offgrid charging of M3

offgrid charging of M3

Hello, I'd do a search in the forum but I don't see how to do that, so I'm starting a new thread.

I'm in Southwestern BC, Canada, living in an off-grid house with microhydro and solar as my electricity source (plenty for running the house). I have had my M3 Long Range AWD for about a month. I charge the car a bit overnight by plugging into a 120v outlet (when only microhydro is powering the house; I set the car to start charging at midnight at 6A draw; I have a 24v system which means 6A translates to 30A; this brings house batteries to about 75% by morning) and then I do a second round of charging during the day when sun hits solar (which adds to what my microhydro is producing; I'll turn up to 8-10A draw from car, which equates to 40-60A on the 24v house system). I can put about 25-35km of range back on the car daily, meaning that I only need to visit a supercharger once every 10 days or so given my daily driving regime.

My question is: is anyone else doing something similar (with just solar, or with microhydro, or other)? If so I'd like to compare notes to optimize my system and car charging capabilities. Please get in touch.

peter.janulf | 14/05/2019

Cant help you here, but I will follow your thread closely when able. Very interesting to se/learn how to make things work as smooth as possible.
/Br, Peter

NEKEV | 14/05/2019

So jealous of your setup. Wish I could do this. We have solar panels but its a grid-tie system. I do very little charging at home since the solar setup is designed to just offset our yearly usage. I do have a couple spare panels lying around though and would love to rig up something to make use of them. Curious to hear how others are doing it.

Tronguy | 14/05/2019

Hm. So, I've got grid-tied solar. In New Jersey, we got these things called Solar Renewable Energy Credits, created by state government fiat. One gets one of these for each MW-hr that is generated by the panels (note: Not delivered to the power company, simply generated). One can sell these on a market for money, and it's substantial money over a year's time. On top of that, excess energy is sold at wholesale rates to the power company, totted up over a year's time; if the running total goes negative, then us solar types pay retail. At the end of fifteen years, no more SRECs, but, by then, they've done their bit to encourage people to put up solar and pay off the cost.
Fine. So, given that there's money involved in the SRECs, one would think that people would cover their roofs with panels to the maximum extent possible. The lawmakers involved Were Aware, and stated that, in the year leading up to the installation, one had to tot up how much energy one used; then, using a fancy formula (state mandated) which took into account panel efficiency, inverter efficiency, wire losses, roof orientation and angle, solar irradiation data from NASA, and what-all, one was only allowed to put the number of panels on the roof that resulted in the equivalent amount of energy being generated, again, over a year's time.
Fine. We ended up with 9 kW of panels. And I found out, later, that there's a systematic error in the mandated equation that results in more generation than use. Typically, this has been, over the past 8 years or so, 2 MW-hr extra per year. So, given that we only get wholesale electricity rates when we sell, that works out to $150 a year or so from the power company.
But, obviously, we're not paying for electricity any more, except for a $4.95 a month connection fee.
So, we don't go retail costs on powering the Tesla until we've used up that extra 2 MW-hr of energy; that's about 8,600 miles of semi-free fueling a year.
Now, given where we are, there's no real point in going off-grid. First, we'd lose the SRECs (we only get those if we're grid tied); second, we'd have to pay for the batteries, and it'd be a long time at $5.00 a month to pay off a couple of grand in power walls and the electronics.
But, if we did go off-grid with powerwalls, with the current setup, we'd still have, plus or minus energy we wouldn't have room to store, that 8600 miles (2 MW-hr) of energy left over.
Have you considered (a) putting up more panels and/or (b) putting in a Powerwall system? My guess is you wouldn't get cash-flow positive for several years at the least, but there it is.

Bighorn | 14/05/2019

Whether the car can charge at 24V would be my concern.

Bighorn | 14/05/2019

You must be using a transformer to convert to 120V at the plug?

coleAK | 14/05/2019

I have a hybrid on/off grid 7kw solar system with a battery back up on our house that I did mostly myself. I have it set up DC coupled and from the inverter I have a secondary breaker panel that can function on or off grid and my inverter will do 240v. I have switched to backup to see how well it will charge and I can get 32A off our wallconnector under full sun in the afternoon this time of year.

@Tronguy, we also have stupid regs in Alaska about over production. The really dumb part it is month to month and it was up to me to calculate how many panels. If I go over too often they can make me remove some panels. This time of year I get 16-18 hours of sun on the panels and in the winter I get almost none.

As per the discussion around grid tie or not. I lose some efficiency with DC coupled. But the issue is if the grid goes down you go dark. We are pretty isolated up here in AK and many homes have back up generators. Mine just runs of the sun instead of NG.

coleAK | 14/05/2019

This is the new version inverter I’m using it will do 240v 52A

kirkbdavis | 19/05/2019

I have a small (~1200 foot) house with a 1600 Watt array in South AZ. Not grid tied. I'm adding an additional 5000 watt array. I have fairly good battery capacity for my solar setup but I only intend to charge while the sun is up at 15 amps @ 120 volts. The inverter (Outback) is 3500 watts. I bought an identical used one from a neighbor when he upgraded his system that I haven't installed yet. The system will allow them to be stacked so I could do 240 Volt charging also.

The panels are relatively cheap (compared to when the 1600 watt array was installed). The best place I've found for prices etc is Northern Arizona Wind and Sun ( They spent a lot of time on the phone consulting about the system for free. Not affiliated just happy with prices and help.