Heating oil energy content: 138,690 BTU / gallon
Typical Oil burner effieciency: 78% – 95%
Electricity energy content: 3413 BTU / kwh
BTU definition: Amount of energy needed to raise 1 lb water by 1*F at 58*F

Water volume conversion: 1 ft3 = 7.4805 gallons
Water density: 62 lb / ft3 = 8.3 lb / gallon

Ex calc:
I use 175 gallons of oil / month in the winter. Using 85% burn efficiency I need about a million BTUs per day in the winter.

Between 140*F and 180*F my 800 gallon heat storage will store: (800 gal) * (8.3 lb / gal) * 40F = 266kBTU.