Size a tank water heater by its first hour rating, the amount of hot water it delivers in your busiest hour, not just its gallon capacity. Size a tankless unit by gallons per minute at the temperature rise your winter water needs. Count the fixtures that run together during your peak hour and match the rating to that.
Key takeaways
- Gallon capacity alone doesn't tell you enough; first hour rating accounts for how fast the tank reheats.
- Your peak hour is the morning or evening stretch when showers, dishwasher, and laundry overlap.
- Tankless flow ratings drop in winter because incoming Philadelphia-area water is much colder.
- Gas tanks generally recover faster than electric ones of the same size, which changes the capacity you need.
- Oversizing wastes energy holding water you don't use; undersizing means cold showers.
What does water heater size actually mean?
Most people think of size as the number of gallons on the label. That's part of it, but not the useful part. Two tanks with the same capacity can perform very differently, because one reheats faster than the other. The number that matters for a tank is the first hour rating, often printed on the yellow energy label. It's how many gallons of hot water the heater can deliver in one hour starting full: the stored water plus whatever it can heat while you're drawing it down.
A tankless heater has no stored water, so it's rated differently: gallons per minute at a given temperature rise. That rise is the difference between the incoming cold water and the hot water you want. The bigger the rise, the fewer gallons per minute the unit can deliver. Both numbers come from the manufacturer and are what we use when we size a replacement.
How do I estimate my household's peak hour?
Pick the busiest hour of your day for hot water. For most families it's the morning rush or the after-dinner stretch. List what runs during that hour and add up roughly how much hot water each one uses. The U.S. Department of Energy publishes general per-use figures you can use for the math; the point isn't precision, it's getting the total in the right ballpark.
- Each shower, at a typical length
- A bath, which uses far more than a shower
- A dishwasher cycle, if you run it in that hour
- A clothes washer on warm or hot
- Hand washing, shaving, and kitchen sink use
How is a tankless unit sized differently?
For tankless, you add up flow instead of gallons. Each fixture that runs at the same time has a flow rate in gallons per minute; a showerhead, a kitchen faucet, and a washing machine all pull water at once if they're on together. Add those up and you have the flow the unit needs to deliver at the same moment.
Then comes the part people miss. In our region the water coming into the house in winter is much colder than in summer, so the unit has to raise the temperature further to reach the same shower. A tankless heater rated for a certain flow in mild conditions delivers noticeably less in January. We size for winter, and we check the gas supply and venting at the same time, because a larger tankless unit burns more gas per minute than the old tank ever did.
What else changes the answer?
Fuel type matters. Gas tanks reheat faster than electric tanks, so an electric unit generally needs more capacity to deliver the same first hour rating. The space you have matters too: a short, wide tank fits under low rowhome ceilings, and a tankless unit fits where no tank will. Think ahead as well. A couple planning a family or adding a bathroom will outgrow a small tank. And a soaking tub is a special case, because it can drain a tank or overwhelm a tankless unit's flow all by itself.
On the other end, don't go bigger just to be safe. A tank that's too large spends all day keeping water hot for nobody, and that costs you every month. Matching the rating to your real peak hour is the sweet spot.
What are the common sizing mistakes, and what's next?
The mistakes we see most: replacing an old tank with the same gallon size without checking whether it ever kept up; picking a tankless unit by its summer flow rating; ignoring recovery rate when switching from gas to electric; and forgetting that a new bathroom changes the math. Sizing also ties into the estimate, because a larger unit, a fuel change, or tankless venting each add scope, and permits apply in Philadelphia and the surrounding townships. Confirm current requirements with L&I or your municipality; we pull the permit and schedule the inspection.
If you're not sure what size you need, that's a normal question and we're glad to answer it. Call us or request a visit. We'll walk through your peak hour, check your fuel and vent situation, and give you options that fit how your house actually uses hot water. No high-pressure sales.
People also ask
Is a bigger water heater always better?
Can one tankless unit handle two showers at once?
Does the number of people in the house decide the size?
Does switching from gas to electric change the size I need?
References: U.S. Department of Energy. Requirements vary by municipality and change over time; confirm current rules with the authority having jurisdiction.