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# Amps to Watts

Member

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. Senior Member Join Date: Jun 2001
Location: CT
Posts: 18,361 There should be a tag on all appliances. If its a 120 volt appliance its the amperage times 120 equaly watts. If its 220 its amps time 220 equals watts. Senior Member

Join Date: Mar 2012
Location: Hooksett, NH
Posts: 1,625 Ohm's Law......http://www.ohmslawcalculator.com/ohm...calculator.php Senior Member

Join Date: Aug 2005
Location: Wilmington, NC
Posts: 4,447 Power= Voltage x Current,

Watts = Volts x Amps

Amps= Watts/Volts Senior Member Join Date: Nov 2007
Location: Richmond & Va Beach
Posts: 1,343 There should be a tag on all appliances. If its a 120 volt appliance its the amperage times 120 equaly watts. If its 220 its amps time 220 equals watts.
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. Senior Member

Join Date: Jun 2007
Location: Spring Hill, FL
Posts: 7,672 110/220 IS volts RMS

From many years ago:

I * V = P (current times volts = power) can be re-written as:

P/I = V or P/V = I Admirals Club Join Date: Nov 2008
Location: Somewhere in the middle of Michigan
Posts: 10,407 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.
Nope. 120 VAC is 120 volts RMS. You need a refresher course before you overload a generator. Here you go.
http://www.ee.unb.ca/tervo/ee2791/vrms.htm Senior Member     Join Date: Mar 2002
Location: Republic of West Florida - the ORIGINAL lone star state
Posts: 17,780 Geek fight. Cool. What about the forces applied when towing a skier from your T-top?

Big Al Senior Member Join Date: Feb 2002
Location: Saugus, Ma. USA
Posts: 11,153 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. Senior Member

Join Date: Feb 2012
Location: Quaint Little Drinking Village with a Huge Fishing Problem
Posts: 1,259 Picture a motor driven pump ........

I have an engine driven pump that needs motor oil in the crankcase. Don't ask me why.  Senior Member     Join Date: Mar 2002
Location: Republic of West Florida - the ORIGINAL lone star state
Posts: 17,780 Engine x Motor = \$ deficiency.

Big Al Senior Member

Join Date: Apr 2009
Location: Michigan
Posts: 3,913 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.
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. 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) Senior Member

Join Date: Aug 2005
Location: Wilmington, NC
Posts: 4,447 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.

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. Senior Member Join Date: Feb 2002
Location: Saugus, Ma. USA
Posts: 11,153 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.
Sure we can make it complicated, but I was keeping it simple. If you have 2 circuits with the same speed but one is a bigger pipe, you'll have more gallons. If you have 2 circuits with the same size pipe but one has a faster flow rate, you'll still get more gallons of water.

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.  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.
Power is an instantanous measurement. (Watts) Energy is power integrated over time (Watt hours or KW hours) Senior Member

Join Date: Feb 2012
Location: Quaint Little Drinking Village with a Huge Fishing Problem
Posts: 1,259 So far I've learned that Amps to Watts conversion is an exact estimate. Member

Join Date: Sep 2005
Posts: 60 Exactly...............this must be why manufacturers don't tag the appliance with wattage because they don't know what the hell it is.... Senior Member

Join Date: Jun 2007
Location: Spring Hill, FL
Posts: 7,672 Try this: ETA: In case you didn't know the unit of power is the watt. So power=watts Senior Member

Join Date: Oct 2010
Location: Pensacola
Posts: 2,122 Geek fight. Cool. What about the forces applied when towing a skier from your T-top?

Big Al
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