Direct charge from solar

Direct charge from solar

Did anyone try direct charge from solar yet?
I know that there is no solution yet from Tesla nor there is a standard for that.
So the only option now is DIY by an enthusiast, but what is Tesla actually - it is a product brought to life by an enthusiast named Elon.
I do have 30 kW of solar power on my roof and the energy goes trough two conversions losing 3% to 5% every time it is converted (numbers are approximate)

solar --> 470 V DC --> domestic power grid 240V AC --> tesla on board --> 400 V DC

shop | May 6, 2013

Most people are not home during the day when the sun is shining so the extra losses you get by using the grid as a big battery isn't that big a deal. Interesting idea, but probably more trouble than it is worth. Even tesla isn't doing this at their supercharger stations.

shs | May 6, 2013

Not only is the grid a "big battery" it is a super efficient one in that with a Time of Use plan one can put juice into the grid at 30 cents a kWh on summer afternoons, and then take it out to charge the MS battery at night for 3.7 cents a kWh. That is a efficiency of 800%.

risquared | May 6, 2013

What state and city are the $$$ numbers from. Here inSan Diego we get nothing $0.00 for pushing power to the grid and pay about $0.15 to $0.20 when pulling power beyond what we generated. PUC is considering rules that will force SDGE paying for the power we produce but the rumor numbers are pretty low in single digit cents. That's why everyone in California is building their solar not to exceed their consumption.

shs | May 6, 2013

I am in PG&E territory. I agree that one does not get much, (I think we would get $.04/kWh) if one has generated excess electricity at the end of the year. As you point out, that does not make it very attractive to over-produce juice. I was trying to point out that charging directly during the day from solar was not very smart if one can sell that juice for 30 cents and then buy it back a few hours later to charge the car at 3 cents.

risquared | May 6, 2013

I'm confused here. Selling the "juice" to whom?

shs | May 6, 2013

You are not "selling" to PG&E but are rather getting a high $ credit for solar production that you use to pay for your other usage that will mostly be a at low $ rate. You could say that PG&E buys back your juice at the high $ rate, and sell it to you at a low $ rate at night. This works great up until your total true-up bill gets to zero. Then overproduction does not payback very quickly if at all. The point is the juice during a hot summer afternoon is worth more than on a winter night and PG&E is willing to pay more for summer afternoon juice from your solar than you pay to heat your house during the winter or charge your car at night.

risquared | May 6, 2013

I have to double check this with SDGE, and with my wife who pays the bills, however I'm under the impression that SDGE does not regard time of the day. They just count kWH -in and kWh -out of the grid and do the balance.

shs | May 6, 2013

Without a TIme of Use rate schedule, it is a very different story. And Time of Use is not for everyone. If you own solar, don't use much AC, heat with electricity in the winter, and charge your car at night, then TOU is a real benefit. If you have not solar, use a lot of AC, don't use electricity to heat in the winter, then TOU is not so good.

Joel N. Weber II | May 6, 2013

I imagine that once approximately everyone in California has solar panels on their roof, there will come a time when there is an excess of electricity from solar panels during the day, at which point, a HPWC variant capable of sending DC from the solar panels to the Supercharger input of the car with just an external DC to DC voltage regulator will make a lot of sense (obviously at lower amperage than an actual Supercharger, but using the same path through the car).

AFAIK, there is currently no supported way to feed DC directly into a Tesla aside from the Supercharger, and obviously Tesla controls the installation of those.

Andre-nl | May 7, 2013

I see little merit in the idea of direct charging from the solar panels. It might be nice for some very limited scenarios, but I don't think that niche warrants all the development work for an actual product.

The fact that daytime energy is more costly than nighttime energy means the energy company WANTS you to feed in your solar energy during the day and charge your MS at night. You are actually helping the grid by doing that. This will change in the future, when the number of solar installations rises exponentially. Some solution will have to be found, but still the fundamental problem of the car not being at home during most days will remain.

Solar inverters are getting very efficient. If you have 30 kW PV (wow!), a suitable modern inverter easily reaches 98.5%. And even in the 2 kW class you nowadays have the Steca coolcept series that achieves the same efficiency. I would imagine the AC-to-DC part in the Model S charger is not significantly worse. Perhaps you lose 5% in total. This number will drop, since even this kind of 'old school' analog electronics can still be improved.

But the best argument for not letting each solar customer balance own demand to owm supply is that much of the fluctuations will even each other out over the millions of consumers and generators hooked up to the grid. The remaining fluctuations are (relatively speaking) much easier and thus cheaper to deal with than when each consumer goes it alone.

So share your green electrons, and let everybody use them! "Give when you can, take when you need" is the motto.

Mark K | May 7, 2013

I fully expect to have a home SuperCharger powered from my roof in the not too far off future.

Once we have good sized batteries at home, we'll have the kW to do it.

Refueling your car in a few minutes midday is very useful.

I think TM will eventually provide an interface to permit access to the DC connection. TM has all the competences to make a kick-ass home storage plant that will take solar to the next level of service.

bt77057 | May 7, 2013

I was going to comment more, but @shs got it right.

But to add, there are TOU (time of use) plans and the normal tiered plans where as was described, kwh in and kwh out.

If you have a second meter, TOU, may not apply, actually, I'm not sure if they even allow that.

One HUGE misunderstanding I had was the difference between selling power and getting credits for power. You DO NOT want to sell power, this is when you overproduce for an entire 12 months. Credits are As described above, generating and getting credits during the costly afternoons, and using during the middle of the night when its cheaper.

Stark | May 7, 2013

Hey bt77057,

Can you elaborate a little on why it is a bad idea to sell to the grid? I'm waiting for my application to be approved so I can sell to the grid. I'm going to get a 20 year contract to sell to the grid (called MicroFit in my area) @ $0.57 kwh. I buy from the grid on ToU at the following rates:

•12.4 cents per kilowatt hour for on-peak usage (from 11 am to 5 pm weekdays);
•10.4 cents per kilowatt hour for mid-peak usage (from 7 am to 11 am and 5 pm to 7 pm weekdays); and,
•6.7 cents per kilowatt hour for off-peak usage (from 7 pm to 7 am weekdays and all day on weekends and holidays).

I'm in Burlington Ontario for reference.

txjak | May 7, 2013

@Stark, that sounds pretty generous! Sounds like a good idea to me.

Here in Austin, Austin Energy pays what they call the "Value of Solar" rate for all the solar energy you produce. It is currently $0.128 per kWh, but they say that they will re-evaluate it annually and change it in October, as needed. This VOS rate is "paid" as a credit on your monthly bill.

Since the main load for AE is during the summer their rate plans charge more for June thru September usage, so any credits accrued from October thru May will be reduced by summer usage. The credit is also reset to zero in October, so there is no real incentive to produce more energy than you will use in a year.

AE does not yet support TOU billing for solar customers, but the last I heard, they may be ready as early as this month. I don't know how other localities handle TOU with solar, but the AE installs a solar-TOU meter in addition to the standard TOU meter in order to match production to time of day. I participated in a pilot study AE did and have you can find more information over here.

Raixie | May 7, 2013

What about solar paint ? Think about the millions of cars lying in the sun at work all day long.

txjak | May 7, 2013

@domric Probably better to put up a shield and paint it and have a nice looking (and cooler) car instead.

bt77057 | May 7, 2013

@Stark, I agree with @txjak, sounds like a pretty good deal you have going there.

The reason I wrote that is, and I'm using summer rates here:

I pay $0.24 during the peak afternoons (which is when I will be generating)
I pay $0.12 during super-off-peak, midnight to 6:00 am (which is when I am charging my car)

If I overproduce, SCE (SoCal Edison) will only pay me about $0.03 per KWH.

In my case, I will size my system such that I pay about $15-$20 per montn.

Joel N. Weber II | May 7, 2013

Given that the cost of moving electricity across the grid is not likely to go down with time (because electric monopolies just don't have the right mindset for that price to go down), and solar panels and batteries keep getting cheaper, there's likely to be some point in time, 5-20 years from now, when suburban homeowners find that buying enough solar panels and batteries to largely not need the electric monopoly will be a great way to save money. At that point, daytime charging will become a fine plan.

Raixie | May 8, 2013

I was thinking more of developping such solar paint for future models.

Brian H | May 8, 2013

On-car solar collection is an expensive waste of effort and resources. It is only supported by the arithmetic-challenged.

mdemetri | May 8, 2013

For those in California, a 2011 law forces all california utilities to pay for excess generation from Solar. Prior to this, excess was wiped clean at the end of each year.

With a TOU plan with SCE, the benefit is even clearer. For example, last month (my first month of TOU while charging my Tesla) I over-generated ~110kWh and my credits were -$110. In other words, I was payed ~$1 per kwh for my excess generation. This was with winter rates (Tier 2 - peak 10am-6pm) 0.31 per kwh, super off peak (12-6am) 0.19 per kwh). This will be even greater when we role over to the summer rate plan in June-Oct when Tier2 peak is 0.70 per kwh and super-off-peak is 0.20 per kwh. When winter roles around again, I will no longer have excess generation and averaged over the year, I should be near zero for kwh. However, my conservative calculations are that because of TOU, SCE will pay me ~$600 per year (and have covered all my other electrical costs including Tesla charging). Sweet!!!

Balwant Singh | May 8, 2013

Why cannot nano technology solar cells be fitted on the car body & charge the car while travelling, much like the Dynamo.

Secondly the car dose not need cooling fans. but instead of these fans can be applied to produce wind power when the car runs above 40 km/h

So all these might not charge the battery fully, but they can definitely sustain the battery power & add more mileage on the run.

risquared | May 8, 2013

I'm no expert in electric power distribution and sales, just consumer. However the numbers flying around here are absolute nonsense in my mind. Why would any utility company pay $1.00 per kWh or even $0.56 per kWh to buy energy from you when it costs about $0.07 to produce (source a pro from SDGE who is personal friend). It is not that simple of course since there is distribution cost, cost to maintain hot reserves etc. But again to me those are fairy tale numbers. I pay $0.07 per kWh in Phoenix AZ (commercial facility) and I pay between $0.15 and $0.25 TOU service in San Diego, CA. SGDE is yet to pay back any money for my solar overproduction. That's why the solar system is designed not to overproduce:-)
My 30 kW system costs about $150,000 installed. The average sunshine time is 5 hours per day in San Diego. My system produces 150 kWh per day on average. If someone is willing to pay me $1 per kWh this means 365 x $150 = $54,750 per year or 36% return on my investment. For such return sir mdemetri the investors will cover every inch of your area with solar panels. So double check your calculations. Same for the guy who claims $0.57 payback.

risquared | May 8, 2013

Even if we forget about the few % in efficiency gain the main advantage of DC charge from solar remains the ability to charge quickly in the middle of the day, or in other words to have a supercharger in your garage. If I manage to connect my 30 kW of DC power directly to feed my Tesla, than I will be adding 90 miles driving range per hour.

Of course if SDGE starts paying high (or any )$$$ for mid-day energy than the economics turn around and solar charge doesn't make sense as many have noticed.

jbunn | May 8, 2013


Right now the nano technology does not exist to put this under a car finish. Also, consider the angle of the sun. Most of the car will never be at an optimal angle even if the car is in full sun. You can't generate significant energy to power a car like this. Best to put collectors on your house.

Secondly, any fan you put on the car to generate electricity will create more drag than usable energy. What you are suggesting is a perpetual motion machine, which is not possible.

txjak | May 8, 2013

@BalwantSingh: For the solar tech to make sense it has to pay for itself. Figure how much it will cost versus how much you will save. There are cars that run on solar alone, but they are usually single occupant, low power, special purpose vehicles to demonstrate solar power. Google solar race car.

For the fan situation, you're essentially talking about a generator running off the airflow created by the car. This will increase drag and decrease mileage. Is it worth it? In the extreme, if there were no drag, you could have a perpetual motion machine. Nobody has been able to make one because of friction/drag. There are vehicles that run on wind power, but they don't generate their own wind.

mdemetri | May 8, 2013

To clarify, $1 per kwh is not a direct number per kwh; this is the number that results from the time of use plan and the net metering agreement with SCE. Just so you understand how it works, lets say during peak time (10am-6pm) I over-generate 30 kwh above baseline and during off peak I use 30 kwh above baseline (in reality there are three timed rates, but for simplicity I will use just two: peak and off-peak). Using revised SCE summer tier two rates (which kick in after baseline) we get -30 x 0.47 per kwh gives -$14.1 for peak time and 30 x 0.09 per kwh gives $2.7 for a net -$11.4. Multiple this by 30 days and you get -$342!!! Thus, even though there was no net generation/use, you still make significant positive cash flow. This is because you are selling high and buying low. Note that the math is more complicated than this and needs to account for the basline usage, but this is essentially the way it works. To confirm the SCE rates see (which were just revised April 1; with the tier 2 summer peak 0.70 per kwh rate lowered to 0.47 per kwh):

At first I did not think that it could work this way, but my first months bill confirmed it. Also note that in the winter, solar generation is much much lower, so you cannot simply multiple by 12 months.

Finally, SCE will not allow you to enter into a net metering agreement if your system is larger than your daily usage. So you need to be close to neutral, but with TOU, you can generate positive cash flow while still covering your electrical usage. Unless you are currently using close to 150 kWh per day on average, your system may be too large to qualify.

Raixie | May 13, 2013

@Balwant, Jbunn and txjak

Solar nano-ink does exist and has proven efficient, even in the shade with infrared!
And it is cheaper than regular panels.

Nano-solar paint and adapted regenerative shock-absorbers could do the trick of free constant recharging.

Raixie | May 13, 2013

What Balwant suggest is not a perpetual motion. It uses the wind and stops with the lack of wind.
Now I don't know if it would be an efficient way to recharge.

Brian H | May 13, 2013

The only wind you don't "pay for" is the wind you feel when the car is stopped. Look up "sailboats" to see how this can be used.