ASK THE EXPERTS: EV Charger Wiring

eGauge EnergyMinder display.

We’re installing an off-grid solar-electric system this spring, and need some advice on off-grid charging for an electric vehicle (EV)—a 90 kWh pre-owned Tesla, which we hope to buy in the next couple of years.

Our daily commute is 5 miles round-trip, so there typically won’t be large daily recharging demands. Our other electrical loads are minimal, and we’d only recharge the EV when it’s sunny. We previously had a 1.5 kW home PV system, which was destroyed in a wildfire. With the additional EV charging load, we’re estimating a 6 or 7 kW PV system. How big of a battery bank would we need?

Would it be better to have a dedicated charging station, or just plug into 240 VAC? Is using a DC charger for the car possible, and would it be more efficient?

Sheryl & Larry Nims • via email

To size a PV array for charging your car, we need to know how much electricity the car’s battery will require to recharge, and how much sun the PV array will get.

The size of the battery bank depends on whether it is used for charging the car and, since you are installing an off-grid system, on household loads.

It may be better to have a dedicated charging station rather than using the plug-in Mobile Connector that comes with a Tesla because you can limit how much power the car draws from your PV system when it’s charging. Charging the Tesla battery with DC electricity would require a charging system that could “talk” to the Tesla battery management system.

Your Tesla should travel about 3 miles per AC-charging kWh; if you work five days a week, the car will need 8.3 kWh per week for the 5-mile round-trip. Adding a few more miles for weekend errand-running could bump that up to a weekly average of 12 kWh for charging (that’s 624 kWh per year).

For example, say that you were using 1,680 kWh of electricity per year (1.5 kW STC × 1.4 × 80%). Adding the car’s load, that total will be 2,304 kWh. If the car is charging only when it is sunny, we will assume that there will be little additional cycling of the household battery system, so a 6 or 7 kW PV system would be more than enough for your driving and the load previously served by the 1.5 kW system. If the battery for your PV system is not used for car charging, then the size would be determined by the usual calculations for “days of autonomy” based on daily household loads (see “Battery Bank Design & Sizing” in this issue).

Teslas come with a Mobile Connector that plugs into a NEMA 14-50 receptacle (like those used for an electric range). When connected to 240 VAC, it draws 40 A (9.6 kW). This Mobile Connector can also be plugged into a dedicated 20 A, 120 VAC receptacle, for a “slow charge” that draws only 1.92 kW. The slow charge would be sufficient to provide overnight charging for the 5-mile commute, but that wouldn’t fit with your plan to charge in the sun.

If you are charging the car on the weekend when it is home during the day, you will want to do it quickly, so this is where your 6 or 7 kW PV system comes in handy. If the house is using 1.5 kW, let’s say you have 5 kW left for car-charging. A 20 A, 240 V charging station, such as the Clippercreek LCS-20, will limit the car to 4,800 watts, leaving enough PV electricity for the house, and allowing the car to charge sufficiently during a sunny day or two.

Charging stations contain a relay that energizes the connector only after it is plugged into the car and does a little “handshake” recognition. The station sends a pilot signal to the charging system onboard the car; the pilot signal tells the car how many amps the station can deliver. By choosing the right charging station, you can tailor car-charging loads to your renewable energy system’s capacity.

See the graph comparison of car-charging loads, PV production for the day, and typical load spikes of domestic water heating from my house one day last July. It shows our Nissan Leaf charging at 5.76 kW during peak PV production hours and our Toyota RAV4 EV (which has a Tesla battery) charging at 9.6 kW off-peak.

Jeremy Smithson • Puget Sound Solar

Comments (9)

John Lackner 2's picture

Why not charge at work? Check for Ev charger rebates. Your employer may be able to get one for free. If you charge the EV when it's dark or cloudy outside you will be using one battery to charge the other. During long periods of clouds you may need that battery power to run the appliances, with the home batteries being drained while charging the car and running the house.

William W Howland's picture

No need to limit youself to a 20A EVSE (Charging Station) with a Tesla because you can set the maximum current draw using the on-board Tesla software. I have a 35A AV EVSE (available on Amazon) that I limit to 30A using the Tesla software. So using a higher rated EVSE (as long as your wiring is adequate) will allow you a greater range of control.

Marc Fontana's picture

The comments about the current limits (as long as the wiring is adequate) may apply to a grid connected home, however, in this situation the available current may be limited by the off-grid Inverter.

Phil Karn's picture

The Tesla's current-limit option is really meant to avoid breaker tripping when you're using ordinary utility outlets. A properly installed J-1772 standard EVSE is already programmed to tell the EV how much current it can draw, and the Tesla will obey this limit just like any other EV. You can still manually reduce the Tesla's draw below this limit, but you can't increase it.

Most J-1772 EVSEs are programmed to deliver 30A continuous on a 40A circuit, so a Tesla connected to such an EVSE will not let you increase the current above 30A even though the onboard charger can handle 40A.

Marc Fontana's picture

I'd consider using a level 2 240v Charging Station with a timer, or use the timer on the Tesla to charge during the day when the home batteries are likely to be charged up and when there is still plenty of sun. Ideally an off-grid PV system where EV charging is handled as a diversion load would prevent excessive draw down of the house batteries, because charging one set of batteries to drain them to charge the EV battery isn't the most efficient way to go. By the way, if your daily 5 mile commute takes you anywhere near the grid, I'd consider the option of charging from grid. With only 5 miles needed per day, you might get enough charge for a week or two from a grid based charging outlet.

WisJim's picture

We have a GE J1772 charging station which uses jumper wires to set the maximum charge rate that it will supply. We use it to charge our Nissan Leaf and have it set at a maximum charge rate of 3.5 kW so we don't overload our Trace 4024 (4kW maximum output) inverter, which runs on a 1500 amp hour 24 volt battery, charged by PVs and a Jacobs wind turbine. We do almost all of our charging during sunny days. We are retired so we usually can manage to charge while we are also getting energy from our system.

Lincoln Gamble's picture

It is important to understand all of the charging losses in a Tesla.

1) A Tesla looses several miles of range per day in standby mode. Given your short commute this may require as much charging as your driving. "Deep sleep" settings can reduce this amount somewhat.

2) The charging inverter uses 250W regardless of charging rate. Charging at 10kW you loose only 3% of the input power, but at lower charge rates you can lose 20% of your input power.

3) The Tesla preheats the battery before charging begins. In warmer weather this loss is fairly small, but at subzero temperatures a Tesla will not charge at all on a 110V circuit; all of the power is going into preheating the battery.

Finally, if you work a typical 9 to 5 schedule you will not be charging while the sun shines on workdays. You will need to take into account the efficiency of your battery/inverter system.

All things considered, you may need to allow for 30+ kWh per week to charge your car.

The good news: With such a short commute your Tesla has enough range for 2 weeks of commuting. One sunny weekend day charging per week may be enough to keep your car happy.

Phil Karn's picture

All excellent points. I've noticed the sleeping power consumption of my own Tesla and wondered why it's so high, but preheating hasn't been much of an issue for me since I live in San Diego. In cooler climates this could probably be reduced by charging when the battery is already warm, such as when the car has just been driven.

I'm curious about the 250W overhead, though. This doesn't seem like a charger conversion loss (which usually varies with power level) so it probably goes to overhead loads in the car, such as the electronics and cooling pumps and blowers.

Phil Karn's picture

The SAE J-1772 charging standard (the increasingly ubiquitous level 2 charging interface used in North America) not only has the EVSE ("charger") telling the car how much current to draw, it can change this figure in real time and the car is required to respond within a few hundred milliseconds. So it should be possible to build an EVSE that would limit the EV's charge rate to what your PV array is currently producing, minus other household loads, to avoid any discharging of your regular house battery. I don't know of any commercially available EVSE that can do this, but it seems entirely doable to this EE.

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