What size water heater do I need

How to size a water heater

Size a tank water heater by its busiest hour: the hot water your household uses in that hour, minus what the heating element reheats meanwhile, divided by the share of the tank that comes out hot. That is the method behind the first hour rating (FHR) on the yellow EnergyGuide label, and Vermont Energy Saver suggests an FHR within 1 or 2 gallons of your peak hour demand.

The calculator works it out in five steps:

1. One shower. Minutes times flow: 8 minutes at 2.0 gpm is 16 gallons at 105 °F. The average American shower lasts 7.8 minutes at 2.1 gpm (Home Water Works); WaterSense showerheads use no more than 2.0 gpm, older standard ones 2.5 gpm (EPA).
2. The hot share. At the mixing valve, tank water blends with cold water. With 55 °F incoming water, a 105 °F shower and the thermostat at 120 °F, the hot share is (105 - 55) ÷ (120 - 55) = 77%.
3. Peak hour demand. One shower per person, back to back. If a bath in the tub needs more than the showers, the bath is the peak.
4. Recovery. In one hour an element heats watts ÷ (1.163 Wh × temperature rise in °C) liters: 28 gallons with 4,500 W from 55 °F to 120 °F. Rheem gives the same rule as wattage ÷ (2.42 × rise in °F).
5. Usable share. Cold water enters at the bottom, so the last part of the tank comes out lukewarm. The calculator counts 64% of the tank, worked out from a Rheem sheet: 40 and 30 gallon tanks rated at 53 and 47 gallons FHR, minus one hour of recovery at the DOE test conditions (58 °F in, 125 °F out), leave 63% and 64% of the tank. In litres the calculator uses 80% instead, from the V40 on European product sheets (litres of mixed water at 40 °C): Vaillant eloSTOR pro 50 and 80 litre heaters give 72 and 126 litres with 15 °C cold water and 65 °C stored, so 72% and 79% of the tank; Ariston Velis WiFi 50 and 80 give 80 and 125 litres at 65 °C, 78-80% with 15 °C cold water. You can change it in the advanced options.

The tank to buy is the smallest common size that reaches the minimum.

Worked example: a family of four, 40 gallons

These are the values the calculator opens with in US units: four people, one 8-minute shower each with a 2.0 gpm showerhead, 55 °F incoming water, 105 °F shower, thermostat at 120 °F, a 4,500 W element.

• One shower: 8 × 2 = 16 gallons; four showers make 64 gallons at 105 °F
• Hot water at 120 °F: 77% of 64, so the peak hour demand is 49 gallons
• Recovery: 28 gallons in the hour
• Minimum tank size: (49 - 28) ÷ 0.64 = 33 gallons

The calculator says a 40-gallon electric water heater, with an estimated first hour rating of 54 gallons: on the EnergyGuide label, look for an FHR of at least 50 gallons. With 40 °F incoming water in winter the minimum rises to 45 gallons and you need a 50; with old 2.5 gpm showerheads it is 52 gallons, so a 65.

In the UK, switch to metric for litres and °C: two people with 8-minute showers at 8 litres a minute, mains water at 15 °C and the cylinder at 60 °C need at least 53 litres with a 1.5 kW element, so an 80 litre water heater. With a 3 kW immersion heater, three people need 62 litres and the 80 litre size still covers them; four people need 106 litres, so a 120.

Water heater size by number of people

Tank size with one shower per person in the busiest hour, 55 °F incoming water, the thermostat at 120 °F and a 4,500 W element:

People	8 min at 2.0 gpm	10 min at 2.5 gpm
1	12 gal peak, 30 gal tank	19 gal peak, 30 gal tank
2	25 gal peak, 30 gal tank	38 gal peak, 30 gal tank
3	37 gal peak, 30 gal tank	58 gal peak, 50 gal tank
4	49 gal peak, 40 gal tank	77 gal peak, 80 gal tank
5	62 gal peak, 65 gal tank	96 gal peak, 80 gal covers 4
6	74 gal peak, 80 gal tank	115 gal peak, 80 gal covers 4

When even an 80-gallon tank falls short, the calculator still names it and says how many showers in a row it covers: move the others to another hour or look at a bigger system. Dishwashers and washing machines running in the same hour add to the peak, so go one size up if that is your morning.

Tank sizes, first hour rating and heat-up time

Electric tank water heaters come in 30, 40, 50, 65 and 80 gallon sizes; the Rheem models in the sources use 4,500 W elements. Estimated first hour rating and time to heat a full tank from 55 °F to 120 °F:

Tank	Estimated FHR	Heat-up from cold
30 gal	48 gal	1.1 h
40 gal	54 gal	1.4 h
50 gal	60 gal	1.8 h
65 gal	70 gal	2.3 h
80 gal	80 gal	2.8 h

These are estimates at the calculator's temperatures. The FHR on the label is measured at 125 °F outlet temperature (ENERGY STAR) and is the number to compare between models.

Common mistakes

• Shopping by gallons alone. Two tanks of the same size can have different first hour ratings, because the rating also depends on the element or burner. Compare the FHR on the EnergyGuide label.
• Ignoring winter. Colder incoming water needs more hot water at the tap: at 40 °F instead of 55 °F the example goes from 40 to 50 gallons.
• Keeping old showerheads. At 2.5 gpm the same family needs a 65-gallon tank instead of a 40.
• Setting the thermostat too low. UF/IFAS gives 120 °F as the best balance of energy and scalding risk, but legionella grows between 20 and 50 °C, 68 to 122 °F (WHO), and the UK HSE asks for hot water stored at 60 °C (140 °F) at least. A hotter tank also stretches the same gallons further.