil boiler? And did you heat DHW with electricity before? Assuming 420 KWH per month for your electric DHW, I get:
420/30.5 = 13.77 KWH per day for DHW
13.77 x 3,412 ~= 47,000 BTU's per day for DHW
Granting 85% efficient for coal at this point I get this as 4.55 lbs per day of coal for DHW
Assuming 3,500 total HDD's to date from October 1st, I get about 26.52 HDD's per day on average for Harrisburg, PA (to date).
800/26.52 = 30.17 days to burn one ton of coal to date, or 66.3 average pounds per day.
Take away 4.55 and you have 61.75 lbs. per day going to home heat.
900/26.52 = 33.94 days to burn 185 lbs of oil, or 4.45 gallons per day.
4.45 gallons of oil = 616,770 gross BTU's for home heat per day via #2 Heating Oil
61.75 lbs. coal x 12,150 BTU's/Lb. = 750,263 gross BTU's per day for home heat via coal
From this it would appear that your coal boiler is only about 82.2% as efficient on a daily basis as is your oil boiler.