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- Placerville, CA, USA
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- Retired PV System Designer
If a woodchuck could chuck all the wood he would chuck,
how much wood would a woodchuck chuck?
He'd chuck all the wood that a woodchuck could,
if a woodchuck could chuck wood.
One prediction on electric cars
- Thread starter gar
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how much wood would a woodchuck chuck?
He'd chuck all the wood that a woodchuck could,
if a woodchuck could chuck wood.
I just knew would rise to the occasion with an equally erudite response.............
mivey
Senior Member
Time or duration both come into play for utility billing determinates. The kW for a meter is measured as energy divided by time and is an average value. The demand interval is important and the smaller the interval, the higher the kW value. The time of the measured demand is also important for coincident and non-coincident billing. The related temperature can also be important even though it does not show up in the units.
Watts is measured by the average of the instantaneous volt * current measurements over a cycle. Plotting the instantaneous voltage and current product over time is revealing about what happens with watts and vars but we don't really use the individual instantaneous values for much. It is the average value that is usually of interest.
mivey
Senior Member
PULL!!!
I don't disagree with any of that.
It doesn't change the fact that the watt is an instantaneous value.
Nor does whatever purpose you use it for.
Like speed is an instantaneous value. Of course you can an average to work out how long a journey will take and ETA.
My satnav does that and is usually pretty much on the money - but not today. It was a 100-mile trip each way, most of it motorway* and subject to the national 70mph speed limit.
OK, 100 miles, basic mental arithmetic and you get about an hour and a half. That's what Myrtle predicted. Today, between roadworks, traffic density and reduced mandatory speed limits it was over two hours.
I was late and I don't do late very well.
To add to my pain, I had to sit through a mandatory half-hour site induction - this for a site I've been going to for nearly 30 years.
Back on topic - electric cars.
The round trip was 200 miles. A shade over, actually.
Not really EV territory.
And that's one of the limitations.
Range.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
oh yikes no.
1W = 1J/sec
1kW = 1kJ/sec
its a rate for sure, and has time in it.
kWh = kJ/s * 3600s = rate x time, in this case the energy consumed when delta t = 1hr
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FionaZuppa
Senior Member
- Location
- AZ
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- Part Time Electrician (semi retired, old) - EE retired.
its not an avg, its the RMS voltage of the sinusoidal 60Hz, a quadratic mean of sine function.
1W is 1A*1V
It's as simple as that.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Yes, but current is a flow rate and therefore has a time dependency.
Rate. Not duration.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
a normalized delta t (duration),
1A = 1C/sec
simply just a count of charge over time.
to increase the energy you add in work thus raising the voltage, or reduce the resistance to increase amps.
yes, in whacky terms, W = AV
J = CV
A = C/s
W = J/s
thus J(t) = CV/sec * t
you can decouple time and look at work only
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Nothing whacky or wacky about it.
One amp times one volt is one watt.
No ifs or buts.
mivey
Senior Member
The topic was an average value. Just because it has the same units as an instantaneous value does not mean the average value becomes an instantaneous value.
If you are talking about DC then perhaps a discussion about instantaneous values can be useful. In the AC realm, we have relatively little use for the instantaneous value of power but focus on average values. Not that the instantaneous value for AC systems has no value as it can be enlightening when trying to understand the mechanism behind the exchange of energy.
I see a hybrid as the real practical alternative...as long as you don't mind the price.
mivey
Senior Member
The product of rms voltage and rms current gets you the apparent power, not the real power. You average the product of voltage and current for each instant to get the real power value we measure for metering.
The topic was electric cars.
And any such suggestion won't come from me - it would nonsensical.
EVs don't, as a rule, have AC systems. The on board vehicle energy source is a battery providing DC for motoring and absorbing DC on the regen or braking phase.
The motors are, I think, mostly of the brushless DC type. I know a little, not a lot, about these. We make some and I got a bit involved in the setting up of the testing.
Very efficient, above 97% in most cases. Sorry, I digress.
I agree. One of my colleagues (AR) had one. A Lexus 450h. Another now has a 200h. And my next door neighbour has a Toyota - same stable as the Lexus.
The electric propulsion system is eerily quiet. The first time I got in the 450h, I thought AR had forgotten to start it - until my brain kicked in!
We drove out of the service area (rest area) where we met and pulled on to the highway. The big V6 kicked in. No, that's wrong. In blended in seamlessly and, all but inaudibly.
Our trip was into London with the usual crawling traffic. EV mode engaged. As AR commented, "You have to be careful with pedestrians. There is no noise to warn them of your approach."
So yes, I agree, hybrids are here and a practical alternative.
mivey
Senior Member
When you were trying to help clear up FionaZuppa's confusion with units it was. Then the discussion turned to POCO charges, utility rates, and peak demand charges. See below:
It was at that point I disagreed with your reply.
Of course, but not what was being discussed at that point.
That is an interesting observation I had not thought about.
It may have been, but not by me. And really, it was something of a red herring in the context of the original post.
Maybe, but it doesn't change the fact that the watt is an instantaneous value.
I'm not disagreeing with you for the sake of disagreeing - you're much too bright for that and show a lot of common sense.
Welcome to the forum!!
A couple of points if I may.
The Tesla may be pollution free at point of use. But very probably not at the source of energy for the batteries. That said, the emissions per mile including those at the source are usually only a fraction of those from a gasoline ICE powered car.
How soon will EVs become mainstream to the point it ousts gasoline cars?
Not very soon in my opinion. Cost, range, and recharge times are obstacles at the moment.
Another potential issue is the electrical supply infrastructure.
If everybody in a street has an electric car, the supply would most likely have to beefed up. Roll that out nationwide and you can begin to see the scale of what would have to be done.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
So Watt, I mean so what? It's a distinction without a difference. My point was that some POCO's charge not only a rate per kWh, but add a peak demand charge based on kW. It makes no difference if they determine that by measuring kWh over a short duration of time; they express it as peak kW and levy a flat surcharge based on the height of that peak.
Of course kW is a rate and it's an instantaneous value. I never said otherwise. Jeez.
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