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The Best Time to Charge an EV

Uploaded: May 12, 2019
I was talking with my mom the other day about electric cars. Since I’d just written a blog post on our power grid, I reminded her not to charge her EV in the evening, since there is a steep ramp in demand then that’s difficult for the utilities to accommodate. She replied that she’s fine with that, she always charges hers in the middle of the night anyway when demand is lowest.

That got me thinking -- is that really the best time? Demand is probably low, but is there much renewable power in the middle of the night? When is our electricity the cleanest? When does it get windy, anyway? (It strikes me as funny how discussions about electricity these days often devolve into discussions about the weather. If you look at the CEC's annual report on electricity generation, it basically starts off with a weather report!)

So, let’s talk weather. Which season has the most renewable power? If you think about solar, you’d guess spring and summer have the most, and fall and winter the least. Hydro is also highest in spring and (early) summer, though only “small hydro” counts as renewable, and there’s not much of it. What about wind? Is that also imbalanced, with spring and summer yielding the lion’s share of renewables?

This chart shows monthly metered renewable supply on our grid for each month over the past few years. (1)


From the March 2019 monthly renewables report found at CAISO

You can see a pretty clear seasonal imbalance in this wave-like graph. We’ve got a lot more renewable energy in the spring and summer (the wave “tops”), including almost twice the solar and wind energy. (2) Large hydropower isn’t shown here (it’s not considered renewable), but you would also expect it to be highest in spring and early summer, making our carbon-free supply even more variable over the year. Only geothermal, biogas, biomass, and nuclear are carbon-free sources that are independent of seasons.

You can also see that our renewable capacity is growing over time, largely from solar. It looks like it’s gone up about 50% just in the last three years. That is pretty great!

So at this point we know we’ve got a big seasonal variation when it comes to renewable power. Seems like it’s better to charge our EVs in spring than in fall. What do variations look like during the day? When does the wind pick up?

Here are some pictures of our renewable power during a day in the middle of each season, spring through winter. (3) The x-axis shows the hour of the day, and the y-axis is measured in megawatts. These are taken from the Daily Renewable Watch reports at CAISO.


You can see that the sun shines in the middle of the day. Reassuring, that! It doesn’t shine for as long in fall and winter, and it’s a bit weaker, but still it looks like a good amount of power. And more good news, our wind power seems to counterbalance the sun somewhat -- it’s stronger when the sun is lower, at least in spring and summer. But winter looks like a problem, unless demand is really low on winter nights. There is no wind! And does the wind provide enough power even in spring and summer? It looks like we get up to 6 gigawatts of renewable power on a spring night. Is that enough? Does it meet demand?

Take a look at the demand curves below, also from CAISO. The x-axis shows the hour of the day, and the y-axis (too small to see) shows demand measured in megawatts. (The y-axes unfortunately have different scales, so use the annotations on the graphs themselves.)


What do you notice? It looks like there is variation in demand throughout the day, and also across seasons. Demand is generally highest in the evening, peaking around 6pm, but there’s also a morning bump (except in summer). Demand is much higher in summer, and lower midday in winter. The good news? We have more renewables when we need them, in summer, and demand is lower in winter when we have less renewable power. The bad news? The numbers don’t look so good. We said we had at best 6 gigawatts of wind power at night, and here we can see we need around 20. Do our other emission-free resources (large hydropower and nuclear) make up for it? Is my mom’s EV getting carbon-free power at night? If not, is there a better time to charge it?

In the graphs below, you can see what kind of power is actually on the grid throughout the day and night, again using those same four days to represent the seasons. Light green represents all the renewables, light blue is large hydropower, and that steady grey line around 2 gigawatts is nuclear. Those are your carbon-free power sources. The orange lines represent natural gas, with the darker orange coming from out of state. I’m including large versions of these, so you can see them more easily. When is our electricity the cleanest? Nights don’t look so good, do they? If you are charging at 2am, then much of the year you are basically filling up your EV with 70% natural gas. Yikes.


Power Supply on May 7, 2018


Power Supply on August 7, 2018


Power Supply on November 7, 2018


Power Supply on February 7, 2019

Cutting to the chase, the chart below shows the emissions rate for each quarter of 2018, measured in pounds of CO2 for each megawatt-hour of energy generated (source). As you probably guessed from the four sample days we looked at, midday is the best charging time throughout the year. Keep in mind that the low-emission window is shorter in fall and winter, more like 10am-3pm than 9am-4pm. Spring (red) has the lowest emissions overall, followed by winter (blue) because of lower midday demand. Fall’s emissions (purple) are worst, but about the same as summer (green) midday.


Do you see anything else interesting in these charts? I was surprised by the big seasonal difference in renewable supply (that wave-like graph near the beginning). How will we get clean year-round electricity, especially as we move our heating to electric power? I was also surprised by the shape of the demand curve. I had expected 2am demand to be much less than peak demand, but it’s two-thirds of peak. What are we doing at 2am to use so much power? Any other thoughts or observations on these charts?

Now that you understand all this (right?), you might be interested in an even more accurate way to determine the best time to charge your EV. Instead of looking at the emissions that are on the grid at a given time, you look at the emissions that would be added to the grid if you were to plug in your car. These are called “marginal emissions.” They sound a bit finicky, but they reflect how the grid actually works, so it’s important to use them for policies or technology that would impact electricity demand at scale. Stay tuned to learn more. In the meantime, consider midday charging!

Notes and References

0. You may be wondering what CAISO is. It stands for California ISO (not helpful), which stands for California Independent System Operator (also not helpful). More helpful: It is a non-profit organization that matches supply and demand of our electric power, and generally manages reliability of about 80% of California’s power grid. The information in this blog post pertains only to the grid managed by CAISO. Other electric markets will have different power mixes. You can find a map of them here.

1. The y-axis in this figure is mis-labeled, and should say TWh (terawatt-hours), not megawatts. A figure of 5 terawatt-hours of energy, measured over a period of 30 days (720 hours) represents an average power of about 7 gigawatts.

2. Does it seem counter-intuitive that we get more wind in spring and summer? In fact, California does have more wind in winter, particularly in the Sierras. But that’s not where we build our wind farms, due to extreme winds, icing, and general inaccessibility. We put them in “wind corridors” that are more accessible (e.g., near highways) and less subject to extremes. And those corridors happen to experience more wind in spring and summer. You can find more information here. Thank you to Michael Nyberg of the CEC for this pointer!

3. The days chosen are all mid-season weekdays: May 7, 2018 (spring), August 7, 2018 (summer), November 7, 2018 (fall), February 7, 2019 (winter). There is some variation within the seasons, but these days turn out to be largely representative except that the fall day was windier than typical.

Current Climate Data (March/April 2019)

Global impacts, US impacts, CO2 metric, Climate dashboard (updated annually)

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Comments

 +   1 person likes this
Posted by Resident, a resident of Another Palo Alto neighborhood,
on May 12, 2019 at 7:31 am

The best time to charge an EV is when it is below 25% of its full charge and for many it will take about 12 hours to charge to 75% charge capacity, the best level to keep the EV charged.

Depending on how many miles per day the EV is driven, makes a difference as to whether this is done on a daily basis or not. If an individual residence has more than one EV, the likelihood is that overnight on any day a car will need to be charged.

Unless PA utilities make overnight electricity cheaper, most will plug in as soon as they return home on their last trip of the day.

The power grid will have to accommodate EV usage. The power grid in the UK knows and expects when usage will occur - the British all like to put their electric kettles on to make tea after the evening soaps end, therefore power surge. Web Link


 +   2 people like this
Posted by eileen , a resident of another community,
on May 12, 2019 at 10:39 am

Looks pretty obvious that we need more charging stations at business locations and more rapid home chargers. To @Resident-- As to at what battery discharge level it is best to charge and to what charge level for an EV, there doesn't seem to be a consensus about this from what I read.


 +   2 people like this
Posted by Curmudgeon, a resident of Downtown North,
on May 12, 2019 at 5:19 pm

"How will we get clean year-round electricity, especially as we move our heating to electric power?"

Until we solve that problem, Palo Alto Utilities needs to carefully reconsider its campaign for 100% electrification. It has a gut appeal but, as you ably point out, our electric energy supply occasions carbon emissions at awkward times. 100% electrification might actually increase our carbon footprint. PA Utilities needs to base its planning and advocacy on realistic emissions analyses instead of easy wishful thinking.

Nuclear power is carbon-free.


 +   4 people like this
Posted by Nayeli, a resident of Midtown,
on May 13, 2019 at 10:26 am

Nayeli is a registered user.

@ Curmudgeon - I had a Chemistry professor in college who argued in favor of nuclear power. He was a staunch environmentalist and often used the classroom to share his thoughts on the matter. He said that the irony was that the biggest opponents of nuclear power were his fellow environmentalists.

I do think that California has taken the wrong steps toward clean energy and a cleaner environment. Instead of pushing for desalinization plants or more wind/solar/nuclear power energy options, we had a governor who focused upon a $100 Billion fast train from Sacramento to somewhere north of L.A.


 +   2 people like this
Posted by Sherry Listgarten, a Mountain View Online blogger,
on May 13, 2019 at 10:55 am

Sherry Listgarten is a registered user.

Great comments! A couple of notes...

@resident. I believe that the guidance for battery charging varies with the battery design. A while ago I read this fascinating article that explains (among other things) that a full battery is not really a full battery. I think it's best to follow the specific car manufacturer's guidelines for charging.

@eileen. I agree! People need convenient options for charging their cars midday. I think if they are available people will use them, if only to save on their home electric bill.

@curmudgeon. I am a big fan of pursuing electrification now (if not ten years ago) because it will not happen overnight. I believe we can walk (electrify) and chew gum (green our electricity) at the same time. Already, California’s electricity is a remarkable 53% carbon-free overall. I’m also curious what alternative you see to electrifying homes. Nuclear still requires electrification. I can’t imagine you are advocating for hydrogen to the home, though I read that some places in Europe have replaced up to 10% of their natural gas with hydrogen.

@nayeli. Keep in mind that California has some of the most aggressive and effective environmental policies in the country, if not the world, which is why our power is so green, our efficiency is so high, our vehicle emissions are so low, etc. Even our dairy farms are subject to recent environmental policies (cf SB 1383).


 +   3 people like this
Posted by Nuke LaLoosh, a resident of Palo Alto Orchards,
on May 13, 2019 at 11:59 am

Nuclear plants take 15 years to build and cost $10-15 billion per (at least, there isn't a recent example to gauge how ridiculously over-budget they would be IRL.)

What could we get for ten billion in renewables, ready to go in a year or two, versus waiting 15 years for the first drip from a nuclear plant? Add in improving renewable and storage technology over those 15 years, and the risk, plus storage for thousands of years, plus huge decommissioning problems, etc...

Nuclear is a pipe dream. Unless a pipe bursts, of course, and then it's a nightmare of unfathomable proportions. I wouldn't want to be down at CalPoly when the earthquake hits. Or on this beach over the next decade of decommissioning

Web Link

Let's look at real solutions available today, not the fantasies of our [adjective removed] college profs from decades ago.


re: battery charging optimization - I'm with Eileen; I read different things. I default on Lithium to the ol' "run it all the way down, charge it all the way back" to maximize battery life (where possible.) Wish I knew....


 +  Like this comment
Posted by Curmudgeon, a resident of Downtown North,
on May 13, 2019 at 1:34 pm

"@ Curmudgeon - I had a Chemistry professor in college who argued in favor of nuclear power. He was a staunch environmentalist and often used the classroom to share his thoughts on the matter."

Ten-four. I used to be against nuclear because of the waste. Then I realized n-waste is manageable, while CO2 dumped into the atmosphered is not. Too bad we don't have Prof. Tom Connolly of Stanford back to help advocate.


"I am a big fan of pursuing electrification now (if not ten years ago) because it will not happen overnight."

Well, then work into the next day. All I advocate is that the effort be based on rational fact-based planning instead of ideological zeal. PA Utilities has barked its 100% no-carbon electricity spiel for so long it has come to believe it (violating the carney's cardinal rule). Premature electrification could actually increase carbon emissions, which is not what we want.


 +   1 person likes this
Posted by Sherry Listgarten, a Mountain View Online blogger,
on May 13, 2019 at 9:25 pm

Sherry Listgarten is a registered user.

"PA Utilities has barked its 100% no-carbon electricity spiel for so long it has come to believe it"

That is not my experience when talking with them. In fact, the last chart in this blog post showing seasonal emission rates comes from their recent emissions accounting proposal.


 +   2 people like this
Posted by Jim, a resident of another community,
on May 14, 2019 at 5:01 pm

Sherry, thank you for another excellent and informative post!

For me, one of the big takeaways from this is that we need to accelerate the roll-out of smart meters in Palo Alto, which would allow CPAU to send customers price signals that would incentivize them to charge their EVs when grid power is cheapest (which also happens to be when it is cleanest!). Statewide we are moving in that direction, with the three major investor-owned utilities set to move all of their customers (20 million of them!) onto default time-of-use rates in 2019 and 2020.
Web Link

Shifting customer demand (through proper price signals) away from the evening peak period and into the mid-day period (when solar is sometimes so plentiful that solar generators have to shut down in order to avoid overloading the grid) will do a lot to help us integrate more renewables onto the grid. It should also reduce utility supply costs overall, which *should* translate to lower rates for all.


 +  Like this comment
Posted by Curmudgeon, a resident of Downtown North,
on May 14, 2019 at 5:56 pm

"... their [CPAU's] recent emissions accounting proposal Web Link."

A perfect case in point. As it states, "This report calculates the carbon content of the City's actual 2018 electric portfolio under a total of six different carbon accounting methodologies: ... ." That's six ways to calculate the carbon content of an allegedly 100% carbon-free electric portfolio.

At any given moment the carbon content of the grid which delivers our kiloWatts has a unique value, which the CAISO publishes Web Link. It is never zero.

In an earlier edition of this blog I called this report Laputan. One must nevertheless admire the ability of our Utilities department to hold seven mutually exclusive concepts simultaneously, none of which are real. The Red Queen would be speechless with envy.

When to charge an EV? Simple. When it's your turn at the charger.


 +   1 person likes this
Posted by Alan, a resident of Menlo Park: Belle Haven,
on May 15, 2019 at 9:33 am

The way I've understood it: some types of power plants can't be throttled; as long as they're online, they will be producing a certain amount of power. Any power not used will be wasted. So, therefore, to some extent, even if you are charging with a dirty source, you may not be incrementally adding to the amount of emissions being created; they were going to be created anyhow. Furthermore, during the day, if extra power is needed and there are not enough renewable sources to provide that power, natural gas power plants may be fired up to provide the extra power. If you charge up during the day, you may be increasing the need to fire those natural gas power plants.

Granted, over time they should be reducing the amount of baseline power produced by "dirty" sources at night, and they may have excess clean power during the daytime; then it would obviously be cleaner to charge during the day.

For someone with home solar with a time-of-use contract and net metering, the financial incentives are to sell your excess power to the grid during the day, and charge at night. I've taken this to mean that we're in the former situation - that they want us to make use of base power at night to minimize cost, and, hopefully, emissions. When that is no longer true, they should change the financial incentives.


 +  Like this comment
Posted by Alan, a resident of Menlo Park: Belle Haven,
on May 15, 2019 at 10:00 am

There's another factor (speaking someone living in Menlo Park, using Peninsula Green Energy): Currently, with a time-of-use rate schedule and net metering, the fastest way to get your money back for your solar panel investment is to sell as much power to the grid as you can during the day, and use the cheaper power at night. Regardless of what time of day has the most "green" energy, this situation is a big incentive for people to use their roofs to increase green energy production. While this is more costly than mass solar installations in the desert, it's limiting the impact of humans on nature. it may be more important to incentivize people to increase green power generation capacity, even if that means leveraging existing dirty power generation to make that financially attractive.


 +  Like this comment
Posted by Curmudgeon, a resident of Downtown North,
on May 15, 2019 at 11:58 am

"The way I've understood it: some types of power plants can't be throttled; as long as they're online, they will be producing a certain amount of power. Any power not used will be wasted."

No. Supply always must equal demand on the grid. There is no "spillway." If some plants cannot cut back, others must reduce their output.


 +  Like this comment
Posted by Sherry Listgarten, a Mountain View Online blogger,
on May 15, 2019 at 5:50 pm

Sherry Listgarten is a registered user.

@Jim. Yes, time of use rates should make a big difference. The trials to date have shown that charging less during low-emission hours is very effective in shifting demand away from peak load. And doing that should lower overall power costs, since the grid will have to ramp up the expensive power plants much less. I hope to see this in Palo Alto!

@Alan. Terrific question. I asked PCE about their incentives for solar, and they responded as follows: “A lot of existing solar customers are grandfathered into a Time of Use (TOU) rate (E-6) that DID incentivize maximizing output during the day with the Summer Peak period between 1-7 PM M-F. That rate schedule is no longer available to new solar customers and the newer TOU rates have shifted the "peak" hours to later in the day to better align with the imbalance of excess solar production throughout the state during the day and incentivize load shifting. As Alan mentions, if you are a legacy solar customer on the E-6 tariff it is financially in your best interest to maximize excess solar production during the day and shift flexible energy use (like charging an EV) to overnight. However if you are a newer solar customer taking service on E-TOU-A,B, or C then your "peak" pricing periods are either 3-8pm or 4-9pm and you could choose to charge your EV during the day directly utilizing excess energy that your solar system may be producing (if that's an option for you) without risking the 'loss' of any financial benefits from net metering.”

I understand that it is possible to switch from E-6 to E-TOU-A, B, or C, but you cannot switch back. If you have more questions, please follow up with PCE directly at info@peninsulacleanenergy.com, and feel free to share here what you learned. Thank you again for the terrific question.


 +  Like this comment
Posted by Naor, a resident of another community,
on May 15, 2019 at 10:32 pm

This is a very good post. It's not easy to distill the grid into layman's terms without oversimplification common in marketing.


 +  Like this comment
Posted by Alan, a resident of Menlo Park: Belle Haven,
on May 16, 2019 at 5:36 pm

@Curmudgeon

"No. Supply always must equal demand on the grid. There is no "spillway." If some plants cannot cut back, others must reduce their output."

Nuclear power plants cannot be throttled very quickly. While that energy might not be on the grid per se, the excess energy may be given off as heat. Sometimes, there are elaborate systems to pump water uphill via reversible generators. An example is the Ludington Pumped Storage Power Plant:

Web Link

They would've have invested hundreds of millions of dollars on this if it weren't true.

Likewise, with wind power, if energy isn't consumed or stored at the time of production, that energy will not be applied to human purposes. A natural gas powered plant can be throttled to match need relatively well, because the amount of gas burned can be varied quickly. This doesn't work as well for coal burning plants.


 +  Like this comment
Posted by Alan, a resident of Menlo Park: Belle Haven,
on May 16, 2019 at 5:57 pm

But ... apart from lost efficiency, there's the problem of cost of power generation equipment. The power company is more cost effective if they don't have to build as many peaking plants.

A simple example: Assume a power plant can produce 1 megawatt hour per day (small plant). Let's also assume their efficiency is perfect, and they can match demand with production perfectly, at all loads, 0 to 100% - no energy loss. Consumption per day is 2 megawatts hours, so you assume you would need 2 power plants. But it doesn't work that way. You might need 3 power plants; run only 1 in the middle of the night, 3 in the middle of the day. To handle this situation, they need to spend 3 times as much building power plants. Clearly, this company would want shift consumption to the nighttime, even if it had no impact on how energy efficient their electrical generation was.


 +   2 people like this
Posted by Michael O., a resident of Stanford,
on May 16, 2019 at 9:39 pm

The best time to charge my EV is when the rates at my home are the lowest: 11 pm to 7 am weekdays, Friday 11 pm to 3 pm Saturday, Saturday 7 pm to 3 pm Sunday. Sunday 7 pm to 7 am Monday. So that's what I'm going to do. Get the electric company to change my rates to different times and that's what I'll do.


 +  Like this comment
Posted by Sherry Listgarten, a Mountain View Online blogger,
on May 16, 2019 at 10:05 pm

Sherry Listgarten is a registered user.

@Alan. Great question. I deferred my answer in the hope you would read my next blog post, coming out soon. But since you ask again: Keep in mind that the utility in your case (which is responsible for handling 3 MWh in the day and 1 MWh at night) probably doesn’t own any plants. Or maybe just one. Because, as you say, it would be pretty inefficient. Instead, it participates in a power market and purchases power from shared resources (plants) that supply power to the grid. It doesn’t have to worry about the cost of building a plant, only the cost of purchasing the power it uses. All it worries about is reducing the sum of those prices. Prices go up when cheaper power (e.g., renewable power) isn’t enough to cover demand. The bigger the gap, the higher the price. Since prices are generally correlated with emissions, a utility that works to minimize prices is also going to be minimizing emissions. But the correlation is not perfect. In particular midday prices seem to be similar to prices in the middle of the night, even though midday emissions are lower. Maybe plants go offline midday for maintenance? I am trying to figure out why that is the case. A carbon tax would help with this...

So there are two issues. (1) Sometimes the TOU rates that utilities provide are not well correlated with current prices. And (2) Prices do not always correlate with emissions, and utilities optimize for price rather than emissions.

@Michael. Yup, you and nearly everyone else would make that choice. I’m curious which utility you use, maybe I can track down their thinking.

Great comments...


 +  Like this comment
Posted by Curmudgeon, a resident of Downtown North,
on May 17, 2019 at 12:42 pm

"Nuclear power plants cannot be throttled very quickly. While that energy might not be on the grid per se, the excess energy may be given off as heat."

It depends on the design. Baseline plants like Diablo Canyon,for example, areKa designed to operate efficiently at a stable burn. Reactors powering ships are designed to vary output frequently. Nuclear peakers would use the latter design approach.

Pumped storage installations have existed for over a half century. But their purpose isn't to sink excess energy; they store energy that is generated for storage. (BTW, they don't get it all back, but that's not caused by grid losses.) The grid remains balanced whether PS plants operate or not. Likewise wind or solar or bio or geo or ... .


 +  Like this comment
Posted by Colin Gould, a resident of another community,
on Jun 6, 2019 at 11:38 am

Great blogs and comments, thanks!

I'm in San Mateo with Peninsula Clean Energy community choice, and have chosen ECO100 which is supposed to contract for 100% renewable/GHG free power (50% solar/50% wind), at least for any consumed energy that doesn't come from my rooftop solar.

I've contacted them to confirm, but I assume this means that even if I charge my EV overnight (at cheap EV ToU rates, which I just switched to),
my energy *should* be emissions free, at least contractually so..?
Obviously won't come from MY roof at night, and won't come from other solar, but supposedly from some wind source?
If not, I'll have to figure on charging during weekend day or something, at a slow rate my solar can cover locally (3KW gen vs 8KW charge), when EV ToU rates are not too high.

Web Link


 +  Like this comment
Posted by Sherry Listgarten, a Mountain View Online blogger,
on Jun 6, 2019 at 12:50 pm

Sherry Listgarten is a registered user.

Colin -- I love your question! First, it is great that you are using PCE, and have chosen ECO100. PCE is doing everything that a clean utility should be doing, not only purchasing clean power, but investing in brand new renewable plants, and so helping to grow the base of renewable power. They also have ambitious goals around ensuring they are purchasing clean power when it is being used, rather than averaging it over the year. That is terrific.

These clean power plants are putting their renewable energy on the grid, per their purchase agreements with PCE. But the grid is not able to direct their output to your home. The power simply goes on the grid, and whoever happens to be plugged in at that point is who gets it. At the end of the day, in California, pretty much regardless of what your utility is doing, people plugged in at the same time are getting the same power, namely whatever is on the grid at that moment. This post describes that in a bit more detail.

You can check here to see what is on the grid. Right now (around noon) it is about 20% gas ("Imports" is generally gas). So it's worse than your "behind the meter" solar, but not by much. So you can charge at a reduced rate from your solar panels, or at a faster rate from the grid. You could purchase a home battery to charge midday and discharge in the evening, but it is expensive. Another option, if you want to charge at night without a battery, is to purchase an emissions-oriented smart EV charger (e.g., JuiceBox using JuiceNet Green), which would help you avoid peaks and generally be smart about when to charge.

It is kind of fun to play around with the power supply link to get an idea of what the power supply is like at different times of day, and where it is coming from. It varies with the seasons, and also some with weekend vs weekday, and will give you some idea of the tradeoffs when you do choose to plug your EV into the grid.

Either way, it is a very good idea to avoid charging from the evening grid (4pm-9pm), and that should be aligned with your TOU rates. If you just do that, it's a great start.

I hope this helps. Thanks again for the terrific question.


 +  Like this comment
Posted by Colin Gould, a resident of another community,
on Jun 7, 2019 at 3:46 pm

Thanks so much Sherry! Amazing information all around, especially the ISO link! Wish EV makers etc would give some (simplified) info like this to help EV car buyers be smarter and greener, given their large investments for a good (and fun) cause! I'll share this blog to local Tesla facebook group, if you don't mind, they'd love to hear!

Based on the emissions & renewables curves from ISO (and my own solar monitoring), to be both cheapest and least actual emissions,
I should try to charge *at home on weekend days (9a-3/5p)* ,
when both my own solar is strong and the grid is low-emission...
least strain on the neighborhood grid (panels straight to car!)
My EV-A rateplan has cheapest off-peak rates all day weekends luckily, same as late night (but that's when renewable sources are low),
so I won't pay any more to reduce actual emissions...
and I won't be losing much solar generation credit $ either.

At 30KWH generation per sunny day, I should be able to fill a half-"tank" each day... my <10k miles driven per year or 180 miles/wk @ 250Wh/mi, = 50KWH charging needed per week. I can do that in two sunny days.
My solar maxes at ~3KW power, while car is ~8KW...
I'd have to charge 'slow' at around ~10-12A/240V, not 24-32A I normally set it to. My tesla Model 3 allows me to set charge rate manually.

That should be good & easy enough manual solution to avoid paying extra $600 or so for JuiceBoxPro w/ JuiceNet Green, but that is an awesome smart-tech option, especially for those who need an EVSE anyway!

Web Link


 +  Like this comment
Posted by Sherry Listgarten, a Mountain View Online blogger,
on Jun 7, 2019 at 4:56 pm

Sherry Listgarten is a registered user.

@Colin -- I think your plan to charge midday is a great one. I think you would also be doing your utility a favor. My understanding is that utilities sometimes lose money on net metering when they pay their customers full rate for midday electricity even though it may not be worth much to them (or in worst case, negative prices, they have actually have to pay to put it on the grid).

It's fine to share this blog with your user group, and I'd be interested to hear if they have any other ideas, or improvements/suggestions/corrections.


Sorry, but further commenting on this topic has been closed.

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On Friday, October 11, join us at the Palo Alto Baylands for a 5K walk, 5K run, 10K run or half marathon! All proceeds benefit local nonprofits serving children and families.

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