How Many Watts Does a Water Heater Use? Tank, Heat Pump, and Tankless

A standard 40 or 50 gallon electric water heater uses up to 4,500 watts at 240 volts while it heats. Rheem's Professional Classic tanks use 4,500 watt upper and lower elements that never run at the same time, so the draw tops out at 4,500 watts, about 18.75 amps. A heat pump water heater uses far less most of the time, and a whole-house tankless electric unit can need 27,000 watts or more.

Water heater watts by type

Sources: Rheem Professional Classic and ProTerra specification sheets; A.O. Smith Voltex specification sheet; EcoSmart electrical requirements. Amps for the Rheem Classic = 4,500 W / 240V. EcoSmart watts are our calculation from its amps.

Why heat pump water heaters use less

A heat pump water heater moves heat from the surrounding air into the water with a small compressor and only uses its resistance element when it needs a boost. Rheem's ProTerra 30 A models are rated at a uniform energy factor (UEF) of 3.83 to 4.07.

Find your own number: the label on the side of the tank lists element wattage and voltage.

How much electricity does a water heater use?

Watts x hours / 1,000 = kWh. A 4,500 W element that runs a total of 3 hours a day uses 13.5 kWh, about $2.30 at 17 cents per kWh. How long it actually runs depends on how much hot water your household uses.

Can a generator run a water heater?

Yes, but it is a big 240V load. An electric water heater is hard-wired, so it needs a 120/240V generator connected through a transfer switch or interlock kit, and enough running watts for the 4,500 W element plus everything else.

• DuroMax XP9500iH: 9,500 starting / 7,600 running watts, 120/240V, inverter, CO alert. Room for the water heater plus a fridge and lights. $1,999
• DuroMax XP13000EH: 13,000 starting / 10,500 running watts, 120/240V, 14-50R outlet. Room for the water heater, well pump, and more. $1,299

One way to stay within your generator's capacity: leave the water heater breaker off and run it only when other big loads are off. Hookup options: How to Hook Up a Generator to Your House.

Can a battery run a water heater?

A home battery with 240V output can, for a while. The Jackery Explorer 5000 Plus puts out 7.2 kW at 120/240V (from $3,149), and the MyGrid 10K (10,496 Wh, 10 kW, $5,999.99 with a 50 A automatic transfer switch) runs a 4,500 W element for about 2 hours (10,496 x 0.9 / 4,500). A tankless electric unit is beyond portable backup power. For whole-house sizing, read What Size Generator Do You Need to Run a House?

Need help sizing? Call 1-888-855-7286. Send us the label from your water heater and what else you want running, and we will match you to a generator or battery system.

Water heater wattage FAQ

How many watts does a water heater use? A standard 40 or 50 gallon electric tank uses up to 4,500 W at 240V while heating.

How many amps does an electric water heater use? About 18.75 amps for a 4,500 W element at 240V. Rheem's ProTerra heat pump models draw 21 A max (30 A versions) or 12 A max (15 A versions).

How many watts does a tankless water heater use? A whole-house electric tankless unit like the EcoSmart ECO 27 draws up to 112.5 amps at 240V, about 27,000 W.

What size generator do I need for a water heater? At least a 120/240V generator with well over 4,500 running watts, such as the 7,600 running watt XP9500iH, connected through a transfer switch or interlock kit.

How much does it cost to run an electric water heater? About $2.30 a day if the element runs 3 hours, at 17 cents per kWh.

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