Heat pump water heater (HPWH)
A heat pump water heater uses a refrigeration circuit to move heat from surrounding air into the tank, delivering 3–4 units of heat per unit of electricity — the most efficient water heater type sold, with UEF ratings of 3.0–4.5.
Most residential HPWHs are “hybrid” units: a heat pump does the everyday work, with electric resistance elements as backup for heavy demand. They cool and dehumidify the space they sit in as a side effect, which is a bonus in a warm garage and a consideration in conditioned space.
Installation needs: enough surrounding air volume (typically ~450–700 cubic feet unless ducted), a condensate drain, and tolerance for compressor noise similar to a refrigerator. 120 V plug-in models remove the wiring barrier for gas-replacement retrofits.
| Certified efficiency | UEF 2.52–4.5, median 3.7 (120 V models 2.8–3.5, 240 V models 3.3–4.5) |
|---|---|
| Compressor draw | Roughly 400–600 W while running — about the same as a household dehumidifier |
| Peak draw with elements | 240 V models list 12–30 max amps (median 22), so up to ~5 kW in electric mode |
| Annual electricity | 681–1,741 kWh certified (median 1,154) — about $116–$296 a year at $0.17/kWh |
| Air volume needed | Roughly 450–1,000 cubic feet of surrounding space unless ducted; 700 ft³ is the common figure |
| Compressor cutoff | Most integrated models stop using the heat pump below 37 °F ambient |
Where the heat actually comes from
A heat pump water heater is a refrigerator running backwards into a tank. A compressor circulates refrigerant through an evaporator that absorbs low-grade heat from the room air, then through a condenser wrapped around or inside the tank where that heat is released into the water. The electricity buys the pumping, not the heat, which is why one kilowatt-hour in produces three to four kilowatt-hours of hot water out.
The heat is borrowed from the surrounding space, and that has consequences. While the compressor runs it removes roughly 3,000–5,000 BTU/h from the room and condenses moisture out of the air — a genuine benefit in a humid basement or a hot garage, where the unit doubles as a dehumidifier. In conditioned living space during winter, that same heat has to be replaced by your furnace, so the net saving shrinks. Garages, utility rooms, and unconditioned basements are the standard recommendation for good reason.
It also explains the slow, quiet operating profile. Instead of a 4,500 W element blasting the tank for 40 minutes, a 500 W compressor runs for two or three hours. Total energy is far lower, but the recovery rate is roughly 10–15 gallons an hour against about 20 for resistance and 40-plus for gas — which is why storage volume, not burner size, is how these units buy capacity.
What it draws, and what circuit it needs
The compressor itself is a small load: typically 400 to 600 watts, comparable to a dehumidifier and well under a tenth of a resistance element. What sets the circuit size is the backup. A 240 V hybrid can fire a 4,500 W element alongside the compressor, and the certified records list maximum draws of 12 to 30 amps with a median of 22 — which, at the NEC 125% continuous-load rule, is a 30 A double-pole breaker on 10 AWG wire.
A 120 V plug-in model is a different animal: 4.5 to 15 amps in the certified data, median 12, so under about 1,400 watts. That fits an ordinary 15 A household circuit and is the whole reason the category exists — replacing a gas heater with one needs no panel work, no new circuit, and no electrician in many cases. The trade is recovery speed and, on most models, no resistance backup at all.
For annual consumption, ignore watts and read the certified kWh. Models on this site range from 681 to 1,741 kWh a year on the DOE test day, a median of 1,154 — roughly $196 a year at national average electricity rates, against about $700 for the electric resistance tank it usually replaces.
The four installation requirements that decide the job
Air volume comes first. Manufacturers specify a minimum surrounding space — commonly around 700 cubic feet, with some models permitting less with louvered doors — because the unit needs a reservoir of air to extract heat from. A sealed closet starves it. Ducting kits solve the problem by drawing from and discharging to another space, at the cost of duct runs and some capacity.
Condensate is second and is the requirement most often missed on a swap. Pulling heat out of humid air condenses water, so the unit needs a drain, and if there is no gravity drain nearby, a condensate pump. Third is temperature: below the compressor cutoff — 37 °F on most integrated models — the unit reverts to resistance heating, so an uninsulated garage in a cold climate quietly turns a UEF 3.9 appliance into a UEF 0.95 one for part of the year.
Fourth is noise and space. Compressor noise runs around 45–55 dBA, similar to a refrigerator but continuous for hours; that is unremarkable in a garage and intrusive on the other side of a bedroom wall. Physically these tanks are taller than what they replace — up to 92 inches in the certified data — and need overhead clearance for the compressor housing plus room around the air intake.
When it pays and when it does not
Against electric resistance the case is overwhelming: a saving in the region of $400–$500 a year at national average rates, which pays back the price difference in two to four years even before rebates. If your current heater is a plain electric tank, this is close to a default upgrade.
Against natural gas it is a real calculation rather than a foregone conclusion. A UEF 3.9 heat pump costs about $196 a year to run; a UEF 0.90 gas tank about $209. Those are close enough that local rates decide it — cheap electricity or expensive gas favours the heat pump, and the reverse favours staying with gas. Where the heat pump usually wins on a gas replacement is when the swap also removes a vent, a gas line, and a combustion appliance from the house.
It does not pay in a few clear cases: a small, sealed mechanical closet with no ducting route; an unheated space that spends winter below the compressor cutoff, unless it is a cold-climate split system; or a household whose demand pattern needs 40 gallons an hour of recovery, where the elements would run constantly and the efficiency advantage evaporates. Utility and state rebates commonly run several hundred dollars and change the arithmetic — check what your utility offers before pricing the job, and verify current federal incentive rules rather than assuming.
Frequently asked questions
How many watts does a heat pump water heater use?
The compressor draws roughly 400–600 watts while running. A 240 V hybrid model can add a 4,500-watt resistance element on top of that, which is why certified maximum draws run 12–30 amps at 240 V and a 30 A breaker is typical. A 120 V plug-in model tops out around 1,400 watts. Over a year the certified models on this site use 681 to 1,741 kWh, a median of 1,154.
Do heat pump water heaters work in a cold garage?
Down to a point. Most integrated units stop using the heat pump below about 37 °F ambient and switch to resistance heating, so in a climate where the garage spends months below that, you lose the efficiency advantage exactly when energy costs the most. Split systems with outdoor compressors — several rated below 0 °F — are the cold-climate answer, along with placing the tank in a basement instead.
How much does a heat pump water heater save per year?
Roughly $400–$500 a year compared with an electric resistance tank at national average rates: about $196 against about $700. Compared with a high-efficiency gas tank the gap is small — around $196 against $209 — so the answer there depends entirely on your local gas and electricity prices.
Are heat pump water heaters noisy?
They run at roughly 45–55 dBA, similar to a refrigerator, but for two or three hours at a stretch rather than in short bursts. In a garage or unfinished basement nobody notices. Directly below a bedroom or against a shared wall with living space, people do — check the model’s sound rating and the location before committing.
Does a heat pump water heater need a drain?
Yes. Extracting heat from room air condenses moisture, so the unit produces condensate continuously while running and needs either a gravity drain or a condensate pump. This is separate from the drain pan under the tank, and it is the requirement most often overlooked when converting a spot that previously held a gas or electric heater.