CHARGING BASICS 104: Why you should not charge electric vehicles at domestic wall outlet

ev-wall-charging-cable-mode-1-fire

Many new EV drivers find it tempting to plug-in their vehicle into home’s regular wall outlet for charging. Some of these enthusiastic EV drivers also buy cheap wall charging cables from online stores – to avoid buying a (comparatively expensive) EV charging station.


This should be completely avoided. Charging your electric vehicle at domestic electrical outlet is not only hazardous, slower, but also not economical - in opposite to what a new EV driver might think. In fact, this direct charging from wall outlet is legally forbidden in many parts of the world.

This post explains reasons why charging at domestic wall outlets is not a good idea!.


1. Safety: Wall outlets do not have sufficient protection

Wall outlets used in Germany, Austria, the Netherlands, Spain and in many other European countries use Type F, also known as ‘Schuko’ outlets. They are rated for 230 V and currents up to 16 A. However, when they continuously deliver 230V x 16A for a longer duration, they heat-up so fast; can reach about 100 degrees Celsius, in less than half an hour.

Wall socket outlets from different manufactures may have different temperature withstanding capacity; unless you’re a qualified electrician, it is not possible to identify these safe operating temperature ranges.

When an electrical outlet continue to deliver high power at high temperatures, there is chance for a thermal runway, often leading to ‘fire hazard’. At worst case, the outlet and the wires connected can fully burn!


Some EV manufactures provide a wall charging cable along with the car purchase. These cables will have a ‘protection circuit’ in between the outlet and the car (technically they are called as Mode-2 charging). However you should note that the EV manufactures' manual still refer them as ‘an emergency charging cable’ and it has to be used only in that way.

Even when you want to use these ‘emergency charging’ Mode-2 cables, you should always consult with a qualified electrician to test and verify the socket outlet you consider. It is necessary that the chosen electrical outlet must be wired from a circuit breaker (a protection equipment) installed at the household’s electrical meter cabinet wiring.


2. Slower charging: Wall outlets can't provide sufficient power

Though the standard Schuko wall sockets are rated up to 16A, to be on a safer side, many countries allow only a maximum of 10A in home outlet electric sockets, which translates into maximum (theoretical) power of just 2.3KW

For example: Tesla Model 3 Standard Range with a 55 kWh battery pack, would require 23 hours 45 minutes to fully charged through a wall socket. Nissan Leaf featuring 40 kWh battery would need 18 hours 30 minutes to be connected to the outlet! [Charging time & cost for popular EV’s can be found here].

So it is important to get a dedicated charging station for home – which can deliver power up to 22 KW (which is nearly 10 times faster charging than Schuko 2.3 KW). Even if you buy a medium speed 11 KW home charger, the same Tesla Model 3 Standard Range can finish charging at 5 hours – which would be perfect for an over-night charging.
tesla-model-3-charging-time-cables-cost

3. High costs due to high energy losses

Theoritically, the domestic electrical outlets can deliver 2.3 KW (230V x 10A), but in reality there will be huge power losses involved. The charging cable between the wall outlet and the electric vehicle impose a resistance to the charging current flowing through them.

The main factors influencing the cable losses are: cable’s diameter, length as well as the duration of charging power and time. Thinner the cable, there will be higher the resistance and higher energy losses as heat.

In addition, what you may not know is: that the electric vehicle’s on-board chargers (internal to the EV) shall work at optimal efficiency only at certain charging speed (current). Every EV manufacturer defines these optimal charging speed requirements suitable for their on-board chargers.

For example: Daimler smart EQ shows different energy consumption at different charging scenarios. Smart EQ featuring 22 KW on-board charger shall give a mileage of 14,5 KWh for 100 km range when it’s charged via a dedicated charging station; but while charging via domestic wall outlet, the same vehicle will require 20,1 KWh for 100 km range – mainly due to the poor efficiency of it's on-board charger at low charging current.

💡This energy loss of 5,6 KWh (20,1 KWh – 14,5 KWh) per every 100 km is significant; translating this energy loss into money, an EV driver who charges from domestic outlet will lose 252 € per year,  in comparison to using a charging station. (assuming 15,000 km annual travel ; electricity rate of 30cents/ KWh)

Source: Screenshot from Daimler Smart EQ media (link)


Note: 5 KWh / 100 km is a substantial drop and Daimler Smart EQ is not the only EV which suffers; (In fact, Daimler is courageous and transparent to display it on their official media release); other EV manufacturers do not show this numbers, but almost all of them will have the same issue while charging at low power domestic outlets.


4. No possibility for load balancing & smart charging

Growth of electric vehicles in phenomenal; in a few years’ time there is going to be more and more EVs in every street. If all those EVs are plugged-in for charging simultaneously in an uncontrolled way, they would increase ‘peak demand’ on the grid, contributing to overloading.

So it will become a necessity to balance household electricity consumption, especially during peak hours. Thanks to smart charging features, most of today’s wall box charging stations can intelligently balance household’s local load (Load balancing) , allow late night charging at cheap rates (flexible / timed / delayed charging), and also optimize loads at grid level (V2G). Some of the home charging stations can also integrate your solar panels and storage batteries - seamlessly enabling a self sustainable energy use.


None of these smart features are possible while charging through domestic outlets ; will end-up just not possible to plug-in the vehicle during peak hours!.

In summary: Using domestic wall outlet for charging electric vehicles is not only hazardous, extremely slower and dumb, but also not economical!. So it is always better to invest on a charging station for your home charging.

11 comments:

  1. This was interesting but do you have the the reference for the poor efficiency of Daimler Smart EQ's on-board charger at low charging current? that sounds like quite a big drop?:

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    1. @David Pickles. I'm glad that you asked :). I have just added the screenshot and official release of Daimler Smart EQ. But, remember, this is not the issue of Smart EQ alone (infact Daimler is courageous and transparent to display it on their website); Every EV's on-board charger has a specific current, only at which they are efficient; the losses may even be higher in some other EVs.

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    2. Pon Paulraj you’re correct re all manufacturers will suffer some loss but I’ve not seen any published figures so I agree that it’s good that Daimler has at least made these available! Thanks!

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    3. @David Pickles the are a few papers c.2016/7 out there showing relative efficiencies of EVs at different charge rates. Some drop as low as 70% at 2.5kW. Optimal (max) power generally better than 95%. Significant difference of particular note when looking at the whole Grid picture. Bottom line remains as install a dedicated EVSE, smart, matched to the cars nominal capability if your supply is able.
      DC chargers efficiencies usually on the datasheet, again upper nineties the norm. Comparing battery uplift against metered (ac) to see losses is always worth a look.

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  2. Dr. Kai-Philipp KairiesFebruary 4, 2020 at 11:12 AM

    Yes, especially the part about conversion losses is sadly true. To make mobility sustainable, we must focus on efficiency - #DC charging can be a good option.

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    1. @Dr.Kai-Philipp, True. That's the reason nowadays we see a lot of home dc chargers (as small as 10KW) coming as well. But still the cable loss could be concern on DC too.

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  3. In my view Mode 2 cables should be banned globally. They put users at risk and encourage bad practice particularly by the inexperienced. Not forgetting the terrible customer experience from nuisance tripping, low efficiency and long charge charge times. While I'm not keen on too much regulation it's time to move away from Mode 2 and make sure all EV drivers have access to dedicated charging equipment installed to a high standard.

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    1. @Luke Dowell, Correct! I would never consider mode-2 as a mainstream charging option. Many EV manuals refer them as 'emergency charging cable', and they've to be used only that way.

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  4. To be honest, 2.3kW is not a too slow power to charge the car for a daily commute as you write "These are too slow and totally not practical even for a day-to-day commute." A daily commute is usually not longer than 100km back and forth. I think a majority only has between 10-30 km to work. If the car is parked from 9 p.m. to 7 a.m. with a 2.3 kW charge, the battery would have gained roughly 20kWh and that is the same as you used for a long commute. A majority of my friends with EV's do not consume above 10 kWh on a daily basis so for them 20 kWh is more than enough.

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    1. Yes, agree , 2.3 KW is not bad for daily commute if someone charges overnight regularly. If he/she misses even a single day or even a few hours, he has to go through 'range anxiety'. Nevertheless, I removed the statement you indicated.

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  5. The plural of EV remains EVs

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