# Vary Charge Rate to Suit Home Solar PV Generation



## Graeme Weston (Jul 28, 2019)

I charge at home using my grid connected rooftop Solar PV. The panels vary generation based on season, time of day and cloud cover. At present I can manually adjust M3 charge rate but if this exceeds generation then the difference is drawn from the grid. Could we have a software tweak which monitors PV generation and varies accordingly so my M3 consumes everything generated free and does not draw from the expensive grid? A CT clamp around PV inverter cable can monitor generation and a wifi device can send this data. Need M3 software to receive data and adjust the charge rate. Maximise sun power - PV 2 EV.


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## garsh (Apr 4, 2016)

That would be a neat technical solution. I don't know of anything like that.

You could just get a Powerwall and then not have to worry about it.


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## Frully (Aug 30, 2018)

That would be handy! I know I've seen someone trick a Tesla Wall Connector's data lines into thinking it is part of a multi-wall-connector breaker-sharing setup using a raspberry pi to send the rs-(232/485?) serial instructions.

The pi is spoofing a wall-connector master and tricks the (only) slave into thinking there is less current available for it, thus communicating to the vehicle to charge at a lower rate. There is no way that I know of to externally tell the car itself to charge at a lower rate. Edit: After reading the documentation linked from the video it seems newer WC firmware doesn't stop the car from charging - the pi has to phone home to mothership using tesla credentials to tell the car to stop charging when less power is available.


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## 3V Pilot (Sep 15, 2017)

I just installed solar and basically do this manually using a Sense home monitor. The car starts charging at 10am and is set to 20 amps, just about the same time the solar is producing enough energy and is on the upswing. Then, if I need to run the A/C or dryer I just stop the car from charging. Nothing automatic but the sunshine here is pretty reliable and there are few clouds or weather to worry about. Not sure I'd even want it automated, I'd rather control the charge and know how long it will take. Very cool idea though if you live somewhere with weather and want to maximize the use of solar.


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## Bigriver (Jan 26, 2018)

garsh said:


> You could just get a Powerwall and then not have to worry about it.


I mostly avoid using the powerwalls relative to charging the cars. The powerwall capacity of 13.5 kWh is dwarfed by the capacity of the cars' batteries. Even if doing only a small charge, it quickly drains the powerwall. And there is an inefficiency in the powerwall that somehow bothers me more if it is used in 1 hour rather than the 12 hour night time cycle.

I have net metering with no difference in grid charges for different times of day. I love when we are charging directly from solar but my next preference is for it to be from the grid.

My solar production often looks like an EKG - varying from 9 kW to suddenly dropping to 2 kW as a cloud passes by. Over and over all day long. Would rapid and frequent changes to the charging current affect the health of the car's battery?


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## JasonF (Oct 26, 2018)

I believe Net metering style solar systems contribute by adding a power supply in phase with the mains power, imitating the way the power company adds generation to the grid. So they don’t need to “shut down” when you use more power than it can provide.

What I started doing with mine is begin the charge at 8 am, so it grabs a few kwh from the morning sun before I leave for work. It’s probably using more than the solar system‘s capacity for that hour and a half, but it has the whole rest of the day to pay it back.


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## orekart (Nov 15, 2018)

Bigriver said:


> ...The powerwall capacity of 13.5 kWh is dwarfed by the capacity of the cars' batteries...


I observe for a daily commute of 70mi (average 270Wh/mi) that 25KWh stored energy capacity and 4275W generation (PV, nominal rated not actual) at home is a good starting figure, just enough to overnight charge the Model 3 and recover during the daytime. To do any better on overnight EV charging and other energy needs I will need to double the stored capacity prior to adding more solar PV. Daytime home energy needs can easily be met just by adding more solar PV.


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## ajdelange (Jun 26, 2019)

Net metering inverters work by increasing their output voltage above the grid voltage by a wee bit and seeing how much current flows to the line as a consequence of this. They then up the voltage a bit more and see if that results in an increase in the power delivered. They repeat this process insuring that at each instant of time the panel is loaded such that it produces the maximum power it can given the insolation it is experiencing. The is called Maximum Power Tracking and there are other algorithms besides the trial and error method I just described. So if the insolation is such that the panels are capable of producing 10 kW, they will produce 10 kW. Now where that power flows obviously depends on the load you present to the system. If you present a 5 kW load then 5 kW go to it and the other 5 to the utility. 

So how to set up charging the car? That depends on what makes you feel good and as well as, of course, your consumption (including the car) and solar panel capabilities. As an example, my system (newly installed) is predicted to produce about 60% of my total demand based on the previous years demand. In June and July it has produced 89.7% and in January and February it will produce much less. The simplest approach is to simply charge as I always have and tell myself (and anyone I am bragging to) that 60% of my electric cost is carried by the sun even though, if I charge at midnight, no current ever goes from the panels to the car. The cost to operate my car is now $0.02/mi as opposed to $0.04/mi before installing the panels and my carbon footprint (if I cared about that) attributable to electricity is around half what it used to be. 

The other extreme is to sit down with smart phone or computer and monitor the panel production adjusting the charge rate such that total consumption for the house (including the charger) is less production. This is obviously rather impractical and pretty boring but I can make the claim that the car is totally charged by the sun if I'm willing to do it. But it doesn't matter how I do it. My electric bill is still going to be the same and I am producing 60% less CO2 whichever way I do it. Another method has been suggested here. That is to set the timer in the car to charge in the morning when the sun is up but (in summertime) the load is low because the A/C's aren't on and to set it at a level which experience tells you will get enough charge into the car while not taking much, if any, from the utility during the charge time. There will be exceptions, of course, as when you garage air compressor comes on at the same time the clothes dryer does but of course you can manage that by not doing laundry when the car is charging and unplugging the air compressor. So a lot has to do with how much management you are willing to do to feel good.

Can it be automated? Well the API for the car allows charge level to be set by an application and, eGauge, for example, have API's which allow the currents from particular CT's and line voltage to be retrieved so an app could be written to do this automatically. I don't think such an app exists and I doubt anyone will produce one unless it's some passionate programmer who really wants to do what you are asking about.


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## alisonl (6 mo ago)

Graeme Weston said:


> I charge at home using my grid connected rooftop Solar PV. The panels vary generation based on season, time of day and cloud cover. At present I can manually adjust M3 charge rate but if this exceeds generation then the difference is drawn from the grid. Could we have a software tweak which monitors PV generation and varies accordingly so my M3 consumes everything generated free and does not draw from the expensive grid? A CT clamp around PV inverter cable can monitor generation and a wifi device can send this data. Need M3 software to receive data and adjust the charge rate. Maximise sun power - PV 2 EV.


Agreed. I was just reviewing my electric bill. Using generated power rather than selling it has definitely made a positive difference. A variable amperage setting to maximize onsite use/storage into charging the car's battery would be fabulous. My electric company charges a line use fee to send the power I sell to them. Then, of course, I pay an additional line use fee to buy the power back when I was using the power to charge my c overnight. Now I simply keep the power on-site by charging my car during power generation saving line use fees both directions. It now costs me even less to operate my car.


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