Water Heater Wattage: Tank vs Tankless Electric
Heating water is one of the most energy-intensive processes in your home. Discover the massive wattage differences between standard tank heaters and electric tankless units.
The Silent Power Hogs
Next to the HVAC system, the electric water heater is the second largest consumer of electricity in the average home, accounting for about 18% of a typical household's total utility bill. Standard electric tank water heaters draw 4,000 to 5,500 watts, while whole-house electric tankless water heaters require an astonishing 18,000 to 36,000 watts.
Understanding the electrical requirements of water heaters is vital if you are planning to upgrade to a tankless system, or if you are trying to size a home backup generator.
Standard Tank Water Heaters (40-80 Gallons)
A standard storage-tank water heater works by keeping a large volume of water hot 24 hours a day. Most residential electric tanks (40, 50, or 80 gallons) contain two copper heating elements: one near the top, and one near the bottom.
Each of these heating elements is typically rated for 4,500 watts at 240 Volts. However, standard tanks are wired so that only one element can turn on at a time. When the top half of the tank gets cold, the upper element fires up, drawing 4,500 watts. Once it reaches temperature, it shuts off and passes control to the lower element.
Because they draw 4,500 watts continuously when heating, standard tank water heaters require a dedicated 30-Amp, 240-Volt electrical circuit.
Electric Tankless Water Heaters
Tankless water heaters do not store any hot water. Instead, they flash-heat the water instantly as it flows through the pipes. To raise the temperature of cold winter groundwater by 60 degrees in just two seconds requires a monumental amount of thermal energy.
A whole-house electric tankless water heater typically draws between 18,000 watts and 36,000 watts. To support this massive load, a large tankless heater often requires three or four dedicated 40-Amp, 240-Volt circuits, pulling a staggering 150 Amps in total. Many older homes require a total electrical service upgrade (from a 100A panel to a 200A or 300A panel) just to install an electric tankless unit.
| Water Heater Type | Typical Running Watts | Voltage & Circuit Required |
|---|---|---|
| Small Under-Sink Tank (2-5 Gal) | 1,440W | 120V / 15A |
| Standard Tank (40-50 Gallons) | 4,500W | 240V / 30A |
| Large Tank (80 Gallons) | 5,500W | 240V / 30A |
| Point-of-Use Tankless (1 Sink) | 3,000W - 6,000W | 240V / 30A |
| Whole-House Electric Tankless | 18,000W - 36,000W | 240V / (2x to 4x 40A breakers) |
Heat Pump Water Heaters (The Efficient Alternative)
If you want to drastically reduce the wattage required to heat water, the modern solution is a Heat Pump Water Heater (also known as a Hybrid Water Heater). Instead of using resistive heating coils, they act like a refrigerator in reverse. They pull ambient heat out of the air in your basement or garage and pump it into the water tank.
While a standard tank pulls 4,500 watts, a heat pump water heater operating in heat-pump mode only pulls about 400 to 600 watts. They are up to 400% more efficient, saving the average family over $300 a year on their electric bill.
Can a Generator Run a Water Heater?
Running a standard 4,500W electric tank water heater during a power outage requires a substantial generator—typically a 7,500W or larger portable generator. Because water heaters are hardwired, you must have a proper transfer switch installed by an electrician to power the circuit.
Running a whole-house electric tankless water heater (which demands 18,000W to 36,000W) is completely impossible for any portable generator, and would require a massive liquid-cooled industrial standby generator.