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Electric cars as peak grid power? Not small ones, but perhaps the Tesla


An article in the LA Times suggests an idea I've seen frequently -- use electric car batteries to meet peak power demand on the grid. After all, you have a car, and it's plugged in, and it has a big battery, so instead of just charging it, have it send juice back to the grid when it most needs it.

The reason this is attractive is that a large part of the cost of the grid is building it to handle the peak load. Most of the capital cost is for that, and fuel costs are based on the real, variable load. Softening the peak is very valuable to the power company -- to the point that power companies give rebates and credits to people who do things that will soften that peak.

This is also one of the virtues of solar. It tends to provide power during the day, which is always when the peak is. However, solar peaks at noon, while the demand peak is the hottest part of the day, which tends to be later in the afternoon. The big peak tends to be around 4-6pm when it's hot, and people have started turning on things in their houses to get ready for dinner. On the spot markets power costs the most then.

Contrast that with the night. Because nuclear plants and some big coal plants aren't easy to dial back, then sometimes even produce more power than is being used, and they end up discarding the power into giant resistors. That makes power at night cheap.

I've never seen it done, but there could even be merit in the idea of mounting fixed solar panels pointing west, so that they catch less power in the morning but do better in the later afternoon when the price of electricity is highest. I presume this doesn't happen because net metering home owners don't get access to the "true" spot power price which would justify this. If they are lucky they do get time-of-day metering so they sell power at a high price in the day and buy it cheap in the evening, but some don't even get that. The harsh reality is that most grids were not built to have a lot of generation at the edges, and power companies are pushing back on net metering and grid-ties that feed back too much power. Indeed, for cost reasons here in California, people should size their solar systems to not quite meet needs, and buy the rest at the cheap "tier 1" price, rather than try to sell back.

Most solar panels are erected facing due south, tilted to the latitude which maximizes total kwh, but peaks at noon. Actually, most are mounted on a section of the roof that is closest to south. If you have to choose between SE and SW, it might be that SW is best, at least for the grid. (Sadly, a number of solar panels are mounted on the front of houses, even if that points north! People are more keen on looking good than doing good. I hope that's rarer than I've been told.)

Anyway, back to the cars

There are a few issues with using the batteries in the car for the peak load.

  1. The peak time is unfortunately a very popular time for driving. People either want to drive in the late afternoon -- it is called the rush hour for a reason -- or they plan to drive soon and want their car's battery to be full to meet their driving needs. They don't want to find their car half-empty at 6pm because it sold power to the grid. A study of car usage patterns detailed the numbers.
  2. The batteries in cars are expensive. Charging and discharging the battery uses up its lifetime. We don't know how long car batteries are going to last but a typical estimate is around 150,000 miles, or about 40,000 lifetime kwh. If it's the 22kwh pack in the LEAF (which costs $12K or so today) that's 27 cents/kwh lifetime. Plus the cost of the electricity that went in to be resold. The peak price ranges from 25-30 cents/kwh in the west but hits as much as 48 cents in New York. So it could be profitable in New York, but barely so. Big, heavy lead batteries are more cost effective.

There are some factors, though, which could change this:

  • Battery packs will get cheaper, and their lifetimes will increase. That will drop the cost of putting a kwh into and out of a battery.
  • Cars like the Tesla model S have huge batteries, far more than they actually need. This, it turns out is quite wasteful, since you buy a lot of battery and rarely use it. If you know you don't plan a 200 mile trip, you might be tolerant that your long-range car is half-empty at 6pm, and happy to sell that excess capacity. You already paid for the capacity, after all to give you that long-trip freedom. You will still shorten the battery life, but you'll be paid for that.
  • Weather forecasts are getting quite accurate, so demand can be predicted and this managed better.
  • The car can also be a backup in the event of grid power outages. There, the 35 cent/kwh price (and loss of driving ability) are minor compared to the burden of having no power in your home.

Calling all cars!

Now, as you might expect on this blog, robocars are also game changers here. The inverters and equipment to feed power back to the grid are expensive, so most people won't have them. But if the robocars have a means to plug in, they can bring the power to where it's needed. A power company, seeing a brownout coming, could send out an alert on the net. "Calling all cars" -- if you have spare capacity, we'll buy it at the following rate. Please drive to the nearest two-way intertie and plug in soon. While ideally some sort of automatic connection would be possible, this could even be a charging lot with human staff who plug in the cars as they arrive and unplug them when they have to leave.

Such charging lots might well exist for cars that need charges at night or other non-peak times. Due to cost, cars will strongly wish to avoid charging at peak cost times. This puts them to use then. Inductive charging also works (at a loss of about 10%) and robotic plug-in is actually quite doable -- there are already robotic gasoline filling stations out there. A robocar charging lot could be dense-pack, valet style, so not take a lot of land. But it would take megawatts -- but that's OK. The robots don't care how convenient it is, so put it next to the transformer station.


Most Tesla owners set the charge time setting in their car for 12 midnight, when electricity rates are at super-off-peak levels. Here in San Diego, SDGE offers an EV-TOU rate discount for electric vehicles, and encourages people to charge only after midnight. Me, I never charge during the day, ever. I don't know any Tesla owners who do.

The grid is not designed to be "fed back into" at scale. I think what we'll see instead is local storage with maybe a trickle back into the grid.. Quite possibly with Tesla batteries. That's the great thing about these big 85kWh Tesla Model S batteries: once the warranty passes, they can have a new life as local storage. Could wind up being a multi-billion dollar business for Tesla and SolarCity.

Not sure I understand, yes, I believe I said that people don't want to charge at peak due to the price.

The grid is not meant to be fed back into, as I said, but that doesn't mean a large battery car like the model S might not provide enough power to run the air conditioner and other items at 5pm at a house (or office) it's plugged into. Indeed, a smart system could make sure no more power is fed back to the grid than it can handle. That's easy, the grid could (under new laws) just not pay for more than it can handle.

Perhaps used electric car batteries can have new life as local storage, and that might be better than recycling them for the lithium, but otherwise they are not great competitors. They are engineered to be light and hold the most kwh per kg and per liter. They are also designed to discharge fast when you zoom, and charge fast when you need it, and this is where the money goes. And they are not expected to last more than the life of the car. Grid storage batteries are different. You don't care about the weight or size that much, and you don't need to charge them super fast or even discharge them that fast either.

The only real win is that the Tesla model S shows people will put way more battery in their car than they need, and as such they might as well find a use for it. I mean how many trips of more than 150 miles do you take in a year? I probably take only a dozen, and so do most people, but everybody is buying the 85kwh Tesla in spite of that.

It should be pointed out that the mere rise of electric cars will help improve the overall efficiency of the power grid. Most will likely be charged overnight, when as you stated demand is currently low. Once electric cars are popular that will make a huge difference to overnight demand.

Serious deployment of electric cars would generate a lot of new electrical demand. That a lot would happen at night is good for the grid, but there is still money to be spent.

Many people will also be keen to charge in the morning after they get to work. You like to keep your battery between about 50% and 80% if you can. The morning is not a peak time, but it's not the night either.

Cars like the LEAF and other 80 mile range electrics essentially make you want to charge wherever and whenever you can.

Would power companies like to sell me that power for cheap? If I'm using my gas furnace, I'd be happy for the power company to turn on an electric heater in my house for a while, if they'll charge me less than I was going to pay for the gas for that period.

Generators use their load banks mostly for testing and emergencies. For example, if a load suddenly disappears (such as power lines go down or a segment of the grid goes down) the power has to be moved somewhere quick. However, from time to time load goes so low that they need to use the load bank at night, though I don't know precisely how often that happens.

Wind turbines use load banks too, because when the wind surges if there isn't a load for it you have to put it somewhere or the equipment can get damaged. You can shut them off, but strangely, due to subsidies (that you don't get if you shut them off) in some areas wind farms have caused the spot price for power to be negative! Not so much that retail customers get paid to take power but that generators who insist on putting power in the grid have to pay to do so.

You can buy this power from them, in the sense that to avoid it happening, they offer strong incentives for heavy power users to do their work at night. But that means "you" the industrial user, not the homeowner.

That's if your power is coal or nuclear. Natural gas and hydro can be turned up and down quickly. Solar just stops generating if there is no load, but wind also needs load banks.

Even coal can be turned down, but slowly. You don't want to thermal cycle your boilers constantly but apparently they can direct the steam into cooling pipes instead of through the turbines if there is no demand.

This is one of the things people imagine for a "smart grid." When there is such surplus power, a message could go out on the net advising people to use power at great prices. Ie. "Hey, electric cars, even if you are waiting to midnight to charge, why not charge now at a better price?" or "Hey, aluminum plant, bump your usage."

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