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How do I convert amps to watts?

Appliances frequently list their power requirements in amps. Most generators list their output in watts. Fortunately, it’s easy to convert from one to the other:

  • Watts = Volts x Amps
  • Amps = Watts/Volts

If you have two of the numbers (e.g. volts, amps) then you can find out the other (e.g. watts). This can help you to determine the rated power that you’ll need from your generator.

 

Starting vs. Running Wattage

Some appliances require extra power to start up, while others maintain the same power requirements constantly.

To calculate your power needs correctly, you need to know which kind of load you are dealing with. (A load is defined as the device that you are powering.)  There are two kinds of loads:

Resistive loads
Resistive loads are pretty simple:  they require the same amount of power to both start and run the equipment. Many resistive loads are involved in heating or making heat of some kind. Examples of resistive loads include:

  • Light bulbs
  • Coffee maker
  • Toaster

 

Reactive Loads
Reactive loads contain an electric motor, which requires additional power to start, but significantly less power to run once it gets going. Typically starting power is 3 times the amount of power to run the application.   Examples of reactive loads include:

  • Refrigerators / freezers
  • Furnace fans
  • Well pumps
  • Air conditioners
  • Bench grinders
  • Air compressors
  • Power tools

 

Some household appliances, like a furnace or refrigerator, have internal fans that come on intermittently. Extra wattage/power is needed to start the fan each time.  Refrigerators also have a defrost cycle that requires power in addition to the compressor and fans.

Reactive loads may also require additional power when the electric motor begins to work.  For example, when a saw begins cutting wood, its power requirement will increase. This is not applicable for most household appliances.

My appliance is a 1000 watt model, but it takes 1600 watts to run it. Why?

Some devices are labeled or promoted with a power number. For example, a hair dryer might say "1000W." This means the hair dryer itself produces 1000 watts of heat energy. But the amount the hair dryer uses from a power outlet is always more than it produces in heat. This is because the device’s energy use is not 100% efficient.

Another example is a microwave oven. It may be marketed as "1100 watt oven" and indeed produce 1100 watts of cooking power, but it will require more than that from a generator.

Translating the data tag:

For some appliances, you can determine the power needed by looking at the data tag supplied by the electric motor manufacturer.

All electrical motors should have a data tag attached to their bodies that give volts, amps, phase, cycles, hp, and sometimes a code.

  • Volts (V) - The volts must be either 120 (110-120) or 120/240. 120/240 means that the motor can be wired to operate on 120V or 240V. Honda generators are either 120V or 120/240V.
  • Amps (A) - Indicates the amps required to RUN the electric motor but doesn't consider STARTING or LOADED power requirements.
  • Phase (PH) - Honda Generators can only power single phase motors.
  • Horsepower (HP) - Rating of how much work an electric motor can perform.
  • Code - This isn't always provided on the data tag. It represents the maximum STARTING power required of the electric motor. You can multiply the code (amps) times the Horsepower of motor to determine starting amps. Find a list of codes and the amps here.
  • Cycles (Hz) - All U.S. electrical appliances run at 60 cycles per second.

To determine the wattage needed, use
Amps x Volts = Watts

Maximum vs. Rated Power

Generators are often advertised at the maximum wattage they can produce. But you’ll also see the “rated power” listed.

  • Maximum power = the maximum output that a generator can produce. Maximum power is usually available for up to 30 minutes. 
  • Rated power - the power that a generator can produce for long periods of time.  Typically 90% of the maximum power.

 

In general, use rated power to determine if a generator will be able to adequately power your applications continuously.

Wattage Estimation Guide

Approximate Starting Wattage Approximate Running Wattage
Air Compressor ½ hp 1600 1975
Air Compressor 1 hp 4500 1600
Bosch Grinder (8 in.) 2500 1400
Concrete Vibrator ½ hp 840 (avg) 840 (avg)
Concrete Vibrator 1 hp 1080 (avg) 1080 (avg)
Concrete Vibrator 2 hp 1560 (avg) 1560 (avg)
Concrete Vibrator 3 hp 2400 (avg) 2400 (avg)
Demolition Hammer 1260 (avg.) 1260 (avg.)
Drain Cleaner 250 (avg.) 250 (avg.)
Drills 3/8 inch, 4 amps 600 440
Drills 1/2 inch, 5.4 amps 900 600
Electric Chain Saw (14 inches, 2 hp) 1100 1100
Hand Drill (1/2 in.) 900 600
High-pressure Washer (1 hp) 3600 1200
Rotary hammer 1200 (avg.) 1200 (avg.)
Table Saw (10 in.) 4500 1800
Fan Duty ¼ horsepower 1200 650
Approximate Starting Wattage Approximate Running Wattage
Split Phase 1/8 Horsepower 1200 275
Split Phase 1/4 Horsepower 1700 400
Split Phase 1/3 Horsepower 1950 450
Split Phase 1/2 Horsepower 2600 600
Capacitor Start Induction Run 1/8 Horsepower 850 275
Capacitor Start Induction Run 1/4 Horsepower 1050 400
Capacitor Start Induction Run 1/3 Horsepower 1350 450
Capacitor Start Induction Run 1/2 Horsepower 1800 600
Capacitor Start Induction Run 3/4 Horsepower 2600 850
Capacitor Start Induction Run 1 Horsepower 3000 1000
Capacitor Start Induction Run 1 1/2Horsepower 4200 1600
Capacitor Start Induction Run 2 Horsepower 5100 2000
Capacitor Start Induction Run 3 Horsepower 6800 3000
Capacitor Start Induction Run 4 Horsepower 9800 4800
Capacitor Start Capacitor Run 1/8 Horsepower 600 275
Capacitor Start Capacitor Run 1/4 Horsepower 850 400
Capacitor Start Capacitor Run 1/3 Horsepower 975 450
Capacitor Start Capacitor Run 1/2 Horsepower 1300 600
Capacitor Start Capacitor Run 3/4 Horsepower 1900 850
Capacitor Start Capacitor Run 1 Horsepower 2300 1000
Capacitor Start Capacitor Run 1 1/2Horsepower 3200 1600
Capacitor Start Capacitor Run 2 Horsepower 3900 2000
Capacitor Start Capacitor Run 3 Horsepower 5200 3000
Capacitor Start Capacitor Run 4 Horsepower 7500 4800