Heat Pump Water Heaters 101
What is a heat pump water heater?
A heat pump water heater (HPWH) heats water by moving heat from the surrounding air into the tank, instead of making heat by burning gas or running an electric coil. Because moving heat takes much less energy than making it, a heat pump water heater can use about a quarter to a third of the electricity of a standard electric tank. You'll also see them called "hybrid" water heaters, because most pair the heat pump with backup electric elements for busy days.
Most people don't think about their water heater until it stops working, which is the worst possible time to learn about your options. That's a shame, because water heating is about 18% of a typical home's energy use, second only to space heating. For a family of four with an electric tank, that can be more energy over a year than driving an electric car.
This guide is for anyone looking for a more efficient option. We'll cover how heat pump water heaters work, what they cost to buy and run, where they fit (and where they don't), and what a newer generation of predictive units adds.
How does a heat pump water heater work?
A heat pump water heater works like a refrigerator running in reverse. It pulls warmth out of the room air, concentrates it with a refrigerant, and hands it off to the water in the tank. A conventional electric tank, by comparison, works like a kettle or a toaster: electricity runs through a metal element that gets hot.
Here's the cycle, step by step:
A fan on top of the unit draws in room air.
That air warms a cold refrigerant, which turns into a gas.
A compressor squeezes the gas, which heats it up.
The hot refrigerant flows through coils wrapped around the water tank, and the water absorbs the heat.
The cooled refrigerant returns to the top, and the cycle starts again. The air the fan blows out is cooler and drier than the air it took in.
How a heat pump water heater works: like a refrigerator in reverse.
When demand is higher than the heat pump can keep up with, most units switch on their backup electric elements. Those elements are reliable but use three to four times more electricity for the same hot water, so how often a unit leans on them is the biggest factor in your real-world bill.
COP and UEF: the two efficiency numbers to know
Coefficient of Performance (COP) is how much heat you get out for each unit of energy you put in, at a given moment. A typical gas tank has an efficiency of around 0.6, because a lot of its heat goes up the flue. An electric resistance tank is about 1.0. A heat pump can reach a COP of 3 to 4, which means three to four units of heat for every one unit of electricity.
Uniform Energy Factor (UEF) is the Department of Energy's lab rating for comparing water heaters apples to apples. It fixes the room temperature, incoming water temperature, and daily hot water draw so every model is tested the same way. Efficient heat pump water heaters typically score between 3.3 and 4.0 or more. For reference, Reservoir Core and Max are rated at a UEF of 3.53.
UEF is a useful yardstick, but your basement isn't a lab. Real-world efficiency depends on your climate, the temperature of the room, and how your household actually uses hot water. Most heat pump water heaters are reactive, meaning they wait for the tank to cool before heating. If a few showers happen back to back, they often fall back on their backup elements to catch up. That's where predictive heating, which we cover below, makes a difference.
Want to go deeper on the parts inside the tank? See How Water Heaters Work (and Why They Fail).
How much does a heat pump water heater cost?
Most heat pump water heaters cost $5,000 to $10,000 installed in New England, before rebates. That's more upfront than a like-for-like gas or electric tank swap, which usually runs $3,000 to $6,000. The difference shows up later: a heat pump has the lowest operating cost of any common water heater type, so the true cost depends on both the sticker price and roughly a decade of energy bills.
Water heater type | Upfront cost, installed | Operating cost | Efficiency |
|---|---|---|---|
Gas tank | $3,000 to $6,000 | Medium | Lowest |
Electric resistance tank | $3,000 to $6,000 | Highest | Medium |
Gas tankless | $5,000 to $10,000 | Low | Medium |
Heat pump (typical) | $5,000 to $10,000 | Lowest | Highest |
Reservoir Core or Max (MA, after instant rebates) | $3,950 to $4,950 | Lowest | Highest |
Upfront costs from our Price, Specs & FAQ page. Reservoir prices include installation. Electrical work, if needed, is an additional $500.
Where the savings are biggest
The payback is fastest when you're replacing an electric resistance tank, because a heat pump does the same job with a fraction of the electricity. We ran the numbers in Electric Resistance vs. Heat Pump Water Heater: Should You Upgrade Now?. Replacing gas or oil can still save money and cut emissions, but the math depends on your fuel and electric rates, so it's worth running for your own home rather than taking anyone's word for it.
Rebates and financing
Incentives can close much of the upfront gap. When you choose Reservoir, we apply rebates directly to your quote: $1,050 in Massachusetts, $1,200 in New Hampshire, and $1,150 in Maine, with no forms or inspection on your end. Massachusetts homeowners can also finance with a 0% Mass Save HEAT Loan, and our 2026 Massachusetts rebate guide covers the rest. Incentives in New England are generous by national standards, but they've been shrinking year over year, so there's more reason to plan now than to wait.
Is a heat pump water heater right for your home?
Most homes can take a heat pump water heater, but three things decide how well it works: the space around it, your electrical setup, and how you handle the cool air it gives off. None of these is a dealbreaker on its own, but they're worth understanding before an installer shows up.
Space and airflow
Because it pulls heat from the air, a heat pump water heater needs enough air to work with: about 700 ft³, the volume of a space 7 by 10 by 10 feet. In a smaller closet or utility room, a louvered door or drywall vent gives it the airflow it needs. Unfinished basements are usually ideal. Heat pump units also need a way to drain condensate, the water they pull out of the air.
Cool air: a perk in summer, a tradeoff in winter
The air coming off the unit is cooler and drier, so it acts like a dehumidifier. That can be great in July, but in January, that same cool air can make a finished basement or a nearby room a bit chillier. Placing the unit in an unheated space avoids most of this. Some newer units, including Reservoir, also let you set a minimum room temperature so the heat pump steps back when the space gets too cold.
Noise
A heat pump water heater has a compressor and fan, so it isn't silent the way a gas or electric tank is. Older models earned a reputation for being loud. Newer ones are much quieter: Reservoir runs at 42 dBA in Quiet Mode and 48 dBA at most, which is in the range of a quiet refrigerator.
Electrical: 240 V vs. 120 V
Most heat pump water heaters run best on a dedicated 240 V circuit, typically 30 A. If you're replacing a gas or oil water heater, you may need an electrician to add one. There are 120 V "plug-in" models, and they can make sense when running a new circuit is hard, but they recover more slowly and give up some capacity. We can install Reservoir at 120 V as a temporary bridge, in heat-pump-only mode, when an electrician can't come out right away. The permanent setup is always 240 V.
Why your plumber might steer you away
Heat pump water heaters have been around since the 1940s, so it's fair to ask why they aren't everywhere. Energy prices fell sharply in the 1980s, which made cheap gas and simple electric tanks the default. Some early-2010s models from major brands had compressor and evaporator problems, and that history still shapes installer attitudes today. During an emergency replacement, a plumber who'd also need to coordinate an electrician will often recommend another gas tank. That's an understandable call on a busy day, and it's a big reason to plan your upgrade before your current tank fails.
What makes a predictive heat pump water heater different?
A predictive heat pump water heater learns when your household uses hot water and heats ahead of time, so it can rely on the efficient heat pump instead of the backup elements. Many newer units add an app and scheduling. Reservoir goes further: we designed it around prediction from the start, and we install every unit ourselves, meaning you get one price and one installation for one appliance.
Here's what that looks like in practice:
Thermal Intelligence learns your routine and heats water before you need it. Think of the tank as a thermal battery, charged when energy is cheap and ready when you need it. (How a water heater stores energy like a battery)
Virtual Capacity lets a 47-gallon tank act like it holds up to 150 gallons, by storing water extra hot and mixing it down to a safe temperature on the way out. Party Mode temporarily maximizes Virtual Capacity when you have guests. (What size water heater do you need?)
TOU Sync and Solar Sync shift heating to the hours when your utility's time-of-use rates are lowest or your solar panels are producing.
Home Protection watches for trouble. Leak Sense detects a leak and shuts off water to the tank, and Freeze Detection (Max only) warns you about pipes at risk of freezing.
No-Wait Shower (Max only) uses a built-in recirculation pump to bring hot water to your faucet before you turn it on. Always-on recirculation can waste energy because it runs all day, like a taxi circling your block in case you need a ride. No-Wait Shower is the taxi that waits at the stand and pulls up right when you're ready.
Quiet Mode keeps the unit at 42 dBA for basements next to living space.
It also tackles the most common reason tanks fail. Conventional water heaters rely on a sacrificial anode rod that corrodes over time, and once it's gone, the tank is next. Reservoir uses an adaptive electric anode instead and carries a 10-year hardware warranty. The Consortium for Energy Efficiency also named Reservoir a winner of its 2026 Integrated Home Competition.
To be clear about scope: Reservoir heats the water that comes out of your tap (and appliances like laundry machine and dishwasher). It doesn't replace your boiler or furnace. But in many oil-heated homes, the boiler also makes the hot water, and taking that job off the boiler can cut a meaningful share of oil use.
Heat pump water heater FAQ
Is a hybrid water heater the same as a heat pump water heater?
Yes. "Hybrid" refers to the pairing of a heat pump with backup electric elements, which almost every heat pump water heater has. The heat pump does most of the work, and the elements help on high-demand days.
Are heat pump water heaters worth it?
For most homes, yes, especially if you're replacing an electric resistance tank. They cost more upfront than a basic tank, but they use far less energy, and rebates can close much of the price gap. The savings are smaller if you have very cheap gas or very little hot water use.
Do heat pump water heaters work in cold climates?
Yes. What matters is the temperature of the room the unit sits in, not the weather outside. A basement in New England usually stays warm enough for efficient operation most of the year. Unheated garages and very cold spaces push units onto their backup elements more often.
Can a heat pump water heater run on a 120 V outlet?
Some models can, but they recover more slowly and give up capacity. Reservoir can run at 120 V in heat-pump-only mode as a short-term bridge until a dedicated 240 V circuit is installed.
How much space does a heat pump water heater need?
About 700 ft³ of air, or a smaller space with a louvered door or vents. Reservoir is 65.5" (1.66 m) tall, 24.8" (0.63 m) wide, and 22.8" (0.58 m) deep.
Will a heat pump water heater make my basement cold?
It can cool the room by a few degrees, which helps in summer and may be noticeable in winter. Place it in an unfinished space when you can. Reservoir also lets you set a minimum room temperature.
How long does it take to install?
With Reservoir, it starts with a free online Virtual Assessment: you share a few photos and details about your home, and we send a quote. Installation is typically a single visit, with electrical work handled as part of the same job if you need it.
Plan your upgrade before your tank decides for you
Heat pump water heaters are one of the most practical ways to cut a home's energy use, and the best time to switch is before an emergency forces a rushed choice. If you're curious whether one fits your home, start with our free, no-obligation Virtual Assessment. It takes a few minutes, and we'll tell you honestly if Reservoir isn't the right fit.
Efficient homes start with curious people.