That should be more like 1,000 btus per gallon. To heat 250 gallons of room temperature wash to distillation temperature in an hour would take around a 200,000 btus (250 gallons x 8lbs x 100 degrees). The boiler would need to be a little larger than 200k btus for practical purposes, though. A million btu boiler would give a faster heat up time, excess capacity to run additional stills, mash cookers, etc, and room to grow. But if you can't afford that then you should be able to get by with substantially smaller unit.