E2 charging efficiency

With the upcoming move to RUCs for EVs I’ve been doing analysis of running costs and I’m curious about the efficiency of our E2 EVSE. I assume the kWhs logged by the E2 is electricity consumed (which in our case is almost exclusively sourced from our solar) but if that is correct then what losses are suffered from electricity into the E2 through to actual charge in our Polestar2’s battery? I realise that the car’s onboard charger will also have efficiency losses that play a part here and I’ll follow up on that in due course but I would be interested to find out what part the E2 is contributing to the overall energy loss.

I plan to monitor input vs. battery charge gained when I next do a significant charge of the car and will post the results I find at that time.

The numbers reported are the amount of energy “sent down the cable” into the car. The charger will also consume a little energy to power the LEDs and comms (from memory it averages at about 2-3W).

There are very minimal losses inside the charger itself. It’s essentially just a big switch with some signalling to tell the car how much power it can draw. There will be some tiny losses in the copper tracks and relay inside the charger, but these tend to be dwarfed by the losses in the wiring from your switchboard to the garage, and from the losses inside the car.

Car losses will be a combination of loss during AC/DC conversion, storage into the battery, and energy used to keep the car awake while charging. It’d be interesting to hear your results. I’ve heard that the “wall to battery” efficiency is usually in the 90-95% range, but haven’t looked at any figures for a Polestar 2.



Thanks for your reply Tom. Another Polestar2 owner from Belgium has carried out an ‘efficiency’ trial using the 16A EVSE (granny ‘charger’) supplied with the car and an MID-certified kWh counter and has posted on the Polestar forum that losses amounted to ~4.5% over a 13.5kWh charge event.

From your reply I would expect a similar result using 7kW AC EVSE like the E2 with the lion’s share of the losses being attributable to the car’s onboard charger due to its AC - DC inversion losses. I’ll update with my results when I get around to doing my own trial.

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Hi Tom,
I’ve had an opportunity to compare charge performance today with a 61%-90% battery SoC gain for our 68kWh Polestar2 battery. Calculated kWhs for that 29% charge is 19.7kWh with the Evnex session showing 19.5kWh (or 1.1% loss which is in the realms of rounding error [1% of SoC is 0.68kWh]).

That seems out of step with the ~4.5% efficiency loss found by the Belgian Polestar owner using a ‘granny charger’ so I’m now curious where Evnex get their charge session amounts from. Can you shed any light on this situation please?

Bear in mind that the car takes some energy just to be awake during charging. This can be up in the 200-250W region, which means that 1A is just being used to power the car circuits. For a 16A charger that means 6% wastage, but more like 3% for a 32A charger (or 12% for an 8A charger - yikes!). My guess is that your Polestar is a bit more efficient than this, maybe just 150-200W to stay awake.

Was it the Polestar which told you that 19.7kWh was supplied or is that an assumption based on the 0.68 * 29? There are various reasons why that might not be super accurate - perhaps it actually went from 61.4% to 89.5%, or perhaps the battery capacity is now slightly lower than 68kWh following some minor degradation.

Our figures come from the onboard energy meter which is accurate to about 0.5% in our tests.



Hi Tom,

I was aware of the effect of ‘rounding error’ as I stated. 1% increments of a 68kWh is a pretty course measurement as you’ve identified but it’s as good a gauge as I have available. My 19.7kWh was just 29% of 8kWh as you have assumed and a longer/larger charge would reduce the effect of rounding errors so next time maybe a 80+% SoC increase will be preferable in that regard. 0.5kWh is close to 2.5% of 19.7kWh but just 0.6% of 80kWh after all.

Calculated battery range is still within <0.5% of when new so I’m satisfied this won’t play a significant part at this stage but again minimised for a longer charge event.

I was aware that some input was required to kickstart the charging which will be part of the reason for the delay after physical connection of the EVSE is done. “Keeping awake” isn’t something I had considered (apart from your BYD efforts recently) but just keeping the car powered up while charging would come at an energy cost I guess.

So, in regard to your “onboard energy meter” comment does this monitor input or output current of the E2?

It monitors output current (i.e. what goes down the cable to the car).