If you are switching from a gas water heater to a heat pump water heater (HPWH), you might notice the new unit is physically larger or that your installer recommends a tank with a higher gallon capacity (e.g., replacing a 50-gallon gas water heater with a 65-gallon HPWH).
Sizing up is a best practice that improves comfort, efficiency, and savings. Even if the gallon capacity is the same, HPWHs are bigger because the heat pump compressor and fan assembly sit on top of the tank.
Here are five reasons why HPWHs are typically larger than gas units.
To provide enough hot water.
A water heater's "first-hour rating" is the amount of hot water it provides in one hour of continuous use, starting with a fully preheated tank. It accounts for the size of the tank (bigger means more preheated water) and the rate at which it heats incoming water (called the recovery rate).
First-hour rating is the best metric to check if a water heater can provide enough hot water for your house.
Gas reheats water very quickly because of its high-temperature combustion. A high recovery rate gives it a good first-hour rating, even with a small tank. And it's the reason gas tankless water heaters are common.
While HPWHs are incredibly efficient, they have slower recovery rates. To ensure you don’t run out of hot water during back-to-back showers, a larger tank provides a bigger buffer of preheated water to compensate for the slower recovery rate.
A common rule of thumb is to go one size up for HPWHs: from a 50-gallon gas water heater to a 65-gallon HPWH - or from a 65-gallon gas to an 80-gallon HPWH.
Heating water is like a pancake breakfast. You have a full table, and everyone's hungry for pancakes! They're going to get full eventually, but you need to deliver pancakes quickly, or they'll start complaining.
Tank size: Good news, you have a head start! Your serving plate is full of pre-made pancakes that are ready to go. This is equivalent to the size of your water heater. Bigger tank = more pre-made pancakes.
Recovery rate: How quickly can you make more pancakes?
First-hour rating: If plates are empty, people will start complaining. How many pancakes can you deliver in one hour? It depends on your number of pre-made pancakes (tank size) and how quickly you can make new ones (recovery rate).
To keep everyone happy (i.e. not run out of hot water), your tank size and recovery rate need to align to deliver a first-hour rating that matches your hot water consumption.
To avoid inefficient backup heating.
Most HPWHs are "hybrid" units, meaning they have backup electric resistance heating elements (like a standard electric water heater) to help when demand is high. However, these elements use significantly more energy than the heat pump.
A small tank depletes quickly, forcing the unit to use expensive backup heating to keep up. Whereas a larger tank holds more preheated water, so it can rely more on the efficient heat pump to slowly and steadily heat the water, keeping you in "heat pump mode" and maximizing your savings.
To improve efficiency in warmer temperatures.
HPWHs are more efficient when the surrounding air is warmer. A larger tank allows the unit to run during the warmest parts of the day (like the afternoon) and store that heat for evening or morning use.
This is particularly effective if you have solar panels, as you can use your own solar production to heat the water during the day.
To take advantage of low-cost (and low-carbon!) electricity.
A larger tank acts like a thermal battery. It allows you to heat a large volume of water when electricity is cheapest (off-peak hours) and store it for use throughout the day.
By "load shifting," the HPWH can coast through times when electricity prices are high without needing to turn on, lowering your utility bill. This can also reduce your peak demand and ease constraints on your electrical panel.
And the carbon intensity of electricity generation changes throughout the day. Off-peak electricity is generated with lower carbon resources, so shifting your consumption away from on-peak hours lowers your emissions even more.
And sometimes, to reduce installation costs (after incentives).
While a larger tank often has a higher sticker price, it may end up being cheaper or comparable after rebates. Many utility and government incentives are specifically targeted at high-efficiency HPWHs. Because the larger tanks offer greater potential for grid load shifting and energy savings, they are often the target of these financial incentives.
How big is big enough?
On average, a household in the United States uses 64 gallons of hot water daily, with the average individual using 20-30 gallons per day.
Consumption by Activity. Daily usage totals fluctuate based on household habits and appliance efficiency. Typical hot water consumption for common tasks includes:
• Showers consume 10 to 20 gallons, depending on the showerhead
• Bathtubs take 15 to 25 gallons to fill
• Laundry uses 7 to 25 gallons per load, depending on the machine
• Dishwashing uses 6 to 10 gallons per cycle
• Hygiene (shaving, hand washing, etc.) takes about 1 to 2 gallons
Showers and baths alone typically account for nearly half of a household's hot water consumption. Consequently, when sizing a water heater, experts recommend calculating the peak hour demand—the highest amount of hot water used during the busiest hour of the day—rather than relying solely on daily averages.
Houses with showers as a primary use benefit the most from drain-water heat recovery (DWHR) systems. They're easy for a plumber to install, contain no moving parts, and will last as long as the rest of the plumbing pipes.
DWHR can reduce hot water usage by 25% and increases the effective hot water supply from your existing water heater.