Traditional electric water heaters heat water quickly, but heat pump water heaters save money by heating water slowly. To size a heat pump water heater properly, you must select a tank one step larger than your traditional electric model. This means choosing a 50-gallon tank for one to two people, a 65-gallon tank for three to four people, and an 80-gallon tank for a household of five or more. You also need to verify that the unit’s First Hour Rating (FHR) meets your home’s peak hot water demand.
Quick answer
Step up one tank size when switching from a standard electric tank to a heat pump water heater. A larger tank ensures you have enough hot water during the slow recovery process and keeps the unit running in its highly efficient compressor mode to maximize your energy savings.
- Best for 1-2 people: 50-gallon tank
- Best for 3-4 people: 65-gallon tank
- Best for 5+ people: 80-gallon tank
- Heat pumps recover hot water at 8 to 20 gallons per hour, which is much slower than the 20 to 25 gallons per hour of standard electric tanks.
- You must match the First Hour Rating (FHR) to your home’s busiest hour of hot water use.
- Running an undersized unit forces the less efficient backup heating elements to turn on, erasing your expected energy savings.
- These units require 700 to 1,000 cubic feet of unconfined space to pull enough heat from the air.
- What size heat pump water heater do I need?
- How do you calculate First Hour Rating for a heat pump water heater?
- Do I need a bigger tank for a heat pump water heater than a standard electric one?
- Is a 50-gallon heat pump water heater enough for a family of 4?
- How does cold climate or incoming water temperature affect heat pump water heater sizing?
- What happens if I undersize my heat pump water heater?
- Physical dimensions and space requirements
- What do owners say?
- Frequently Asked Questions
What size heat pump water heater do I need?
You need a tank that can hold enough hot water to cover your household’s busiest hour of the day. Because heat pump water heaters (HPWHs) heat water slowly, you cannot rely on the tank to refill and heat up while you are actively using it. The tank capacity must hold the bulk of the water you need upfront.
The general sizing rule of thumb for heat pump water heaters is based on household size. However, this assumes standard water usage. If you have a large soaking tub or multiple teenagers taking long showers, you should increase the size further.
| Household Size | Recommended HPWH Tank Size | Typical First Hour Rating (FHR) |
|---|---|---|
| 1 to 2 people | 50 Gallons | 45 – 55 Gallons |
| 3 to 4 people | 65 Gallons | 60 – 75 Gallons |
| 5 or more people | 80 Gallons | 75 – 90 Gallons |
How do you calculate First Hour Rating for a heat pump water heater?
The First Hour Rating (FHR) is the golden rule of sizing. The FHR tells you exactly how many gallons of hot water the heater can supply in a single hour starting with a full tank. You must calculate your home’s Peak Hour Demand to ensure it stays below the unit’s FHR.
To calculate your Peak Hour Demand, identify the busiest hour of the day for hot water in your home. Usually, this is in the morning when people take showers before work and school. Tally up the total gallons used during that specific hour using the worksheet below.
| Fixture / Appliance | Average Gallons Used | Your Peak Hour Tally |
|---|---|---|
| Standard Shower (10 minutes) | 20 to 25 gallons (at 2 to 2.5 GPM) | ___ x 20 = ___ gallons |
| Bathtub Fill | 15 to 20 gallons | ___ x 15 = ___ gallons |
| Running Sink (Shaving/Washing) | 2 gallons per minute | ___ x 2 = ___ gallons |
| Dishwasher Load | 4 to 6 gallons | ___ x 5 = ___ gallons |
| Washing Machine (Warm/Hot) | 10 to 15 gallons | ___ x 10 = ___ gallons |
| Total Peak Hour Demand: | = _____ Gallons |
If your calculation shows you use 65 gallons of hot water between 7:00 AM and 8:00 AM, you need a heat pump water heater with an FHR of at least 65. If you choose a tank with an FHR of 50, the last person to shower will get cold water.
Do I need a bigger tank for a heat pump water heater than a standard electric one?
Yes. You almost always need a bigger tank for a heat pump water heater. The reason comes down to the recovery rate,the speed at which the unit heats incoming cold water.
Standard electric water heaters use dual heating elements that consume a lot of electricity but heat water fast. They typically recover 20 to 25 gallons per hour. Heat pump water heaters move heat from the surrounding air into the water using a compressor. This process uses a fraction of the electricity but takes much longer. Heat pump water heaters generally recover hot water at about 8 to 20 gallons per hour using only the compressor.
Because the unit cannot heat water fast enough to keep up with back-to-back showers, it needs a larger reserve of hot water sitting in the tank ready to go.
Is a 50-gallon heat pump water heater enough for a family of 4?
Usually, a 50-gallon heat pump water heater is not enough for a family of four. A household of three to four people typically requires a First Hour Rating (FHR) of 60 to 75 gallons. A 50-gallon heat pump unit generally offers an FHR of 45 to 55 gallons.
Decision Matrix: Will a 50-gallon tank work for four people?
- If your family staggers showers (two in the morning, two at night) and uses low-flow 1.5 GPM showerheads, then a 50-gallon tank will provide enough hot water.
- If your family takes four showers back-to-back in the morning using standard 2.5 GPM showerheads, then a 50-gallon tank will run out of hot water, forcing you to upgrade to a 65-gallon or 80-gallon unit.
How does cold climate or incoming water temperature affect heat pump water heater sizing?
Cold climates heavily impact heat pump water heater sizing because the incoming groundwater is significantly colder. The colder the incoming water, the longer the heat pump must run to bring it up to 120 degrees Fahrenheit.
Consider a worked example comparing a warm climate to a cold climate. We assume a four-person household taking four showers a day, using US average electricity rates of 17 cents per kWh.
- Warm Climate (Florida): The incoming city water is 70 degrees. The heat pump only needs to raise the water temperature by 50 degrees. A 65-gallon tank recovers quickly enough to keep up with demand in compressor-only mode.
- Cold Climate (Maine): The incoming winter well water is 40 degrees. The heat pump must raise the water temperature by 80 degrees. The compressor must work nearly twice as long. A 65-gallon tank might struggle, making an 80-gallon tank the safer choice to prevent cold showers.
If you live in a northern state, it is always safer to round up to the next tank size.
What happens if I undersize my heat pump water heater?
If you undersize your heat pump water heater, it will not run efficiently. Most modern units are “hybrid” models. This means they have an efficient heat pump on top and traditional electric resistance elements inside the tank as a backup.
Upsizing an HPWH tank allows the unit to run in heat-pump-only mode longer, maximizing the COP (Coefficient of Performance) which often ranges from 3.0 to 4.0. This means the unit creates three to four units of heat for every one unit of electricity it consumes.
Running an undersized HPWH forces the less efficient electric resistance elements to turn on frequently to meet your hot water demand. When the backup elements turn on, the COP drops back down to 1.0. This completely negates your expected energy savings.
From the Shop
We recently inspected a unit for a homeowner who complained their electricity bill barely dropped after installing a new heat pump water heater. They had swapped an old 50-gallon electric tank for a 50-gallon heat pump tank for their family of five. Because the tank drained so quickly every morning, the unit constantly switched into “High Demand” mode, running the power-hungry electric resistance elements just to keep up. Once they upgraded to an 80-gallon unit, the system stayed in heat-pump-only mode, and their water heating costs dropped by over $350 a year.
How does hybrid mode affect the sizing requirements?
Hybrid mode acts as a safety net. If you have guests staying over and your hot water demand spikes, hybrid mode will turn on the electric elements to heat the water faster. However, you should not rely on hybrid mode for your daily sizing requirements.
Your tank should be large enough that it can handle your normal, everyday peak demand using the heat pump alone. If your normal daily routine requires hybrid mode to trigger, your tank is too small.
Physical dimensions and spatial requirements for larger heat pump tanks
Sizing up your tank capacity means sizing up the physical footprint. You cannot simply squeeze an 80-gallon heat pump water heater into the same tight closet that held a 50-gallon standard tank.
First, check your ceiling height and door widths. An 80-gallon heat pump water heater is often over six feet tall and several inches wider than a standard unit. Add extra space for the plumbing connections on top and the condensate drain line.
Second, measure the air volume in the room. Heat pump water heaters work by pulling heat out of the surrounding air. Most manufacturers recommend 700 to 1,000 cubic feet of unconfined space for adequate airflow. If you put the unit in a tiny closet, it will turn the closet into a refrigerator, choke off its own heat supply, and stop working efficiently. If you must install it in a closet, a licensed contractor will need to install louvered doors or ductwork to bring air in from other rooms.
What do owners say?
Owners like
- Massive drop in electricity bills when properly sized to avoid resistance mode.
- Dehumidification effect in the basement is a nice bonus of the larger units.
- Wi-Fi app controls make it easy to boost capacity/switch to high demand mode before guests arrive.
Common complaints
- Ran out of hot water because we bought the same size as our old electric tank.
- The 80-gallon unit was too tall for our basement ceiling and wouldn’t fit.
- Recovery takes forever if you drain the tank and leave it in ‘Eco’ or ‘Heat Pump Only’ mode.
- The unit blows cold air, making the immediate space chilly in winter if it’s too small for the room.
Summarised from owner reviews and long-term user reports; individual experiences vary.
Frequently Asked Questions
heat pump water heater sizing
Proper heat pump water heater sizing requires selecting a tank that is roughly one size larger than a traditional electric water heater. A 50-gallon tank works for one to two people, a 65-gallon tank suits three to four people, and an 80-gallon tank is ideal for five or more people. You must also ensure the unit has 700 to 1,000 cubic feet of air space around it.
heat pump water heater sizing guide
To use a heat pump water heater sizing guide, you first need to determine your household’s peak hour demand. Count how many showers, baths, and appliances run during your busiest hour. Multiply each shower by 20 to 25 gallons. Your total gallon usage for that hour must be lower than the First Hour Rating (FHR) listed on the water heater’s energy guide label.
heat pump water heater sizing tool
A heat pump water heater sizing tool calculates your required capacity based on household size, fixture flow rates, and climate. When using one, always input the flow rate of your specific showerheads (e.g., 2.5 GPM) rather than relying on default low-flow settings, as this gives a more accurate picture of the First Hour Rating you actually need.
