# Amps to Watts

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**1**Member

Thread Starter

Join Date: Sep 2005

Posts: 60

**Amps to Watts**

Some of you electrical guys probably know why, but I wish appliances posted the wattage as well as amperage on the tags. Most people are shocked(no pun intended) to hear the genny pull down when they turn on the coffee pot or hair dryer.

I used the conversion formula for each circuit/appliance when installing the transfer switch but why? Had to pull out the fridge etc to get the amps and do the math.

I also heard that after a hurricane or extended power outage that a lot of people had to replace refrigerators and other appliances because they had overloaded their generators and did not supply enough power to them causing damage.

I used the conversion formula for each circuit/appliance when installing the transfer switch but why? Had to pull out the fridge etc to get the amps and do the math.

I also heard that after a hurricane or extended power outage that a lot of people had to replace refrigerators and other appliances because they had overloaded their generators and did not supply enough power to them causing damage.

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**3**Senior Member

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**5**Senior Member

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If the device uses direct current [DC], then Volts x Amps=Watts.

This is not true for alternating current [AC]devices.

For AC, you first must convert the AC volts to Volts RMS] RMS is Root Mean Squared. Since alternating current changes its direction of flow 60 times per second [60 hertz, or 60 cycles] the voltage is not always the same. It varies with the cycle, both in intensity and direction of flow.

For "perfect" AC [a sine wave] the formula is a/ square root of 2. a is the peak amplitude of the wave, which for 120 volts would be 60. So 60 divided by 1.4142136 [the square root of 2] would be 42.4264.

For an A/C device using 120 volts, multiply the amps x 42.4264 to yield the wattage.

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**7**Admirals Club

Join Date: Nov 2008

Location: Somewhere in the middle of Michigan

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This is not correct.

If the device uses direct current [DC], then Volts x Amps=Watts.

This is not true for alternating current [AC]devices.

For AC, you first must convert the AC volts to Volts RMS] RMS is Root Mean Squared. Since alternating current changes its direction of flow 60 times per second [60 hertz, or 60 cycles] the voltage is not always the same. It varies with the cycle, both in intensity and direction of flow.

For "perfect" AC [a sine wave] the formula is a/ square root of 2. a is the peak amplitude of the wave, which for 120 volts would be 60. So 60 divided by 1.4142136 [the square root of 2] would be 42.4264.

For an A/C device using 120 volts, multiply the amps x 42.4264 to yield the wattage.

If the device uses direct current [DC], then Volts x Amps=Watts.

This is not true for alternating current [AC]devices.

For AC, you first must convert the AC volts to Volts RMS] RMS is Root Mean Squared. Since alternating current changes its direction of flow 60 times per second [60 hertz, or 60 cycles] the voltage is not always the same. It varies with the cycle, both in intensity and direction of flow.

For "perfect" AC [a sine wave] the formula is a/ square root of 2. a is the peak amplitude of the wave, which for 120 volts would be 60. So 60 divided by 1.4142136 [the square root of 2] would be 42.4264.

For an A/C device using 120 volts, multiply the amps x 42.4264 to yield the wattage.

http://www.ee.unb.ca/tervo/ee2791/vrms.htm

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**9**Senior Member

Join Date: Feb 2002

Location: Saugus, Ma. USA

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Let's put it in a boating (at least water related) example. Picture a motor driven pump which pumps water in a loop. The water exits the pump, goes through a loop of pvc pipe, and then enters the pump again.

The loop is the circuit.

The speed of the water is like the voltage.

The diameter of the pipe is like the amperage.

The total water passing through (gallons per hour) is a product of the speed of the water x the size of the pipe, so it's like wattage. Increasing the speed (higher voltage) or increasing the pipe diameter (more amperage) will increase the wattage. That's why generators care about watts, since it's like the total gallons per hour being pumped.

The loop is the circuit.

The speed of the water is like the voltage.

The diameter of the pipe is like the amperage.

The total water passing through (gallons per hour) is a product of the speed of the water x the size of the pipe, so it's like wattage. Increasing the speed (higher voltage) or increasing the pipe diameter (more amperage) will increase the wattage. That's why generators care about watts, since it's like the total gallons per hour being pumped.

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**10**Senior Member

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**12**Senior Member

Join Date: Apr 2009

Location: Michigan

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The loop is the circuit.

The speed of the water is like the voltage.

The diameter of the pipe is like the amperage.

The total water passing through (gallons per hour) is a product of the speed of the water x the size of the pipe, so it's like wattage. Increasing the speed (higher voltage) or increasing the pipe diameter (more amperage) will increase the wattage. That's why generators care about watts, since it's like the total gallons per hour being pumped.

Voltage is the pressure of the water (not speed).

Amperage is the flow of the water

The diameter of the pipe is better thought of as the resisance

Watts have nothing to do with time. It's power at a moment, so not like gallons per hour.

As a side note on how generators control speed and voltage/power. They set the gain so the voltage at 60hz is 240v (if 240v genny). If it gets hit with a load, it starts pulling down the voltage and speed. The system is closed loop on the speed so it opens up the throttle (quickly) to keep the frequency at 60hz, then keeping the desired voltage.

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**13**Senior Member

Join Date: Jun 2010

Posts: 706

For AC

VARS = voltage * current (volt Amp reactive)

Watts = voltage * current * power factor

The power factor is the sine of the phase difference between the voltage and current.

power factor varies between 1.0 for a pure resistive load to 0.0 for a pure inductive or capacitive load.

SO you can have current flow (amps) with out any power. (watts)

VARS = voltage * current (volt Amp reactive)

Watts = voltage * current * power factor

The power factor is the sine of the phase difference between the voltage and current.

power factor varies between 1.0 for a pure resistive load to 0.0 for a pure inductive or capacitive load.

SO you can have current flow (amps) with out any power. (watts)

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**14**Senior Member

Join Date: Aug 2005

Location: Wilmington, NC

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I would disagree with a few of these statements (Al wants a geek fight!).

Voltage is the pressure of the water (not speed).

Amperage is the flow of the water

The diameter of the pipe is better thought of as the resisance

Watts have nothing to do with time. It's power at a moment, so not like gallons per hour.

As a side note on how generators control speed and voltage/power. They set the gain so the voltage at 60hz is 240v (if 240v genny). If it gets hit with a load, it starts pulling down the voltage and speed. The system is closed loop on the speed so it opens up the throttle (quickly) to keep the frequency at 60hz, then keeping the desired voltage.

Voltage is the pressure of the water (not speed).

Amperage is the flow of the water

The diameter of the pipe is better thought of as the resisance

Watts have nothing to do with time. It's power at a moment, so not like gallons per hour.

As a side note on how generators control speed and voltage/power. They set the gain so the voltage at 60hz is 240v (if 240v genny). If it gets hit with a load, it starts pulling down the voltage and speed. The system is closed loop on the speed so it opens up the throttle (quickly) to keep the frequency at 60hz, then keeping the desired voltage.

x2 I like your analogy better, and the equations used for both systems are identical.

However, you are wrong on a few accounts. Power by definition is work per period of time.

Force = Mass x Acceleration

Energy(work) = Force/distance

Power = Energy/time

For example 1 hp equals the average amount of work a horse can do in 1 days time. Power = work/time, and 1 HP = 745 Watts

pressure x flow rate is also power in a fluid system. This is how you'd measure the power of a hydraulic motor, such as those in agriculture equipment skid steers etc.

Now let me step off my nerd pedestal, I knew my MS in Mechanical Engineering would pay off on this forum sometime.

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**15**Senior Member

Join Date: Feb 2002

Location: Saugus, Ma. USA

Posts: 11,153

Voltage is the pressure of the water (not speed).

Amperage is the flow of the water

The diameter of the pipe is better thought of as the resisance

Watts have nothing to do with time. It's power at a moment, so not like gallons per hour.

As a side note on how generators control speed and voltage/power. They set the gain so the voltage at 60hz is 240v (if 240v genny). If it gets hit with a load, it starts pulling down the voltage and speed. The system is closed loop on the speed so it opens up the throttle (quickly) to keep the frequency at 60hz, then keeping the desired voltage.

That is far easier to visualize, and in my example there would be very little water pressure anyway. And watts are related to time similar to gallons per hour when your meter reads kilowatt hours. Peopel who don't understand the subtleties can easily grasp that passing through a skinnier pipe will make the water go faster (transformer) or two pumps and two circuits next to each other increase the amperage but the voltage stays the same (like batteries in parallel).

This was meant to be a SIMPLE analogy that didn't require math and would be easily visualized.

So quit crapping on my example.

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**16**Senior Member

Join Date: Jun 2010

Posts: 706

x2 I like your analogy better, and the equations used for both systems are identical.

However, you are wrong on a few accounts. Power by definition is work per period of time.

Force = Mass x Acceleration

Energy(work) = Force/distance

Power = Energy/time

For example 1 hp equals the average amount of work a horse can do in 1 days time. Power = work/time, and 1 HP = 745 Watts

pressure x flow rate is also power in a fluid system. This is how you'd measure the power of a hydraulic motor, such as those in agriculture equipment skid steers etc.

Now let me step off my nerd pedestal, I knew my MS in Mechanical Engineering would pay off on this forum sometime.

However, you are wrong on a few accounts. Power by definition is work per period of time.

Force = Mass x Acceleration

Energy(work) = Force/distance

Power = Energy/time

For example 1 hp equals the average amount of work a horse can do in 1 days time. Power = work/time, and 1 HP = 745 Watts

pressure x flow rate is also power in a fluid system. This is how you'd measure the power of a hydraulic motor, such as those in agriculture equipment skid steers etc.

Now let me step off my nerd pedestal, I knew my MS in Mechanical Engineering would pay off on this forum sometime.

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**20**Senior Member

Join Date: Oct 2010

Location: Pensacola

Posts: 2,122

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