What Size Water Heater Do You Need?
Quick answer: The gallon capacity of the tank isn’t the best way to size a water heater. Instead, use First Hour Rating (FHR), which measures the amount of hot water a unit can deliver during one hour of continuous use.
Most households, from a one-bedroom condo to a house full of teenagers who shower like it's a competitive sport, need an FHR somewhere between 40 and 75 gallons. Read on to figure out where your household falls.
Tank size vs First Hour Rating
The gallon number stamped on a water heater is its physical tank capacity, which measures how much water it can hold at once. That's the number most people shop by, but it’s not representative of how hot water gets used in homes.
The number that better predicts whether you'll run out of hot water mid-shower is the First Hour Rating (FHR): a standard measure, required on every water heater's EnergyGuide label, of how many gallons of hot water the unit can deliver in one hour of continuous use. FHR accounts for both the water already sitting in the tank and how fast the unit can reheat more while you're using it. Two tanks that hold the same 50 gallons can have different FHRs, depending on how quickly each recovers.
In short: tank capacity tells you what's stored; FHR tells you what you can use without running out.
Figuring out your peak hour demand
Before you can match a water heater to your home, you need a rough number for how much hot water your household uses in its busiest single hour. For most homes, that's a weekday morning: showers, the dishwasher running from last night, maybe a load of laundry starting before everyone leaves.
Ballpark gallons for common fixtures during that hour:
Shower (8 minutes, standard showerhead): 14-18 gallons
Bath: ~20 gallons
Dishwasher cycle: ~6 gallons
Clothes washer load: 7-10 gallons
Bathroom sink, shaving/washing face: 2-4 gallons
Add up what runs in your busiest hour, and you have your target FHR. A household with two back-to-back showers and a dishwasher running is looking at something like 30-40 gallons. Add a third shower and a load of laundry, and you're closer to 55-65.
Rule-of-thumb sizing by household size
If you want to skip the math, you can estimate FHR based on the number of people and bathrooms in your home:
# of people | # of bathrooms | Typical FHR |
|---|---|---|
1-3 people | 1-2 bathrooms | 35-50 gallons |
3-4 people | 2-3 bathrooms | 50-75 gallons |
5+ people | 3.5+ bathrooms | 75+ gallons |
These ranges will vary depending on your household routines and even the plumbing fixtures you choose. A luxury soaking tub will use much more hot water than a low-flow showerhead.
Reservoir’s FHR: 75 gallons
We tested Reservoir in Balanced Mode in a controlled lab setting and got a 75-gallon FHR.
The standardized test parameters don’t allow us to test in Party Mode (where the heat pump and other additional heating elements run in parallel), but with those settings, Reservoir could actually put out more than 75 gallons of hot water in one hour.
Reservoir N51 has:
47-gallon physical tank: the water actually stored
75-gallon First Hour Rating: what you can draw on in your busiest hour, combining stored water with the compressor's ongoing recovery
150-gallon Virtual Capacity: what the unit can supply in Party Mode by running the heat pump and all three additional electric heating elements in parallel
In practice, that means a single Reservoir tank handles either a solo apartment dweller's long, luxurious shower or a busy family’s weekday morning, without you having to guess which model number to buy.
FAQ
Is a 50-gallon water heater big enough for a family of four? Yes, if its FHR is high enough, and that's the number that matters, not the tank size. A family of four typically needs an FHR in the 50-75 gallon range.
What's the difference between tank capacity and First Hour Rating? Tank capacity is how much water the unit physically holds. FHR is how much hot water it can deliver in one hour of high-throughput use, combining stored water with how fast it recovers. FHR is the number to shop by.
Do heat pump water heaters need a bigger tank than gas or electric models? Not necessarily. A heat pump's recovery rate is slower than gas, but it recovers continuously throughout the day rather than in short bursts. What matters is the FHR and virtual capacity of the specific unit, not the fuel type or tank size alone.
What size households does Reservoir work for? For households of 1-4 people, Reservoir is a great fit. For households of 5+, it could work. To see if it would work in your home, take our online Virtual Assessment and one of our team members will get back to you.