I use between 5-600 gallons per year. It has gone down since I switched from oil domestic hot water to a hybrid hot water heater. I want to get the heat pump eventually, just haven't done it yet. The 2 tank set up was done by the previous owner. He had switched the house from electric baseboard to oil hot water baseboard. The boiler he installed was a 145,000 btu oil/wood/coal fired monstrosity that he had bought used from somewhere. It was so inefficient that we had to keep the window in our bedroom cracked in winter because the heat going up the chimney through our bedroom closet made our room too warm even when it was only set at 65°. I remember when that thing died. It was March 2001, during one of the snowiest stretches ever. There was so much snow that they couldn't take the old boiler out until mid-April.