Water Heater Sizing: Tank vs Tankless for Every Household

Two Different Sizing Methods for Two Different Technologies

Tank and tankless water heaters solve the same problem — hot water on demand — but they do it in fundamentally different ways. A tank stores a reservoir of pre-heated water. A tankless unit heats water instantly as it flows through. Because the mechanisms differ, the way you size them is completely different. Get the sizing wrong on either type and you end up with cold showers, callbacks, and unhappy customers.

Sizing a Tank Water Heater: First-Hour Rating

The critical spec for a tank water heater is the first-hour rating (FHR) — the number of gallons of hot water the unit can deliver in the first hour of use, starting with a full tank at temperature. FHR combines the tank volume with the recovery rate (how fast the burner or element can reheat incoming cold water).

To determine the FHR you need, add up the hot water demands during your household's peak hour. For most families, this is the morning rush:

• Shower: 10 gallons per use
• Bath: 20 gallons per use
• Dishwasher: 6 gallons per cycle
• Clothes washer (warm): 7 gallons per load
• Hand washing / face washing: 2 gallons per use
• Kitchen sink (food prep): 4 gallons per use

Example: A family of four with two morning showers, one kitchen sink use, and a dishwasher start = 10 + 10 + 4 + 6 = 30 gallons FHR minimum. A standard 40-gallon gas tank with a 40+ gallon FHR handles this easily. Add a third shower and the FHR jumps to 40 gallons — now you're looking at a 50-gallon tank.

Common Tank Sizes and Who They Serve

• 30 gallon: 1–2 people, apartment or condo. Tight for a household with a dishwasher.
• 40 gallon: 2–3 people. The most common residential size. Adequate for most couples and small families with moderate usage.
• 50 gallon: 3–4 people. The standard upgrade when a 40-gallon can't keep up. Handles back-to-back showers and a dishwasher cycle.
• 75–80 gallon: 5+ people or homes with high-flow fixtures like soaking tubs. Also consider if multiple bathrooms run simultaneously.

Remember: tank size alone doesn't tell the whole story. A 40-gallon gas tank with a high-BTU burner can have a higher FHR than a 50-gallon electric tank with slow recovery elements. Always check FHR, not just gallon capacity.

Sizing a Tankless Water Heater: GPM and Temperature Rise

Tankless units don't store water — they heat it on the fly. Sizing is based on two numbers: flow rate (GPM) andtemperature rise.

Temperature rise is the difference between the incoming cold water temperature and your desired output temperature. If groundwater enters your home at 50°F and you want 120°F at the tap, you need a 70°F rise.

Flow rate is determined by how many fixtures you want to run simultaneously:

• Shower: 2.0–2.5 GPM
• Kitchen faucet: 1.5 GPM
• Bathroom faucet: 1.0 GPM
• Dishwasher: 1.0–1.5 GPM
• Clothes washer: 1.5–2.0 GPM

Add up the simultaneous demands. Two showers plus a kitchen faucet = about 6 GPM. At a 70°F rise, you need a tankless unit rated for at least 6 GPM at 70°F rise. This is a critical detail — many tankless units advertise their GPM at a lower temperature rise. A unit rated “9.5 GPM” might only deliver 5 GPM at a 70°F rise. Always check the specs at your actual temperature rise.

Gas vs. Electric: A Practical Comparison

Tank water heaters

Gas tanks heat faster. A typical 40-gallon gas unit recovers about 40–50 gallons per hour. A 40-gallon electric unit recovers only 20–25 gallons per hour with standard 4,500W elements. This means gas tanks can be sized smaller for the same household demand because they recover faster.

Tankless water heaters

The difference is even more dramatic with tankless. A gas tankless unit can deliver 8–11 GPM at moderate temperature rises — enough for a whole house. An electric tankless unit typically maxes out at 3–5 GPM, which may only cover one shower and a faucet simultaneously. In cold climates where incoming water is 40–50°F, electric tankless units struggle to deliver adequate flow at the required temperature rise.

Electric tankless also requires significant electrical infrastructure. A whole-house electric tankless unit may draw 100–150 amps — often requiring a panel upgrade and multiple dedicated circuits with heavy-gauge wire. Factor this into the installation cost.

Energy Factor and Annual Cost

The Uniform Energy Factor (UEF) measures overall efficiency, accounting for the energy used to heat water plus standby losses (heat lost through tank walls). Higher UEF means lower operating cost:

• Standard gas tank: UEF 0.58–0.65
• High-efficiency gas tank: UEF 0.67–0.80
• Standard electric tank: UEF 0.90–0.95
• Heat pump water heater: UEF 2.0–3.5
• Gas tankless: UEF 0.82–0.97
• Electric tankless: UEF 0.96–0.99

Electric units have higher UEF because they don't lose heat up a flue. But electricity costs more per BTU than natural gas in most areas, so a lower-UEF gas unit often costs less to operate. Run the numbers for your local utility rates before making a recommendation.

Installation Considerations

Switching from tank to tankless (or vice versa) isn't just a swap. Each type has installation requirements that affect cost and feasibility:

• Gas tankless needs proper venting. Most use category III stainless steel or PVC venting, which is different from the standard B-vent used by tank heaters. Some units require dedicated concentric vent kits. Budget $500–$1,500 for venting on a retrofit.
• Gas line sizing. A tankless unit draws significantly more BTU when firing than a tank heater (199,000 BTU vs. 40,000 BTU is common). The existing gas line may need to be upsized from 1/2″ to 3/4″ or even 1″ to handle the flow.
• Electrical requirements for electric tankless. As noted above, whole-house electric tankless can require 2–3 dedicated 40A circuits with 8 AWG wire. Verify panel capacity first.
• Condensate drains. High-efficiency condensing tankless units produce acidic condensate that needs a drain line. If there's no floor drain nearby, this adds plumbing work.
• Space and mounting. Tankless units mount on a wall and free up floor space — a real advantage in small utility closets or garages. But they need clearances to combustibles and access for maintenance.

The Cost Comparison

Upfront, tank water heaters win on price. A standard 50-gallon gas tank installed runs $1,200–$2,000. A whole-house gas tankless installed runs $3,000–$5,000. The tankless premium comes from the unit cost, venting, and potentially upsizing the gas line.

Over time, tankless closes the gap. Annual energy savings of $100–$150 are typical, and tankless units last 20+ years compared to 10–15 for tanks. Over a 20-year span, total cost of ownership is often comparable — but only if the tankless unit is properly sized and maintained (annual descaling in hard-water areas).

The Bottom Line

For a family of 2–4 with moderate usage and natural gas available, a 50-gallon gas tank is hard to beat on value. For larger families, homes with high simultaneous demand, or situations where space is limited, a properly sized gas tankless unit delivers endless hot water and long-term savings. Electric tankless is best suited for point-of-use applications or mild climates where the temperature rise is small.

Use our Water Heater Size Calculator to determine your first-hour demand and get a recommendation for both tank and tankless options based on your household size and fixture count.