How many amps does a water heater use? You will need to know the current draw if you want to adequately size a breaker, a wire, or if you want to run a water heater with a generator.
Water heater amp usage is quite easy to figure out. Check the label on the water heater and you should find the specified water heater amperage.
Even if you don’t find it, you can deduct how many amps does a water heater use from wattage and voltage using the basic electric power equation:
I (Amps) = P (Watts) / V (Volts)
Namely, we know that most water heaters (30-gallon, 40-gallon, 50-gallon, 60-gallon, 70-gallon, and 80-gallon) use 4500 watt water heaters that run on 240V circuits. Here is how we can calculate the water heater amp draw:
Amp Draw (4500 Watt Heater) = 4500 Watts / 240 Volts = 18.8 Amps
In short, most commonly sized water heaters use 18.8 amps when running at 100% output.
Obviously, smaller 10-gallon and 20-gallon water heaters will draw fewer amps (even if they run on 120V circuits), and bigger 80+ gallon hot water heaters will draw more amps.
Here is the full chart that tells you how many amps do different hot water heaters use:
Water Heater Amps Chart
| Water Heater Size: | Amperage: |
| 10 Gallon Water Heater | 8.3 Amps – 16.7 Amps (120V) |
| 20 Gallon Water Heater | 8.3 Amps – 16.7 Amps (120V) |
| 30 Gallon Water Heater | 18.8 Amps |
| 40 Gallon Water Heater | 18.8 Amps |
| 50 Gallon Water Heater | 18.8 Amps |
| 60 Gallon Water Heater | 18.8 Amps |
| 70 Gallon Water Heater | 18.8 Amps |
| 80 Gallon Water Heater | 18.8 Amps |
| 90 Gallon Water Heater | 18.8 Amps Or More |
| 100 Gallon Water Heater | 18.8 Amps Or More |
As you can see, most water heaters will run on 18.8 amps.
Example: How many amps does a 50 gallon water heater use? Well, it has a standard 4500 watt heater running on a 240 volt circuit. That means it will draw 18.8 amps of electric current.
The smallest water heaters (10-gallon and 20-gallon) can run on as low as 8.3 amp current (these have the smallest 1000 watt heaters). Most commonly, 1600 watt heaters are used for smaller boilers. These run on 13.3 amps when operating at 100% heating output.
The big 100-gallon water heater might require a lot more than 18.8 amps to run. If it has a 10,000 watt heater, the amp draw will be closer to 41.7 amps.
If you are sizing a water heater breaker or a wire, be sure to check exactly what is the maximum amp draw for your water heater. This will allow you to adequately choose a big enough breaker or wire size and will avoid going against the code or even avoid your circuit catching fire.