40 vs 50 Gallon Water Heater
Ten gallons of storage is worth less than you think. The fuel the tank burns is worth far more.
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Buy 50 gallons if the tank is electric, and 40 is usually enough if it is gas. A 40-gallon gas tank such as the Rheem Performance 40 delivers about 65 gallons in the first hour; a 50-gallon electric tank such as the Rheem Performance Platinum delivers 63. Ten extra gallons of storage does not beat a gas burner's recovery. The A. O. Smith Signature 100, 50 gal gas at a 91-gallon first-hour rating is the pick when you want both.
The question almost always arrives in the same form: the old unit is 40 gallons, somebody suggested going up to 50, and nobody can say whether it will make a difference. It depends entirely on what heats the water, because storage and recovery are two different things and the first-hour rating is the number that combines them.
First-hour rating, not capacity
First-hour rating is how many gallons of hot water a tank can deliver in one hour starting full — the storage plus whatever it can reheat while you are drawing. A gas burner puts 36,000 to 50,000 BTU/h into the water. A pair of residential electric elements puts in far less, and only one element runs at a time. That is the whole story, and it is why capacity alone tells you very little.
| Unit | Capacity | Fuel | First-hour rating | Recovery |
|---|---|---|---|---|
| Rheem Performance 40 gal gas | 40 gallons | Natural gas, 36,000 BTU/h | 65 gallons (PROG40-36N data); 68 on the retail model | 36.4 gal/h |
| A. O. Smith Signature 100, 50 gal gas | 50 gallons | Natural gas, 50,000 BTU/h | 91 gallons | Not published separately |
| A. O. Smith Signature 100 (40,000 BTU version) | 50 gallons | Natural gas, 40,000 BTU/h | 77 gallons | Not published separately |
| Rheem Performance Platinum 50 gal electric | 50 gallons | Electric, 240 V | 63 gallons | 21 gal/h at a 90°F rise |
| Rheem Gladiator 50 gal electric | 50 gallons | Electric, 5,500 W double element | 63 gallons | Not published separately |
Read the first column against the fourth. The smallest tank on the list beats both of the largest ones, because it burns gas. If your replacement is electric, those ten gallons are doing real work and you should take them. If it is gas, the ten gallons are mostly standby loss.
Quick picks
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| # | Product | Best for | Price |
|---|---|---|---|
| 1 | A. O. Smith Signature 100, 50 gal gasThe straightforward 50-gallon gas replacement, and the reason to pay attention to the burner size: the 50,000 BTU version is rated at a 91-gallon first-hour rating against 77 gallons for the 40,000 BTU one. | Most gas tank replacements | Check price on Amazon No live price feed on this site, so we show none. #ad |
| 2 |  Rheem Performance 40 gal gasThe honest small-house gas tank: 36,000 BTU/h, around a 65-gallon first-hour delivery, and a recovery rate of about 36 gallons per hour — which is why a 40-gallon gas tank serves a family that would need a 50-gallon electric. | Smaller gas households | Check price on Amazon No live price feed on this site, so we show none. #ad |
| 3 |  Rheem Performance Platinum 50 gal electricA plain, well-warranted 50-gallon electric tank: a 63 gallon-per-hour first-hour rating, 21 gallons per hour of recovery at a 90°F rise, and a 0.93 UEF. | Straight electric replacement | Check price on Amazon No live price feed on this site, so we show none. #ad |
| 4 |  Rheem Gladiator 50 gal electricThe electric tank to put above a finished floor: integrated leak detection with an automatic shutoff valve that limits a leak to around 20 ounces, plus a 12-year tank and parts warranty. | Electric tanks in finished spaces | Check price on Amazon No live price feed on this site, so we show none. #ad |
The four tanks in full
Pick 1 — Most gas tank replacements
A. O. Smith Signature 100, 50 gal gas
The straightforward 50-gallon gas replacement, and the reason to pay attention to the burner size: the 50,000 BTU version is rated at a 91-gallon first-hour rating against 77 gallons for the 40,000 BTU one.
| Capacity | 50 gallons |
|---|---|
| First-hour rating | 91 gallons (50,000 BTU model); 77 gallons (40,000 BTU model) |
| Burner | 50,000 BTU/h ultra-low NOx, or 40,000 BTU/h |
| Dimensions | 61 in H x 22 in dia (50,000 BTU); 60-3/4 in H x 22 in dia (40,000 BTU) |
| Warranty | 6-year tank and parts with 1-year labor (50,000 BTU); 9-year tank and parts with 2-year labor (40,000 BTU) |
What is good
- A 91-gallon first-hour rating from a 50-gallon tank covers back-to-back showers that empty an electric tank of the same size
- Widely stocked at Lowe's, so same-day replacement is realistic when the old one fails
- Ultra-low NOx burner meets the stricter air-district rules in California and Texas
- 22-inch diameter fits most existing alcoves without re-plumbing
What is not
- The longer 9-year warranty is on the smaller burner, so warranty and recovery pull in opposite directions
- Atmospheric venting needs a working chimney or B-vent
- Standing pilot and flue losses mean it idles energy away all day
Skip it if: Skip it if your water heater sits in a sealed closet or a space with combustion air problems. An atmospheric gas tank needs real combustion air and a real flue — the power-vent or direct-vent models exist for exactly that case.
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Specifications above from Lowe's listing, Signature 100 50-gal 50,000 BTU; Lowe's listing, Signature 100 50-gal 40,000 BTU 9-year. We have not tested this unit ourselves.
Pick 2 — Smaller gas households
Rheem Performance 40 gal gas
The honest small-house gas tank: 36,000 BTU/h, around a 65-gallon first-hour delivery, and a recovery rate of about 36 gallons per hour — which is why a 40-gallon gas tank serves a family that would need a 50-gallon electric.
| Capacity | 40 gallons |
|---|---|
| Input | 36,000 BTU/h |
| First-hour delivery | 65 gallons (Rheem PROG40-36N data); 68 gallons on the Performance retail model |
| Recovery | 36.4 gallons per hour |
| Energy factor | 0.62 (0.58 UEF on the retail Performance model) |
What is good
- Gas recovery means a 40-gallon tank behaves like a much larger electric one
- Cheapest route to a working hot water supply for one or two bathrooms
- Short and tall variants both exist, so it fits awkward spaces
- Parts on the shelf at every supply house in the country
What is not
- Published figures vary between the professional and retail versions of the same nominal unit — read the EnergyGuide label on the box you are buying
- 0.58 to 0.62 energy factor is the low end of current gas tanks
- 40 gallons is genuinely tight above three people
Skip it if: Skip it for a household of four or more with two bathrooms. The first-hour rating is adequate for one bathroom running at a time, and the complaint that follows a too-small tank is always the same: the last shower is cold.
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Specifications above from Young Supply listing, Rheem PROG40-36N; Rheem Performance 40 gal retail listing. We have not tested this unit ourselves.
Pick 3 — Straight electric replacement
Rheem Performance Platinum 50 gal electric
A plain, well-warranted 50-gallon electric tank: a 63 gallon-per-hour first-hour rating, 21 gallons per hour of recovery at a 90°F rise, and a 0.93 UEF.
| Capacity | 50 gallons |
|---|---|
| First-hour rating | 63 gallons per hour |
| Recovery | 21 gallons per hour at a 90°F rise |
| Efficiency | 0.93 UEF |
| Voltage | 240 V |
What is good
- Publishes recovery at a stated temperature rise, which most retail listings do not
- 0.93 UEF is at the top of what a resistance tank can do
- Drop-in replacement dimensions for the previous generation
- Available in short and tall variants
What is not
- 21 gallons per hour of recovery is a third of what a gas tank does
- No leak detection on this model
- Resistance heating is the most expensive way to make hot water in most of the country
Skip it if: Skip it if you have 7 feet of headroom and an unconditioned space to put it in. A heat pump model in that spot uses roughly a third of the electricity for the same hot water, and the payback is measured in a few years, not decades.
No live price feed on this site, so we show none. #ad
Specifications above from Home Depot specification sheet, Rheem Performance Platinum electric (PDF). We have not tested this unit ourselves.
Pick 4 — Electric tanks in finished spaces
Rheem Gladiator 50 gal electric
The electric tank to put above a finished floor: integrated leak detection with an automatic shutoff valve that limits a leak to around 20 ounces, plus a 12-year tank and parts warranty.
| Capacity | 50 gallons |
|---|---|
| Elements | 5,500 W double element, 240 V |
| First-hour rating | 63 gallons |
| Efficiency | 0.92 UEF |
| Leak protection | 360° LeakSense detection with factory-fitted automatic shutoff; leaks limited to no more than 20 oz |
| Warranty | 12-year tank and parts, 3-year labor |
What is good
- The auto-shutoff valve is the single most valuable feature on any tank installed over drywall
- 12-year tank and parts with 3-year labor is among the longest electric warranties sold at retail
- 5,500 W double element gives faster recovery than the usual 4,500 W pair
- Leak alerts arrive before the ceiling does
What is not
- A 63-gallon first-hour rating from 50 gallons is ordinary — electric recovery is slow, full stop
- Running cost is electric-rate running cost; a heat pump model costs far less to run
- The shutoff valve is one more serviceable part in the cold inlet
Skip it if: Skip it if the heater lives on a concrete slab next to a floor drain and you have the headroom for a heat pump model. You are paying for leak protection you can get from a 20-dollar pan, and giving up the running-cost saving a heat pump would deliver.
No live price feed on this site, so we show none. #ad
Specifications above from Home Depot listing, Rheem Gladiator XE50T12CS55U1. We have not tested this unit ourselves.
When 40 gallons is the right answer
- Gas, one to three people, one bathroom. A 65-gallon first-hour delivery covers two showers and a dishwasher without a pause.
- The alcove was built for a 40. Within one manufacturer's family the 50-gallon version is usually several inches taller, and in a closet built to the old unit that difference turns a swap into a re-plumb. Water heater dimensions has the published figures.
- You are paying to heat a tank you never empty. Standby loss scales with stored volume, every hour of every day.
- You want the long warranty. On the Signature 100 the 9-year tank-and-parts cover sits on the 40,000 BTU burner, not the 50,000 one — warranty and recovery pull in opposite directions in that family.
When 50 gallons is the right answer
- Electric, any household above two people. Electric recovery is 21 gallons an hour at a 90°F rise. Storage is the only lever resistance heating gives you, so take it.
- Three or more people on any fuel. Water heater size for a family of 4 is a 50-gallon gas or an 80-gallon electric conversation, not a 40-gallon one.
- Two bathrooms used back to back. Two showers in a row plus a laundry load puts you past a 40-gallon electric tank's useful output.
- Cold inlet water. Every gallon has to be lifted further in winter, which cuts real first-hour delivery below the rating plate figure.
What the two sizes cost you to run
We do not publish invented running-cost figures; the inputs vary too much by state. What the rating plates do tell you is the direction. Standby loss on a 50-gallon tank is larger than on a 40-gallon tank of the same construction. A gas tank's 0.58 to 0.62 energy factor looks poor next to a 0.93 UEF electric tank, but gas energy is cheaper per BTU almost everywhere, which is why the gas unit usually still wins on the bill. What a water heater actually costs sets out the published ranges with attribution.
Match the size to the house
| Your situation | Buy | Why |
|---|---|---|
| Gas, 1-2 people, one bathroom | 40 gallon gas | 65-gallon first-hour delivery and less standby loss |
| Gas, 3-5 people | 50 gallon gas, 50,000 BTU | 91-gallon first-hour rating is the biggest number on this page |
| Electric, 1-2 people | 50 gallon electric | 63 gallons in the first hour, and recovery you cannot speed up |
| Electric, 3+ people | 50 gallon heat pump, or 80 gallon resistance | Resistance recovery cannot keep up; storage or a heat pump has to |
| Tank sits over a finished floor | Rheem Gladiator 50 | Leak detection with an auto shutoff that limits a leak to about 20 ounces |
| The old tank never ran out | The same capacity again | A first-hour rating you know works beats a specification you are guessing at |
Whichever size you land on, the install list is the same: a pan under it, an expansion tank if the house is on a closed system, and straps in a seismic zone. Water heater installation kit covers the parts, and 40 gallon water heaters and 50 gallon water heaters carry the full model lists for each size.
What goes in with it
A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.
- Expansion tank
Required by code on a closed system, which is most homes with a pressure-reducing valve.
- Drain pan
Cheap insurance anywhere the tank sits above something you care about.
- Anode rod
The part that decides when the tank leaks. Replacing it is the highest-value hour of maintenance there is.