What Size Water Heater Do I Need?

The three numbers that matter

Every tank heater has these, usually on the label or in the manufacturer's spec sheet.

Capacity (gallons)

How much water the tank holds. It is the headline number and the one most often used alone, which is where sizing goes wrong. Capacity tells you what is already hot when demand starts and nothing about what happens after that.

It also overstates what you get. You never draw a tank down to the last gallon of useful hot water — as hot water leaves the top, cold water enters the bottom and the outlet temperature falls. Usable hot water is meaningfully less than the rated capacity, which is one reason a "50-gallon" tank does not give you fifty gallons of shower.

First-hour rating

The one to compare. The first-hour rating is how much hot water the heater can deliver in one hour starting from a fully heated tank — capacity plus whatever it can reheat during that hour. It is the closest thing to a real-world performance figure, and it is why fuel type matters as much as tank size.

Recovery rate

How fast the heater reheats, usually expressed as gallons per hour at a given temperature rise. It is what carries you through back-to-back demand once the stored water is gone. Gas burners generally recover considerably faster than standard electric elements, which is the mechanism behind the first-hour difference.

Why gas and electric of the same size perform differently

This surprises people replacing an old heater, and it is the single most useful thing on this page.

A gas burner can put heat into the tank quickly. An electric heater uses resistance elements, which recover more slowly. So for the same capacity, a gas unit typically carries a higher first-hour rating than an electric one.

The practical consequence: a 40-gallon gas heater can out-deliver a 50-gallon electric over a busy hour, even though the electric one holds more water. If you are switching fuel types, comparing gallons to gallons will mislead you — compare first-hour ratings, and expect to need more capacity on electric to match what a gas unit did.

Switching fuel changes the installation too, not just the heater — venting, gas supply or electrical capacity all come into it. The gas versus electric guide covers that side.

Working out your busiest hour

Forget bedroom counts. What matters is the worst hour your household actually puts the heater through, because that is when you notice.

Think through a real weekday morning: who showers, whether any of it overlaps, whether the washing machine or dishwasher runs at the same time. Then do the same for the worst case — guests staying, or everyone leaving at once.

Rough draws to reason with, which vary a lot by fixture and habit:

• A shower is the big one, and it scales with flow rate and length. A long shower on a high-flow head can use several times what a short one on a low-flow head does.
• A bath draws its hot water all at once rather than over minutes, which is harder on a tank than the same volume spread out.
• A dishwasher or washing machine adds to the hour if it runs during it. Many machines heat their own water, so check before counting it.
• Hand washing and kitchen use is small by comparison and rarely changes the sizing decision.

Add up the worst realistic hour, then look for a first-hour rating at or a little above it. Sizing to your average hour is how households end up with cold showers twice a week.

Why bigger is not automatically better

There is a real cost to oversizing, and it is worth knowing before you buy up.

A tank keeps its contents hot whether you use them or not. Standby loss is a continuous, small expense, and a larger tank has more surface area losing heat. Oversizing by one step is usually unremarkable; oversizing by a lot means paying to keep water hot that nobody draws.

Then there is fit. Water heaters have to go through doorways and into the space they occupy, with clearances — combustion air and venting on a gas unit, service access on both. A taller or wider replacement sometimes does not physically work where the old one sat, and a "tall" model of the same capacity is not interchangeable with a "short" one in a tight closet or a crawlspace.

On electric, a larger unit may also push you into different circuit requirements. On gas, it can change the venting or the gas supply demand. None of that is a reason to undersize — it is a reason the decision gets made with the installation space in view rather than from a catalogue.

Where tankless changes the question

A tankless heater has no capacity figure at all, because it does not store water. It heats on demand, so the question stops being "how many gallons" and becomes "how many gallons per minute, at what temperature rise".

Two things follow.

Simultaneous demand becomes the limit. A tankless unit sized for one shower will struggle with two at once. The sizing question is how many fixtures might run together, not how long any one of them runs — and in that respect it is the opposite of a tank, which can cover a big simultaneous draw and then needs time to recover.

Incoming water temperature matters. A tankless heater raises water by a certain number of degrees at a certain flow. The colder the water arriving, the lower the flow it can deliver at your target temperature — so the same unit performs differently in winter than in summer. Sizing on a summer figure is a common way to end up disappointed in January.

See tankless water heaters for how those systems differ to install and service.

Things that look like a sizing problem and are not

Worth ruling out before buying a bigger heater, because a replacement will not fix any of these.

• Sediment. A heavy layer on the tank bottom takes up volume and insulates the burner or buries the lower element. A tank that has quietly lost capacity to sediment behaves like an undersized one. See the maintenance guide.
• A failed lower element, on electric. With only the upper element working, you get a short burst of hot water that runs out fast — the classic signature, and very easy to mistake for a tank that is too small.
• A failing dip tube. It is meant to deliver incoming cold water to the bottom of the tank. Damaged, it mixes cold straight into the hot outlet, and the heater seems to have shrunk.
• A thermostat set low. Lower stored temperature means less hot water available after mixing.
• A crossed connection. A plumbing fault that lets cold into the hot side reduces delivered temperature everywhere and has nothing to do with the heater.

If your hot water used to be adequate and got worse, suspect one of these before capacity. Something changed, and a tank does not shrink. Troubleshooting is the place to start.

What Sonlight does when sizing a replacement

The existing heater's label is the first reference point — capacity, fuel, and first-hour rating if printed — because it tells you what the household has been living with. Then whether that was actually adequate, which only you can answer: if the old heater was fine, matching its performance is a sound decision; if you were running out, it needs to go up.

Then the space, the venting or electrical supply, and the access route, since those decide which models are installable rather than merely suitable. Sonlight installs tank units from Bradford White and A. O. Smith and tankless from Navien, and the manufacturer's own sizing data for the specific model is the authority over any rule of thumb on a web page, including this one.

A quote is given in writing before any work begins; how pricing works explains the dispatch fee that covers the visit itself.

For a replacement see water heater replacement or tankless water heaters. If the symptom may be a fault rather than size, start with troubleshooting or water heater repair. The gas versus electric guide covers fuel choice, maintenance covers sediment, and a leaking heater is a separate problem. See all service areas.

Replacing a Water Heater in the Tri-Cities?

Bring the old unit's label details and whether it kept up, and the sizing conversation gets much shorter. See water heater replacement.