Water Heater Size Calculator

Tank Size (gal) and Peak Hour Demand depend only on Household Size and Peak Hour Usage -- Incoming Water Temp, Desired Hot Water Temp, and Electricity Cost have no effect on either output, since tank sizing here is driven purely by how many gallons the household draws in its busiest hour, not by temperature or energy price. Peak Hour Usage sets gallons per person (10 for Low, 12 for Medium, 15 for High), so Household Size determines the recommended tank the same way regardless of what temperature you enter. Tankless GPM Needed responds to Household Size only up to 3 simultaneous fixtures -- past a household of 3 people, this figure stops rising with household size (it only rises again if you also switch to High peak usage, which adds one more simultaneous fixture), because the model assumes no more than a handful of fixtures run at once regardless of how many people live there.

Temperature Rise is simply Desired Hot Water Temp minus Incoming Water Temp, so raising the desired temperature or lowering the incoming temperature by one degree changes the rise by exactly one degree either way -- they move it identically, just in opposite directions. Annual Cost scales with Household Size, Temperature Rise, and Electricity Cost together, but is completely unaffected by Peak Hour Usage, since the cost model tracks average daily energy use rather than peak-hour flow rate.