How To Stop Your EV Draining Your Home Battery

Guide to preventing an EV from draining a home battery system

I think AI is going to design my next house.

Don’t want to waste the energy in your home battery charging your electric car? You’re not alone. Discover a straightforward wiring tweak that fixes this—no fancy gear needed.

Battery to Battery Charging Is Not Optimal

A modest-range version of the modest Nissan Leaf stores 40kWh of electricity, while a Tesla Powerwall 2 stores 13.5 kWh. You need at least $45,000 for 4 Powerwalls to charge a Leaf fully overnight from stored energy whilst leaving a margin to account for losses and keep the lights on at home.

With these numbers in mind, it’s more than fair to assume you don’t want to be charging an electric car at home using a solar battery, not least of all because it’s a lot of battery cycles that don’t make good use of these very expensive assets.

Unfortunately, most home EV chargers can’t stop your home battery from depleting into your electric vehicle.

The Value Of Solar Charging

Solar charging is undeniably efficient. No matter the size of your PV system, diverting solar energy directly into your electric vehicle is like converting sunlight into money. You’ll save on your electricity bills, help relieve stress on the grid, and reduce greenhouse gas emissions—a triple-bottom-line winner.

Smart Management Is Key

If you have a solar power system with an inverter and EV charger from the same happy compatible family of products, they’ll have smarts built in to manage energy flows and display them in an app.

For instance, Fronius has the Gen24 solar inverter, battery (BYD), and Wattpilot EV charger. This can be programmed with priorities for solar-only charging of either EV or home battery while also offering time schedules to bar or encourage taking energy from the grid.

Fronius Wattpilot app

Retail Electricity Tariffs Add More Complexity

To maximise your savings, there are ways to manipulate energy consumption according to wholesale spot prices. With the right retail deal and APIs (Application Programming Interface, for computers to talk to each other) your solar battery or EV charger can soak up cheap energy and even get paid to do it.

Not All Charging Devices Play Nicely

For instance, Tesla’s wall charger is affordable and reliable but lacks smart features. The intelligence lies within the Tesla vehicle itself, meaning you’d need third-party software like ChargeHQ to manage energy flow for any other EV.

Don’t worry if you have an older solar power system. Again, smart technology like ChargeHQ or a well-designed charger with its own CT coil for measurement can add the intelligence your system lacks.

Home Solar Has Consumption Monitoring Too

Confusingly called a “smart meter”, a home battery system (with or without solar panels) needs to know what’s coming and going from the property. The mains supply is monitored either by passing the energy through a dedicated meter or passing the mains cable through another CT, so the inverter at the heart of the system can see what’s going where.

fronius system diagram

Full house of Fronius energy. Most importantly, all the components feed communications back to the solar inverter.

Fundamental Rules Apply

The programming of a home battery means it stands between the house and the electricity meter, trying to minimise consumption from the grid. It does this by charging from solar panels (or otherwise when energy is cheap), and then as the sun recedes, the home battery is discharged to meet loads in the house.

EV charging can be a big load, using half of your entire connection to the street. Even if you have your Electric Vehicle Supply Equipment (EVSE) programmed to only use solar system yield, measuring and differentiating energy flows can be difficult, especially as the house battery has a somewhat limited ability to meet all the demands. Flicking on the kettle and toaster might mean grid energy is needed to augment what the battery supply can do.

Where Problems Arise

It’s typical for an EV to draw up to 7kW when charging with a 2kW lower limit to make hardware efficient and keep losses low.

  1. Solar Output And Battery Status: Imagine your solar power system is generating 5kW of electricity. At this point, your home battery is fully charged, and the household is consuming only 1kW.
  2. EVSE In Action: Your EVSE (the charger) then harvests the remaining 4kW that would otherwise be exported to the grid.
  3. Changing Household Load: Now, you switch on a 2kW kettle and the household load rises to 3kW.
  4. EV Charging Adjusts: The EV charging rate dips to 2kW to match the available solar yield. At this point the solar yield matches the total consumption.
  5. Solar Output Decline: A cloud passes, causing the solar yield to drop by 1kW, leaving you with a 1kW energy shortfall.
  6. Where It Goes Pear Shaped:
    • The home battery, designed to bridge such gaps, kicks in to supply the extra 1kW.
    • Oblivious that this load is not essential for household use, the home battery keeps up supply to the EVSE.
    • Ultimately, your home battery, thinking it’s fulfilling its role, empties itself into the car.

By walking through this scenario, you can see how your home battery and EV charger may not always be on the same page, leading to less-than-ideal outcomes. If you like Irish accents and watching a bloke hitch his pants up, slog through this YouTube explainer. The useful part is between minutes 3:50 and 14:30.

How Tesla’s Ecosystem Manages The Issue:With a Powerwall and Tesla car, the control software dynamically decides if your car should tap into the Powerwall. On a flat-tariff with ‘self-consumption’ mode enabled, it will readily channel Powerwall energy into your electric vehicle. However, with a time-of-use tariff and the app set to time-based control, it typically reserves the Powerwall’s juice for peak hours, opting to use grid power for electric car charging.

The Solution: Create A Pecking Order

Unless there’s a war, bushfire, flood or virulent plague, chances are you’ll never want to charge your electric car with your house battery. So, the simple way to fix the problem is to put the EVSE outside the house so to speak:

Fronius solar power system with generic electric car charger

With a smart (or dumb) electric vehicle charger connected outside the solar consumption meter, the battery system keeps to itself, while the car can still soak up export before it gets to the grid.

The smart EV charger uses a CT to monitor and divert solar energy exports. Installing that CT and connecting the actual charger to the mains upstream of the battery/solar consumption meter means the house battery will never see the EVSE. It simply doesn’t know it exists and will not discharge the battery to meet the load.

The charger will still monitor for export, but it will get second bite at the solar electricity yield and the house battery will be charged first unless you program the solar/battery inverter to delay it’s charging with a schedule. However, this scenario, filing the small tank first, is perfectly rational.

The only real downside is  whatever native solar monitoring app you have will be blind to EV consumption.

About Anthony Bennett

Anthony joined the SolarQuotes team in 2022. He’s a licensed electrician, builder, roofer and solar installer who for 14 years did jobs all over SA - residential, commercial, on-grid and off-grid. A true enthusiast with a skillset the typical solar installer might not have, his blogs are typically deep dives that draw on his decades of experience in the industry to educate and entertain. Read Anthony's full bio.

Comments

  1. And this becomes also a bit more complicated when having a 3 phase supply combined with either a 1 phase or 3 phase car charger. But other than that a workable solution.

    • My 2c worth: I wired the car chargers on the grid side of the generation & storage. I monitor the whole system using an IoTaWatt to measure the consumption, & Home Assistant aggregates all the data from the Solar PV, the Battery (Zenaji 11.5Kw + Victron Multiplus), & grid flow. The OCPP ZJBeny EVSE is controlled by a software module in Home Assistant. My rural system is complicated by 2 phase rural supply & the dwelling being some distance from the metering & generation.
      Home Assistant allows the merging of disparate equipment into a manageable whole.
      The Zenaji battery is discharged totally most nights between abt 5 & 8PM. It also manages peak demand. When o/p power was affordable, we used to charge the Zenaji in the early morning for the Breakfast rush, but now O/P is almost as much as shoulder TOU tariff, so no longer economical. However, if my Energy provider offers an EV rate in the Midnight to dawn as one of the Gentailers does, I will take up the offer & charge my battery enough to cover the morning rush. Our household is 7 people in 3 families & our average daily grid draw is abt 25Kw in a fully Electric house with 2 of EVs, where the Kona does 30K Kms/Yr, mostly on solar PV power. Our other EV, the shopping trolley (12 y/o Imiev) now does abt 8K Kms/yr. Glad I am not buying dinosaurs!
      Home Assistant is a very powerful Open Source home management solution that can control many many devices (even the robot Vac!) It is a bit of a nerd sphere tho, but the on-line help is amazing.
      One thing not often considered with EVs is that the charging efficiency drops at levels under abt 2Kw. We charge the Imiev from a 10A charger, plugging it in mid morning for abt 4>5 hours. The Imiev runs about 25Ks to town & return mostly.

  2. Erik Christiansen says

    Great article, Anthony. That elephant has long been hiding in the strawberry patch, camouflaged only by red painted toenails. And it casts an even bigger shadow over off-gridders, ‘caus flat battery = the ice cream melts, if you don’t go out and start the generator in time.

    Your nifty EVSE idea might even be able to be adapted to the needs of us off-gridders, with a small tweak? If EVSE feedback were taken from the house battery’s current shunt, instead of a (here non-existent) grid CT, then it should even more directly snaffle only as much PV yield as can be taken without battery discharge. There’s just the small issue of voltage scaling, I figure. (The CT’s AC output will be rectified and amplified to a usable level in the EVSE, so if we matched level and polarity, it shouldn’t notice any difference.)

    That does though still leave the matter of priority; charging the house battery to cover the next 48 hrs, or charge the EV for a trip to town. Not discharging the house battery is not the same as giving it the lion’s share of PV yield. And the priority changes according to how close it is to shopping day.

  3. I have the ZJ Beny EV charger and the DLB box which works very well monitoring our Solar system export and controling the charger,
    I also added a timeclock into the stop button circuit, to limit charging to a certain period during the day. Plus I added a LED current monitor (as I like to see what’s happening).
    I got the non-OCPP version charger with WiFi and App (only one or the other not both) but I found the app pretty useless.

  4. Tim Falkiner says

    I simply put the Tesla Powerwall to 100% and charge during sunlight with my Zappi which draws 7 kW so I might get something like 4 kW from the panels and 3 kW from the grid. In Victoria, however, I will be required to go on to a variable tariff so I will be obliged to charge at night on off-peak rates. As my ex-oil company executive neighbour would point out, this will turn my EV into a coal-burner.

  5. Polestar Stan says

    Hi – love your work and sunny disposition!

    Next month, I’m getting a 7kw Trina solar system, a Polestar 2 and a Tesla Powerwall with a Fronius Gen 24 inverter and a WattPilot 22go EVSE.

    I was planning on smart solar charging the car using either or both of the Fronius apps but how will the Tesla Powerwall fit into all of this?

    Thanks!

    • Tristan, 7 kw is now a minimal system I feel. Sometimes that is all you can fit on the roof. 10+kw better. btw, do not worry about grid feed limit: in Rural NSW now some are limited to 3Kw export. When you charge your car you will easily use the excess. (Also try to use power during the day: washing M, dishwasher, pool : use them during the day.) Overpower the inverter if allowed (increases the ramp up in the morning & later in the afternoon). Spec a bigger inverter, & manage any shading (with chainsaw, or use Micro-inverters or optimisers). Even a TV antennae can reduce output from shadow.

  6. Your solar will also not ramp up if it has for example a 5kW export limit, as it won’t see the EV Charger load, meaning there could be production left untapped if there is low house loads and a 10kW inverter

    • Finn Peacock says

      Great point – I never thought of that. If you are export-limited, the better solution is software control of the battery.

      • The export limit is easy to accommodate: the metering is on the grid side of the EV draw, but the EV is on the grid side of the battery. That way the export is throttled if necessary.
        All of the previous monitoring situations can be accommodated with Home Assistant, an Open Source Home Management system that runs on something like an Intel NUC or a Raspberry Pi. Home assistant can interface with most inverters & control systems, as well as things like DC current shunts for off-grid situations. Well worth investigating. As I said before, there is a very helpful community, so even software mods are possible for users, usually by modding existing modules (Python: a relatively easy language to learn).
        Roy, it is possible to control the ZJBeny with Home Assistant. I have a DLB box, but it was easier to control the charger directly with Home Assistant (in my case with OCPP, but the registers are available from ZJB to control the śmart’ ZJB using software. Here is an example of a user interfacing a WallBox charger: Google the text: home assistant ev charger control modbus tcp. (Links are not trustworthy!)
        The IoTaWatt is also well worth considering: another Open Source project monitoring platform that interfaces with Home Assistant & others. Up to 14 inputs & can run on multi phase systems. Can measure AC input/output power. Now sold in Australia too. Really reliable too: mine has been running for over 5 years!
        One thing I hate is having too many monitoring systems competing. That is why I do not use the ZJBeny DLB box: better to use Home Assistant to read the data from the inverters, etc, the IoTaWatt, then control the loads to balance the system. Loads such as pools, pumps, EV Chargers, ACs, etc can be controlled directly or using something like a Tuva controlled switch. If you wish, it is all accessible with a mobile phone too! (A step too far for me…)

      • Hi Finn, I’ve currently got a 8kw solar PV and Powerwall 2 setup (via your excellent referral)! I also now have a Tesla M3 Highland on order and really hope that Tesla’s “Charge on Sunshine” app feature will become available in Australia, are you aware of any timeline rumours?

  7. A simple and easy approach is to restrict the charging period and current. I have a 4 kW off grid system with 15 kWh of storage. I set the car to charge between 9:00 am and 4:00 pm and limit the charging power draw to 1.8 kW. I monitor the state of charge, the average generation and the other loads during the day. The air conditioner is the main concern, but that is a discretionary load. I can reduce that load if I want to. I also check the forecast generation (depends on cloud cover) when I am planning my charging strategy. The battery is typically at about 70% at 9:00 am and back up to 95% by 2:00 pm. If the battery is losing charge I can reduce the charging rate to 1.4 kW. In practical terms, I expect to gain about 50 km of range in one day. I can easily get 80 km if I extend the charging period and draw the battery down to about 50%.

  8. Any updates/news on being able to use that Nissan leaf as the house battery overnight? Last time I checked it wasn’t allowed/yet to be regulated in certain states or something.

    • Anthony Bennett says

      As I understand it the one approved charger, the wallbox quasar, has been discontinued. It was only compatible with a CHAdeMO equipped vehicle and car makers are now coalescing around CCS plugs for charging instead. Frustratingly the standards for CCS2 to allow bi-directional charging haven’t been written/negotiated/settled yet.

  9. Thanks, this is just the sort of info I was looking for. We have an Enphase system so can’t use Charge HQ atm. We have a power diverter for the hot water which is great but now thinking of a solar smart EVSE soon (and battery in the future). How the EVSE integrates into the system seems quite critical!

    • Mike Peacock says

      Hi Solar quotes team. Great article topic! Any chance you could clarify a couple of points? Please bear in mind I’m no electrician.

      With the example combo of the Fronius Gen24 inverter, BYD battery & Wattpilot EVSE (all from the same ‘happy family’) I think Anthony is saying: although it’s a good setup that includes ‘the smarts’ it still can’t address the “EVSE draining the home battery” issue….unless the EVSE/CT clamp is connected upstream of the battery consumption meter. Is that right?

      So if Anthony’s proposed configuration is used I’m guessing the ‘generic EV charger’ in the 2nd diagram could be a Wattpilot but the Fronius App would unfortunately include consumption for EV charging as part of the export record. If so it’s not the end of the world to me considering the system addresses this tricky issue.

      (I’m particularly interested as one of Solarquote’s highly recommended installers is installing this combo for us next month and I want to be sure they nail this issue with a configuration that works for us. They seemed a little uncertain on this point initially)

      Regards

      • Anthony Bennett says

        Hi Mike,

        I’ll reach out to Fronius to be sure but you might also try Fakebook. There’s a group for Wattpilot users because of course there is… https://www.facebook.com/groups/780328227201448

        • Mike Peacock says

          Thanks Anthony, it would be great if you can get confirmation or comment from Fronius. I guess as more and more people seek to solar power their home & vehicle with Fronius being a prime contended the answer to this is critical point. I’ll watch with interest.

          Also cheers for the Wattpilot users FB group link. I’ll try them & report anything of value.

    • Hi, suggest you investigate Home Assistant. With the OCPP module you can bypass an external monitoring supplier (such as SolarHQ, which seems to be a good option too, but unnecessary)

  10. I am a little confused here. I was of the understanding that a solar battery would normally be wired so that it only supported some circuits in the house but not all of the house. If you wired the charger so that it was on one of the circuits that is not supported by the battery, wouldn’t it still be able to charge from the solar panels or the grid but not the battery and not affect the export limit?

    • Anthony Bennett says

      Hi Alan,

      It’s true a solar battery would normally be wired so that it only supported some circuits in the house but that is when there’s a grid outage.

      ie not all of the house is backed up.

      However when the grid is operating normally then any circuit in the house can use he solar yield, and energy from the battery, in preference to grid power. That’s the main function of “behind the meter” generation while blackout protection is usually a nice bonus.

      You raise a good point about export limit though. To make best use of a large solar array (which is what I always advocate) the EV charger needs to be downstream of the solar consumption meter. This has the potential to reintroduce the problem of the house battery being consumed by the EV unless perhaps the charger has an external CT which can be placed upstream of the consumption meter.

      • Good comment. My battery system is Victron controlled, where there can be an issue with the Neutral link. All my household circuits are downstream of the Victron Multiplus 2, but my EV chargers are on the grid side of the Victron so if the power goes off, the EVs do not charge. I think it is still possible to charge off the battery, but that depends on settings & the grid being available. (I do not want to charge from the battery because it is less efficient: if the battery efficiency is 95% & the EV charger in the vehicle is 95%, the round trip expense increases (% are example only, not my system!). However you do want to ´trap´ any Grid export & re-direct to the EV if possible. Home assistant, or a good manual intervention can achieve this. One thing not often mentioned is that the occasional grid import to charge the vehicle is still more efficient than ICE vehicles & cheaper to run overall.

      • We have a system that is as your initial diagram, without the ohmpilot – PV, Fronius Gen24+, wattpilot & smartmeter, BYD battery, EV.
        Initially, I though we’d use the PV Surplus charging mode most often but, in fact, we use the Default mode (non-smart, essentially). The battery acts as a buffer in the event PV generation drops below that necessary to supply the load (house + EV).
        Your scenario has a ‘passing cloud’ dropping the production – that’s transient & we found that the battery supplies the deficit then quickly recharges as the cloud passes.

        We do have a fall-back however. Home Assistant (which has integrations for Fronius inverter/smartmeter & wattpilot) is set to automatically stop the charging if certain conditions are met (eg load exceeds PV generation for 15 minutes & EV charging is set to Default & Battery SoC <70%, then stop EV charging).

        Works for us with 3-phase, 13kW PV, 10kWh battery, 10kW inverter, EV charges at 7kW single phase.

        • I should add that the Fronius app (or Home Assistant, if preferred) allows limiting home battery discharge to a specified % SoC (when charging on Eco & PV Surplus setting).

          For example, if I set 35% min SoC, EV charging won’t start until the house battery>35%. Once reached, if there is PV surplus (above a user-set minimum, eg 1.4kW), EV charging starts Of course, charging stops if the SoC drops back to <35%.

          This setting controls the problem of the EV draining the house battery.

    • This now not always the Case. The Sungrow 10kw hybrid allows all household circuits to be backed up for grid failure, and up to 10Kw to be used at once if one is that keen.

  11. Mike Peacock says

    A belated thanks to Bernie for your reply on his setup. From your comments Bernie it sounds like: for a Fronius setup in standard configuration (with the EVSE connected downstream of the battery/solar consumption meter) Fronius largely solves the EVSE draining the home battery if you get cunning with the settings in its own App. And then Home Assistant will completely ‘nail it’ if you want to go further. Our system will be almost identical to yours (but 1 phase not 3) so it’s personally very reassuring.

    I in summary I guess the initially described concept of moving the CT clamp to upstream of the battery/solar consumption meter is perhaps better suited to other brands/component mixes that are less component interconnection.

  12. Michael Paine says

    I have just had a Wallbox Pulsar Max (a new model) installed with the configuration recommended above. It seems to be working in that the Tesla system does not “see” the car charging and try to use the Powerwall for this.
    The Wallbox app allows me to control charging remotely. I have this set to charge during off-peak times but can over-ride this at any time, such as when there is “excess” solar.
    I will look at ChargeHQ (now a $7 monthly subscription) for smart charging of the car when there is a large amount of energy being exported to the grid. The Wallbox is OCPP compatible for this purpose.
    In the meantime I am corresponding with the developer of the Powerwall Monitor app (https://www.fantasticsprinkles.com/tesla-powerwall-monitor.html ) to see if he can add a notification when the grid export exceeds a nominated value (in my case around 4kW). I can then decide whether to start charging during the day.

  13. I am getting a 13.2kw system installed this month with a Sungrow 10kw hybrid inverter and 12.8kw Battery.

    I am going to install the Sungrow EV charger when it’s available in Australia early next year. I have done some research on the setup and it looks like the Sungrow EV charger can be put into green mode which uses excess solar however it will also use grid or battery if there is no excess solar. To prevent the battery from being used while charging the EV, there is a setting that can prevent the battery being used during set times. The ideal setting would be to have the EV charger set to a time schedule to charge during the day and set the battery time schedule to prevent usage during the day. This would solve the issue of the battery charging the EV.

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