Volt-Amps and Watts

[curiouscarbon]

for DC circuits where the voltage is mostly "constant" over time, Volts x Amperes = Watts seems to work well.

this graphic has been handy for me a few times:



for AC where voltage is sine wave like




thank you all for helping me learn!

 

[Tulex]

I was taught to remember that if the unit is named after a person it is capitalised;
So V for volts (Alessandro Volta)
A for amps (André-Marie Ampère)
W for watts (James Watt)
K for kelvin (William Kelvin)

But unfortunately there are exceptions with name clashes; and because M is used for Mega (as m=milli) and G for Giga (g=gram), I guess that's why people put K for kilo so often.

So is MV Milli Vanilli?

 

[Plum Crazy Rob]

for AC where voltage is sine wave like

I wish I knew more Nerds to share this hilarious graphic with ...

 

[LakeHouse]

The value of 120 vac is the RMS or root mean square of the sign wave. Sign wave starts at 0 deg peaks at 90 deg and drops to zero crossing the zero axis and peaks at minus 180 degs or 270 degrees if you prefer. Then returns to zero and repeats. At 45 degs we use the cosign of 45 degs times the peak voltage to come up with 120 vac.

Again just simpler to say VA=WATTS.

There's just so much wrong with this.

The value of 120 vac is the RMS or root mean square of the sign wave.

This part is pretty much correct. 'Sine' wave, but the intent was clear enough.

Sign wave starts at 0 deg peaks at 90 deg and drops to zero crossing the zero axis and peaks at minus 180 degs or 270 degrees if you prefer. Then returns to zero and repeats.

Sign Sine wave starts at 0 deg peaks at 90 deg and drops to zero crossing the zero axis and peaks at minus 180 degs, then peaks again with opposite polarity at or 270 degrees (or negative 90 degrees) if you prefer.

At 45 degs we use the cosign of 45 degs times the peak voltage to come up with 120 vac.

cosine, not cosign... But this is just wrong. RMS is not 'the value at 45 degrees'. For a sine wave the RMS is the peak voltage divided by the square root of 2, and this coincidentally is the voltage at 45 degrees (because cos(45) is equal to 1/2^0.5).

Again just simpler to say VA=WATTS.

And you've missed the entire point of this whole post. VA would equal Watts if the voltage and current were in phase. For DC systems they are because the voltage doesn't vary.
But for AC systems they may not be in phase: for inductive loads (most ac motors, for example) the voltage and current both follow sine waves but they peak at different times.
Apparent Power (VA) is the RMS of (voltage curve * current curve). Note that this isn't the same as the RMS of the voltage curve * RMS of the current curve.
Whereas Real Power (W) is the average of (voltage curve * current curve).
In the extreme case where the voltage and current curves are separated by 90 degrees, the Real Power (W) can be zero while the Apparent Power (VA) is certainly not:

 

[scrubjaysnest]

There's just so much wrong with this.

This part is pretty much correct. 'Sine' wave, but the intent was clear enough.


Sign Sine wave starts at 0 deg peaks at 90 deg and drops to zero crossing the zero axis and peaks at minus 180 degs, then peaks again with opposite polarity at or 270 degrees (or negative 90 degrees) if you prefer.

cosine, not cosign... But this is just wrong. RMS is not 'the value at 45 degrees'. For a sine wave the RMS is the peak voltage divided by the square root of 2, and this coincidentally is the voltage at 45 degrees (because cos(45) is equal to 1/2^0.5).

And you've missed the entire point of this whole post. VA would equal Watts if the voltage and current were in phase. For DC systems they are because the voltage doesn't vary.
But for AC systems they may not be in phase: for inductive loads (most ac motors, for example) the voltage and current both follow sine waves but they peak at different times.
Apparent Power (VA) is the RMS of (voltage curve * current curve). Note that this isn't the same as the RMS of the voltage curve * RMS of the current curve.
Whereas Real Power (W) is the average of (voltage curve * current curve).
In the extreme case where the voltage and current curves are separated by 90 degrees, the Real Power (W) can be zero while the Apparent Power (VA) is certainly not:

View attachment 191300

Thanks for the spell checks and corrections.

 

[Checkthisout]

Not familiar with the Kelvin-Watt-hour...

It's the amount power in solar units produced in a fortnite in a leap year.

 

[scrubjaysnest]

At least someone understood my joke!

And I let you pull my chain!

 

[Danke]

It's the amount power in solar units produced in a fortnite in a leap year.

Does that apply to solar panels or inverters made in China ? They use a leap month instead of a leap year. If so, what’s the mathematical formula for that?

 

[SeaGal]

Does that apply to solar panels or inverters made in China ? They use a leap month instead of a leap year. If so, what’s the mathematical formula for that?

According to the depths of my Solis manual, there is a concept of WMM (the Watt Month), not to be confused with Wmm (the Watt minute) (or even confused by US readers who put month before date, hence the Wdd).

So, a Chinese WMM = 29+17⁄32 Wdd, where a Wdd = 24 x Wh. Then we have to factor in the leap months (7 of them in 19 years) = 235 months in 19 years as well as the 24 solar solar terms (roughly equivalent to seasons), one of which represents 'Pure Brightness' so we have to assume that has a positive effect on PV output; lets assume +10% for one month out of 12, leaving 216 regular months in 19 years and 12 pure brightness months in 19 years...

So, starting with a Chinese-Watt-hour (CWh), I get that back to (our) kWh using the following formula:-

kWh = ((((24 x CWh / 1000) x (29+17⁄32) x 216 / 19) + (24 x CWh / 1000) x (29+17⁄32) x 1.1 x 12)) / 365.25 / 24)

Caveat.... There is a slight error in the above formula re my assumption of our leap years - I haven't taken into account that year 2100 will not be a leap year, but I suspect my Chinese Solis inverter won't last that long

 

[wsaharem]

I can see occasionally that someone puts down just watts when they really mean watt-hours. It often can be inferred by the context of the post. But I sure do agree that some have a difficulty with understanding watt over time is how we measure electrical production or commonly called power. Especially it is the case when they have a bit more complication like running watts for less than an hour. 1000w for 15minutes? How is that possible?

Although the math is fairly basic it seems that many balk at applying it.

If kWh = kW * time and is 1 hour, if trying to figure out 1 minute, is it as simple as dividing the kWh number by 60? Yesterday was trying to get amps from Emporia, but that is all in kWh or Watts at the min / sec level. Or, if Emporia gives it to me in Watts, not kWh, then it's watts / Volts = current amp draw?

 

[MisterSandals]

kWh = kW+time

KWh = kW x time (in hours)

 

[wsaharem]

KWh = kW x time (in hours)

Thanks, corrected.

 

[OzSolar]

If kWh = kW * time and is 1 hour, if trying to figure out 1 minute, is it as simple as dividing the kWh number by 60? Yesterday was trying to get amps from Emporia, but that is all in kWh or Watts at the min / sec level. Or, if Emporia gives it to me in Watts, not kWh, then it's watts / Volts = current amp draw?

If you're trying to get amps on Emporia then

Click click on the hamburger on the top left, select "Units of Measurement" then select amps. Walla!

And yes, Watts/Volts = Amps

For example

1200 watts / 120v = 10 amps
or
1200 watts / 240v = 5 amps

 

[wsaharem]

If you're trying to get amps on Emporia then

Click click on the hamburger View attachment 194419 on the top left, select "Units of Measurement" then select amps. Walla!

And yes, Watts/Volts = Amps

For example

1200 watts / 120v = 10 amps
or
1200 watts / 240v = 5 amps

That’s very cool. Thanks for that!!