The results are a starting point for talking to a plumber, not a design. A plumber still has to check venting, gas or electrical capacity, space, and local permit rules.
Before you buy
- Replacing a water heater can require a permit. See which Montgomery County municipalities require a permit to replace a water heater.
- Switching from a tank to tankless, or from gas to a heat pump, can change venting, gas-line, electrical, drainage, and space requirements. Ask the plumber to confirm each one in the written estimate.
- Bring these numbers to the estimate: "Our busiest hour is about X gallons. What first hour rating (or gpm at what rise) are you quoting?" A competent answer names the model's rating and explains how it compares.
How we calculated this
Busiest hour. We add up the hot water used in your rush hour. This follows the Department of Energy's peak-hour method: a tank water heater's first hour rating should be at least that number.1 A shower's hot water is its length × the shower head's flow × the share of hot water in the mix. We assume a 105°F shower, so with 50°F water coming in and a 120°F tank, about 79% of the flow is hot. Dishwasher loads (7 gallons), front- and top-loading washer loads (15 and 25 gallons), and kitchen sink uses (3 gallons) use the DOE's per-use values.1 A tub bath is counted as 20 gallons of hot water, which is our planning assumption.
Typical tank size. We model each common size's first hour rating as 70% of the tank's volume plus 80% of an hour of recovery at your temperature rise. That lines up with typical published ratings. We assume gas burners of 36,000–76,000 Btu per hour at 78% efficiency, and 4,500-watt electric elements. We suggest the smallest size that covers your busiest hour and doesn't run short on the busy-day simulation. Real models vary, so the label's first hour rating is what to compare. Heat pump water heaters recover more slowly in heat-pump-only mode.
Tankless. Flow = the fixtures running at once × their flow rates. Rise = your setting minus the incoming water temperature.1 The DOE suggests assuming 50°F incoming water unless you measure it, but tap water is colder in winter. Measure it in January for a worst case. Gas input = gpm × 8.33 × 60 × rise ÷ 0.93. Electric kW = that heat ÷ 3,412 ÷ 0.98. Business fixture flows are federal maximums: 0.5 gpm for public lavatory faucets, 2.2 gpm for kitchen faucets, and 2.5 gpm for shower heads.2
Cost. Heat per day = gallons × 8.33 × temperature rise. Yearly cost = heat ÷ UEF × your rate × 365 days. One therm is 100,000 Btu, and one kWh is 3,412 Btu. The default gas rate is Pennsylvania's 2025 average residential price of $15.04 per thousand cubic feet,3 converted at about 1,037 Btu per cubic foot. The default electric rate is Pennsylvania's July 2026 average residential price of 21.72¢ per kWh.4 Both are statewide averages that include delivery. Your utility's rate will differ.
- U.S. Department of Energy, Energy Saver, "Sizing a New Water Heater." The live page was unavailable, so values come from the archived copy of 2 July 2026. Last verified 2026-09-30.
- Federal plumbing product water-use limits, 10 CFR 430.32 (showerheads and faucets), and the ASME A112.18.1 public lavatory limit.
- U.S. Energy Information Administration, Pennsylvania Natural Gas Prices, annual residential price. Retrieved 2026-09-30.
- U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A. Retrieved 2026-09-30.