Charging from PV-System

Charging from PV-System

My Roadster arrives in a couple of days. Already I am figuring about the charging. I have 18kWp Solar Power on the roof which is connected to the State Grid.
Now, as we do have periods of no power (at night or cloudy days), I use a "Current measuring Relay" in the PV system to switch a contactor which in turn will only connect to a bigger Load (e.g. the Tesla) when there is enough power from the solar panels.
This means, I do not want to charge the Roaster at night or when it is an overcast day. This system works perfect with a 9 kW hot water boiler.

So I wonder if it is possible to use it for the Roadster: to plug-in, slide the switch and let the relays do the job. Would the Tesla SW lock me out altogether if I slide the switch and if the power from the Contactor is dropped?

PS: Will Tesla do something for the 3-phase systems? Here in Europe almost every home has 240/400 V

ida-intl | 27 June, 2011


I'm in CT, USA. I've charged my Tesla Roadster with PV array on my business buildings since I've owned it. In CT (and each state in the USA is different) we have a meter measuring how much kwh I pull from the grid, and a 2nd one on how much kwh I push to the grid. At end of a billing cycle, I pay on the net kwh usage (even though we have three types of rates during a cycle: peak, off-peak, and shoulder). If I push more kwh than I pull (as I have since PV system went live in 2009Mar) then I have a kwh credit that carries over (such as into Dec-Mar when I don't generate enough to meet my needs). If I still have a credit of kwh in March of the year (as I've had each year) then the utility sweeps it and pays me based on average wholesale rate of renewable energy over the past year (pennies). In summary: the grid acts like a big battery, or bellows as I generate more in summer than winter, and more during day than at night. Thus, for me no issue to have a timer on the charger, which I think would be your best method.

I would avoid a more elaborate relay trigger for charging, because with an array you can have clouds that cause the generated output to fluctuate wildly. Also, the charging sequence requires sometimes to prepare the battery via heating or cooling, so the on/off sequencing would not be productive.

ida-intl | 27 June, 2011

One more thing, my charger is 220v/50a. Also, the time-to-charge capability in the car should solve the aspects for the most part if you follow my recommendation to simply charge during time the PV array would possibly be active and sending power.

joe.gruber | 27 June, 2011

Thank You for comments and Ideas!

I am aware of the fluctuation problem. For that reason there is a large hysteresis setting on the relay so it will not get erratic and jump while clouds affect the output.

I am just wondering what the Firmware will do or react to such setup. (Would it reset... or continue?)
In any case, I am sure it will be interesting to find out (by trail and error) and will keep You posted.

The reason to take less power from the grid is because I am not subsidized. The ratio is about 5 : 1 (for the power I produce I get about 5 cents/kWh, and for the power I consume I pay about 20 cents/kWh)
This "unjust" fact makes me (try) to use as much as I can from my PV-production. Really, it becomes more a matter of principle than an economic issue. (even that I am not one of those poor people over here who pay about 8 USD for a Gallon of fuel)

In my case, the Roadster would definitely be the largest energy consumer.... so why not my own!

Good luck and take care!


VolkerP | 29 June, 2011


Tesla has charged a roadster with a "smart charger" that tells the vehicle the available power in short intervals. Read
Charging from PV on a cloudy day would require exactly that kind of fine grained power control. As demonstrated, this is no problem for the roadster's built-in charger, just a data link and a firmware upgrade is needed.

BTW: after 7 years, when your roadster battery is "worn out" (e.g.70% capacity left), you can replace it with a new one and use the old in a stationary installation as a buffer. Saving 15cent on every kWh buffered, it will pay off the required charging & cooling devices quite fast.

psusi | 29 June, 2011

Wow. In most states in the US, the power company is required to use "net metering", meaning that if you generate an excess kWh of power, then the meter runs backwards by 1 kWh, effectively canceling out a kWh you already paid for when you weren't generating excess.

In your case, I think you are screwed because you can not directly connect the solar panels to the car charger. The panels must be connected to an inverter, and if the inverter is a grid-tie type ( which it sounds like you have ), then it will not function unless it is actually connected to the grid.

Even if you have a non grid-tie inverter that will function when you try to connect it to the car charger, you have a load matching problem. The car charger will kick on and always try to draw full power to charge the battery, but the amount of power you can get out of the pv array varies with the sun. If it dips below the amount that the charger wants, then the inverter will shut down.

psusi | 29 June, 2011

Now that I think about it you might have an option, though it is an expensive one. You can install a house hybrid inverter/charger and stationary battery bank, like the Xantrex XW system. You can configure it to charge the batteries from the solar panels, and activate the load shave mode where it will start discharging the batteries to satisfy a high load, like plugging in the tesla. It also means you will be able to keep the house power on when the grid goes down. Cost is in the area of $10k-$20k though.

VolkerP | 29 June, 2011

Integrating higher amounts of energy from regenerative sources into the grid resembles building an "island" type grid. You need to balance generation from a varying source with load and you need some buffer capacity. I hope that there will be incentives for running a buffer system in your basement soon. With a little extra software, it can either help balance load in the grid or run your home as an independent system.

wright151 | 12 November, 2011

What we need is the ability to direct DC charge a Tesla battery pack from a non-grid tied solar array without (Inefficient) inverting to AC and then back to DC. I would think there could be a simple solid state gate circuit that would limit dc current draw to maintain DC voltage above the required critical value. I would think in Europe at $8.00/gallon gas, the PV sourced power option would be quite attractive.

Douglas3 | 23 November, 2011

You would need a lot of spare capacity in the array to avoid dropping out when a cloud wanders by. Practically speaking it would require a battery system between the PV and the car. Also the Roadster is not designed for DC charging.

seabobbase | 4 October, 2013

Hi, I have the same Problem.

I want to Charge the Roadster easy going with 220/drawing 10 amps.
That is what my Basic charging cable is alowing max anyway. It is the swiss Version limited to 10 amps.

Since it takes very Long to Charge the car I figured I could instal as many PV Panels that will supply approx. 2,5 KW continuously pumping it in the Inverter. The Inverter will supply the 220/ at 10 amps most of the time since I will install a sufficient number of Panels. Panels are cheaper than batteries for buffer.
No in case the sun stops due to lots of clouds or sudden short rain, I would like the Inverter to swich to the grid/regular power from the house for the time of the solar black out.

This way I coud Charge the car by simply parking it under the solar panneled car port. Since I can't use the Roadster all the time and charging with 10 amps takes 20 hours anyway, there is enough time to charge, and I prefer to Charge it by sun instead of paied power.

Does anybody know if an Inverter will work directely from pv System without buying a battery charger and batteries?

To buy Panels and a good Inverter would be cheap and everlasting :-)

Maybe the Inverter would Need somthing like a 12 V input from a charger. how would the Inverter know that the voltage and currant is coming from a charger instead of a Batterie?

best regards form Germany