Utilities companies taking a beating while I charge my S

[ljwobker]

I think the larger issue isn't some relatively small fraction of energy lost. Rather, it is the load placed on the wider grid if many cars plug in at 20 kW during peak hours or when there's no surplus renewable power. In this time of declining electricity consumption (thanks to efficiency improvements), EVs are a very good thing for the utilities. But they are going to want to take steps to manage EV demand patterns.

Long term isn't this simply managed with demand-based pricing? Many places already have time-of-use charging for residential power. Most commercial customers pay more for power than residences, and where there's not explicit time of use rates, part of this is an assumption that commercial users draw more power during high demand times of the day.

The utilities and the grid operators like flat usage curves, because most of the generation capacity generates flat supply curves (they call this "base load" generation)... Although as solar and wind grow in supply share this may change -- if these sources become substantial parts of the input, it may change the demand/pricing curve to match.

The utopian setup is that almost everyone (commercial or residential) has a moderate capacity energy storage device, and the utility has the right and ability to push and pull energy out of this distributed storage as needed. But for this to really happen, the cost savings of having all this flexible storage and supply has to be higher than the cost to implement it all.

 

[wycolo]

Energy losses in a conductor are "I Squared R losses" - - easier to say it this way and sound like an engineer, too.
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[abasile]

Long term isn't this simply managed with demand-based pricing? Many places already have time-of-use charging for residential power.

Yes, exactly. In addition to putting residential customers on TOU (time of use) plans, some utilities might attempt to institute demand charges of some sort (basically extra fees for higher power draws). In California as in many other places, though, the utilities have to gain the approval of the relevant PUC (public utilities commission) to change their rate plans. So this won't happen overnight.

 

[David_Cary]

NC here. I have residential demand pricing (we are usually so far ahead of CA so this doesn't surprise me). The demand pricing has been available optionally for a while but not talked about much. In 2013, the utility had a solar rebate incentive and to get this mandated demand pricing for 5 years.

So smart meter was installed and I pay about $5 per kw demand (over 15 min) a month. This only counts peak time periods (we just have peak and off peak). So I can draw 20 kwh at night with no charge. But if I draw that in peak, I pay $100 a month for the privilege. My typical peaks are 5 but creep to 7 in the winter (electric hot water, heat pump). I look at the demand charge as my grid battery cost. Much cheaper than a battery for now.

The upside is that I pay $.05 for off peak kwh and $.06 for on. Very little difference, the demand is the difference. Usual rate is around $.105.

AZ also has residential demand charges.

The rate plan is very good for high users of electricity. Here in NC, the utility tries to charge based on cost and this rate plan presumably tracks their costs. So small users pay relatively more per kwh. Exact opposite of CA.

Interesting thing about the rate plan - off peak for 8 months a year includes 1-4 pm. Presumably, there is enough west facing solar that is underutilized outside of the summer. Fantastic time to run the heat pump as it is the warmest of the day. Things like this are the future of electricity charging (IMO)

 

[wycolo]

MrClown is saying that for a given charging setup, the %power lost to heat is the same whether you charge at 40 amps or 80 amps. $$ cost is also the same since the KWHs will be the same. The main difference will be that the wiring and EVSE equipment will operate cooler at the slower rate and possibly last longer. Thanx for the clarification.

Copper is no more efficient a conductor than aluminum as long as they are equivalently sized for resistance/unit length. But Cu is generally easier to handle and connect being smaller than Al. Hope this is still true.

Still not sure exactly why 'utilities are taking a beating' here. You are 'living better electrically' (GE's slogan from the '50s).
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[Cottonwood]

MrClown is saying that for a given charging setup, the %power lost to heat is the same whether you charge at 40 amps or 80 amps. $$ cost is also the same since the KWHs will be the same. The main difference will be that the wiring and EVSE equipment will operate cooler at the slower rate and possibly last longer. Thanx for the clarification.

Not true.

Let's take two cases with some typical numbers, each with a 240V source and 0.1 Ohm wiring resistance (R[SUB]w[/SUB]):

1. At 40 Amps, the Voltage lost in the wiring is 40 Amps time 0.1 Ohm or 4 Volts. 4 Volts * 40 Amps is 160 Watts. The power to the car is 236 Volts * 40 Amps or 9,440 Watts. The total power is 9,600 Watts, so 1.67% of the power is lost to heating the wires and 98.33% goes to the chargers in the car.
2. At 80 Amps, the Voltage lost in the wiring is 80 Amps time 0.1 Ohm or 8 Volts. 8 Volts * 80 Amps is 640 Watts. The power to the car is 236 Volts * 40 Amps or 18,560 Watts. The total power is 19,200 Watts, so 3.33% of the power is lost to heating the wires and 96.67% goes to the chargers in the car.

The 80 Amp case has twice the percentage power loss in the wires as the 40 Amp case.

For a give set of wiring, the percentage power lost to the wires rises linearly with the charging current (I). The actual power in the wires goes up as I[SUP]2[/SUP]*R[SUB]w[/SUB], but the total power goes up as I, so the percentage power lost to heat in the wires rises linearly with current (I).

 

[Johan]

Cottonwood is correct here. (I^2 is not cancelled by I^2 but divided by I yielding I). So it's not just that the power loss is being concentrated in a shorter period of time at a higher current, thus heating the wires more but the loss is IN ADDITION larger in absolute terms when doing let's say a full 80 kWh charge.

The other, untelated issue, is that the car's internal chargers and the charging process as a whole on the car side is more efficient at and above 40A than on lower currents. This has been extensively tested and measured for both the Roadster and the Model S.

So all in all, for total efficiency (considering both power feed and the car side of things) 40A is likely a sweet spot for a Model S (both 60 and 85).

 

[bollar]

The other, untelated issue, is that the car's internal chargers and the charging process as a whole on the car side is more efficient at and above 40A than on lower currents. This has been extensively tested and measured for both the Roadster and the Model S.

So all in all, for total efficiency (considering both power feed and the car side of things) 40A is likely a sweet spot for a Model S (both 60 and 85).

I'd like to see data for the Model S. In my testing, I've found no statistically significant difference at 20, 40, 60 or 80 amps. Perhaps someone with a larger dataset and different procedures can come up with different results.

 

[richkae]

Ohm's law says that the power lost in a resistor goes up with the square of the current.

For example, if you draw 80A and are losing 1600W to resistance in the wire, if you cut your draw to 40A, you will only lose 400W to resistance in the wire.

P = I^2 * R
1600W = 80^2 * R
R = 0.25 Ohm

0.25 Ohm * 40^2 = 400W

So really, the easiest thing to do is to charge at a slower rate unless you need to. It will also be easier on your equipment, too.

But at 40 amps you charge 2x as long as you do at 80amps.
So you will have half the energy loss at 40amps ( not a quarter ) that you do at 80amps, because energy lost (L) at 80amps will be L = 1600W * time and at 40 amps it will be L = 400W * time * 2

 

[MrClown]

Cottonwood, you're right. I made a mistake in my logic. The apparent resistance of the load (the charger) must decrease by half to get the current to double in the first place. The only resistance that is constant is that of the wire. Correcting for that mistaken logic does make my math yield the same result as yours.

I would still say that the power lost on the service feeder wires for wk's setup that I came up with before is a good approximation. The voltage drop he sees is due mostly to the regulator, not to actual power lost in the wire. Thus the losses for the utility are much less than he originally thought.

 

[3mp_kwh]

Are utilities taking a beating?

So smart meter was installed and I pay about $5 per kw demand (over 15 min) a month. This only counts peak time periods (we just have peak and off peak). So I can draw 20 kwh at night with no charge. But if I draw that in peak, I pay $100 a month for the privilege. My typical peaks are 5 but creep to 7 in the winter (electric hot water, heat pump). I look at the demand charge as my grid battery cost. Much cheaper than a battery for now.


The upside is that I pay $.05 for off peak kwh and $.06 for on. Very little difference, the demand is the difference. Usual rate is around $.105.


AZ also has residential demand charges.


The rate plan is very good for high users of electricity. Here in NC, the utility tries to charge based on cost and this rate plan presumably tracks their costs. So small users pay relatively more per kwh. Exact opposite of CA.

When a utility charges a 1 cent spread, peak to off-peak, and hits you with a demand charge, it makes your paying back their costs heavily dependent upon your load profile. I think eliminating demand charges at night makes a lot of sense (low load on the street/equipment). What doesn't make a lot of sence is Duke running so much baseload, at night, that they needed to scale a rate plan to suit when, and how, they generate their power. Technically, I think you could argue they're the ones using your battery.

 

[wycolo]

Energy to routinely charge your Tesla overnight is the same whether you charge at 40A or 80A - same energy and same $$ cost on your electric bill. Just to clarify. This should be made perfectly clear to every EV owner.

Power and [I squared R] etc confuse the issue for the EV owner. I said 'power' when I meant 'energy' in my last post. Also we can no longer edit 'old' posts!
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[wk057]

Energy to routinely charge your Tesla overnight is the same whether you charge at 40A or 80A - same energy and same $$ cost on your electric bill. Just to clarify. This should be made perfectly clear to every EV owner.

Power and [I squared R] etc confuse the issue for the EV owner. I said 'power' when I meant 'energy' in my last post. Also we can no longer edit 'old' posts!
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My original post was not about what it cost me, the EV owner. It was about the added losses on the utility side at 80A vs anything lower like 40A. I have no incentive to charge at 40A vs 80A, but the utility company would likely prefer that I do so.

 

[AWDtsla]

Granted, that comes out to about 1 kWh they lose for every 80 miles I drive, so it's not a whole lot. But I'm guessing if you added this up for every Model S owner the utility companies are probably losing a measurable amount of power here.

Keep in mind generation costs are a small part of your total electricity bill, and those numbers already have profit built into them. So its not nearly as bad for them as you thinking.

Secondly, this is a good reason to charge at low amperage on a regular basis unless you really have to leave in a few hours. This should be semi-obvious...

 

[mibars]

My original post was not about what it cost me, the EV owner. It was about the added losses on the utility side at 80A vs anything lower like 40A. I have no incentive to charge at 40A vs 80A, but the utility company would likely prefer that I do so.

Every utility company would love to see all clients draw constant power all day, with ideal power factor and no harmonics. That way they can deliver most energy with least losses on their side. But we don't live in ideal word and they take that variable load into account. So if you want to be nice to them you can try spreading load over longer periods, not only it decreases losses, but also makes infrastructure last longer.

Ah and since other clients aren't ideal too they actually prefer to have you draw more power off peak to balance the load.

 

[Tedkidd]

I have the E5

Can do a ton with the software too. I like it.

What kind of work do you do? Or just for stuff like this?

 

[wycolo]

Utility Co won't be taking a beating on the 14kv line until you sag ~20v below your no load '240v' nominal. Be the first Tesla to burn out their pole pig!
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[wk057]

What kind of work do you do? Or just for stuff like this?

It's useful for electronics hardware dev.

Utility Co won't be taking a beating on the 14kv line until you sag ~20v below your no load '240v' nominal. Be the first Tesla to burn out their pole pig!

Maybe. I plan on adding a second HPWC for the fiance soon if I can get the load calcs to work out, so, we'll see.

 

[tga]

It's useful for electronics hardware dev.

The local Home Depot has a Flir i7 for rental. I've picked it up a couple of times. I'd love to buy one, but I can't really justify the expense.

However, Santa brought me a Seek Thermal camera. I'll post a review at some point, and try to compare it with the i7.

A Thermal Camera for Your Smartphone | Seek Thermal