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Charging losses in electric cars

How high are they really, and what can be done about them?

Charging losses in electric cars occur during every charging session. An ADAC measurement reveals major differences between a domestic socket, a wallbox and a fast-charging station. Charging via a wallbox is particularly efficient. With DC charging, battery temperature and preconditioning influence the losses.
© ADAC/Martin Hangen
THE KEY POINTS IN BRIEF
• A wallbox instead of a domestic socket: charging losses are significantly reduced.
• DC charging is technically efficient: however, battery temperature can reduce this benefit.
• Consider charging power: faster AC charging reduces time-dependent ancillary losses.

Do you know how much elec­tric­i­ty your elec­tric car uses? Of course, you might think – the on-board com­put­er shows the con­sump­tion, after all. But that’s not the whole sto­ry. When elec­tric cars are charg­ing, ener­gy is lost – ener­gy that you have to pay for but which doesn’t end up in the bat­tery. And the on-board com­put­er sim­ply doesn’t show this loss. Charg­ing loss­es can­not be com­plete­ly pre­vent­ed. But in times of ener­gy short­ages and high elec­tric­i­ty prices, you should make sure to keep these loss­es as low as pos­si­ble. And this is actu­al­ly pos­si­ble for every user – you just need to know how.

Reducing charging losses in electric cars

Reduc­ing charg­ing loss­es in elec­tric cars starts with choos­ing the right charg­ing method. This is because not all the ener­gy you pay for actu­al­ly reach­es the bat­tery dur­ing charg­ing. Some of it is lost due to tech­ni­cal process­es. The on-board com­put­er usu­al­ly does not dis­play this addi­tion­al ener­gy con­sump­tion. As a result, many dri­vers under­es­ti­mate the actu­al charg­ing costs.

The ADAC has inves­ti­gat­ed var­i­ous charg­ing sce­nar­ios. It com­pared AC charg­ing at home with DC charg­ing whilst on the move. The results show sig­nif­i­cant dif­fer­ences:

  • The high­est charg­ing loss­es occur when using a domes­tic sock­et.
  • A wall­box oper­ates much more effi­cient­ly.
  • Con­ver­sion loss­es are par­tic­u­lar­ly low dur­ing DC fast charg­ing.
  • Bat­tery tem­per­a­ture can cause addi­tion­al loss­es dur­ing fast charg­ing.
  • A high AC charg­ing pow­er reduces time-depen­dent aux­il­iary pow­er con­sump­tion.

Why do electric cars experience charging losses?

Charg­ing loss­es arise from sev­er­al tech­ni­cal process­es. An elec­tric car’s trac­tion bat­tery stores direct cur­rent. The mains sup­ply, on the oth­er hand, pro­vides alter­nat­ing cur­rent. Dur­ing AC charg­ing, the vehi­cle must there­fore con­vert the cur­rent. This task is car­ried out by the on-board charg­er. This is where some of the charg­ing loss­es occur.

In addi­tion, var­i­ous sys­tems require ener­gy dur­ing the charg­ing process. These include con­trol units and the 12-volt elec­tri­cal sys­tem. Charg­ing cables, wiring and the bat­tery also cause minor loss­es. The longer a charg­ing ses­sion lasts, the greater the impact of these aux­il­iary con­sumers can be. This effect is par­tic­u­lar­ly evi­dent when charg­ing from a domes­tic sock­et, where the charg­ing pow­er is com­par­a­tive­ly low.

Charging your electric car at home: socket or wallbox?

Elec­tric car dri­vers have a range of charg­ing options avail­able at home. House­hold sock­ets and wall­box­es are par­tic­u­lar­ly com­mon. A house­hold sock­et is tech­ni­cal­ly sim­ple to use. How­ev­er, it is not the best solu­tion for reg­u­lar charg­ing.

A wall­box, on the oth­er hand, offers high­er charg­ing pow­er. Depend­ing on the vehi­cle and instal­la­tion, charg­ing rates of up to 11 or 22 kilo­watts are pos­si­ble. This sig­nif­i­cant­ly reduces charg­ing time. At the same time, the per­cent­age of charg­ing loss­es is reduced.

Charging losses at the domestic socket

Charg­ing from a domes­tic sock­et result­ed in the high­est ener­gy loss­es in the ADAC test. There were sig­nif­i­cant dif­fer­ences between the vehi­cles test­ed.

Loss­es of up to 24.2 per cent were mea­sured for the Mer­cedes CLA EQ. For the Tes­la Mod­el Y, by con­trast, they stood at 12.7 per cent. The VW ID.7 record­ed a fig­ure of 15.3 per cent. The results thus reveal con­sid­er­able dif­fer­ences between the vehi­cles.

One rea­son lies in the charg­ing pow­er. At low pow­er, the charg­ing process takes con­sid­er­ably longer. Dur­ing this time, var­i­ous elec­tri­cal sys­tems remain active. This increas­es the ener­gy required by aux­il­iary con­sumers. The domes­tic elec­tri­cal instal­la­tion can also play a role. Long cables can cause addi­tion­al loss­es. In the case of old­er instal­la­tions, a spe­cial­ist firm should there­fore check their suit­abil­i­ty. This applies par­tic­u­lar­ly in the case of a per­ma­nent­ly high load.

The Wallbox reduces charging losses

The wall­box oper­ates much more effi­cient­ly dur­ing AC charg­ing. In the ADAC test, loss­es of 11 or 22 kilo­watts ranged between 5.1 and 7.0 per cent. This meant that the loss­es were rough­ly half those of a domes­tic sock­et. The high­er charg­ing pow­er also reduces the oper­at­ing time of oth­er elec­tri­cal appli­ances.

A wall­box also offers greater con­ve­nience and safe­ty. This makes it par­tic­u­lar­ly worth­while for reg­u­lar charg­ing at home. Any­one wish­ing to reduce charg­ing loss­es with their elec­tric car should there­fore use a wall­box wher­ev­er pos­si­ble.

Photovoltaic systems: How high are the losses?

More and more elec­tric car dri­vers are charg­ing their vehi­cles using solar pow­er they have gen­er­at­ed them­selves. Sur­plus PV elec­tric­i­ty is of par­tic­u­lar inter­est here. The vehi­cle is often charged at a reduced charg­ing pow­er. This allows the use of one’s own solar pow­er to be max­imised. In the ADAC test, charg­ing loss­es dur­ing sur­plus PV charg­ing ranged between 8.0 and 12.8 per cent.

These fig­ures are high­er than those for fast wall­box charg­ing. Nev­er­the­less, the method can still make eco­nom­ic sense. The rea­son lies in the low-cost or self-gen­er­at­ed elec­tric­i­ty. As a result, the addi­tion­al ener­gy loss­es are less sig­nif­i­cant.

How to reduce AC charging losses

When it comes to AC charg­ing, there is a sim­ple rule of thumb. The high­er the charg­ing pow­er, the short­er the charg­ing time. This reduces the rel­a­tive ener­gy demand of oth­er elec­tri­cal appli­ances. There­fore, the high­est pos­si­ble charg­ing pow­er is usu­al­ly more effi­cient. Any­one wish­ing to reduce charg­ing loss­es in an elec­tric car should there­fore make sen­si­ble use of their wall­box. A high charg­ing pow­er is par­tic­u­lar­ly ben­e­fi­cial for short charg­ing ses­sions. How­ev­er, a low­er pow­er set­ting may still be advis­able for charg­ing using sur­plus solar pow­er, where the focus is on util­is­ing your own solar-gen­er­at­ed elec­tric­i­ty.

DC charging on the go: Why are the losses lower?

With DC fast charg­ing, the pow­er con­ver­sion works dif­fer­ent­ly. The charg­ing point already sup­plies direct cur­rent to the vehi­cle. The on-board charg­er there­fore does not car­ry out the same con­ver­sion as with AC charg­ing. This reduces the asso­ci­at­ed con­ver­sion loss­es. ADAC mea­sure­ments con­firm this advan­tage. The con­ver­sion loss­es at the fast-charg­ing point aver­aged around three per cent. How­ev­er, oth­er ener­gy loss­es occur dur­ing fast charg­ing. The tem­per­a­ture of the bat­tery is par­tic­u­lar­ly impor­tant in this regard.

Battery temperature affects charging losses

Lithi­um-ion bat­ter­ies oper­ate par­tic­u­lar­ly effi­cient­ly with­in a spe­cif­ic tem­per­a­ture range. High charg­ing cur­rents also gen­er­ate heat. The vehi­cle there­fore some­times has to active­ly cool the bat­tery. This requires addi­tion­al ener­gy.

At low tem­per­a­tures, a dif­fer­ent prob­lem aris­es. The bat­tery often needs to be warmed up before fast charg­ing. This ener­gy is not sub­se­quent­ly avail­able for dri­ving. Nev­er­the­less, it is drawn from the mains and paid for. In the ADAC test, warm­ing up the bat­tery result­ed in addi­tion­al loss­es of between two and eight per cent. The out­side tem­per­a­ture there­fore has a sig­nif­i­cant impact on the effi­cien­cy of fast charg­ing.

Pre-conditioning saves time, but not necessarily energy

Many elec­tric cars can pre­con­di­tion their bat­tery before a fast-charg­ing stop. This involves bring­ing the bat­tery up to the cor­rect tem­per­a­ture whilst the car is still on the move. This can speed up the charg­ing process at the charg­ing point. How­ev­er, this does not elim­i­nate the ener­gy require­ment.

The heat­ing ener­gy required is sim­ply used before the charg­ing stop. This increas­es ener­gy con­sump­tion dur­ing the jour­ney. Con­se­quent­ly, less addi­tion­al ener­gy is required for heat­ing at the charg­ing point. Over­all, there­fore, a dis­tinc­tion must be made between time sav­ings and ener­gy effi­cien­cy.

How high are the losses during DC fast charging?

The ADAC test­ed four elec­tric cars under var­i­ous con­di­tions. These includ­ed the Hyundai Ion­iq 6 and the Renault Mégane E‑Tech Elec­tric. The Tes­la Mod­el Y and the VW ID.3 were also test­ed. In each sce­nario, 30 kilo­watt-hours were charged. Three dif­fer­ent con­di­tions were exam­ined:

  • a warm bat­tery after a jour­ney or pre­con­di­tion­ing
  • a cold bat­tery after a jour­ney or pre­con­di­tion­ing
  • cold bat­tery with­out pri­or dri­ving or pre­con­di­tion­ing


The mea­sure­ments took into account the ener­gy drawn from the mains. In addi­tion, the ener­gy was mea­sured at the charg­ing point. The actu­al ener­gy stored in the bat­tery was also a key fac­tor. Any ener­gy not sub­se­quent­ly used for dri­ving was clas­si­fied as a loss. Over­all, ener­gy loss­es dur­ing DC charg­ing ranged between five and 15 per cent.

AC or DC: Which charging method is more efficient?

In terms of ener­gy loss alone, DC charg­ing can offer advan­tages. This is par­tic­u­lar­ly true when the bat­tery is at the opti­mum tem­per­a­ture. As soon as heat­ing or cool­ing becomes nec­es­sary, this effi­cien­cy advan­tage dimin­ish­es. There­fore, actu­al effi­cien­cy depends heav­i­ly on the con­di­tions.

When it comes to costs, the pic­ture is dif­fer­ent. Pub­lic DC charg­ing is often sig­nif­i­cant­ly more expen­sive than charg­ing at home. The effi­cien­cy advan­tage usu­al­ly can­not off­set this price dif­fer­ence. DC charg­ing can only be cheap­er if the prices per kilo­watt-hour are iden­ti­cal. For every­day use, there­fore, AC charg­ing at home remains par­tic­u­lar­ly attrac­tive.

Reducing charging losses in electric cars: the best tips

Unnec­es­sary loss­es can be avoid­ed by tak­ing a few mea­sures. Choos­ing the right charg­ing pow­er is par­tic­u­lar­ly impor­tant.

  • Where pos­si­ble, use a suit­able wall­box at home.
  • Charge using the wall­box at the high­est pos­si­ble pow­er.
  • Avoid reg­u­lar­ly charg­ing from a domes­tic sock­et.
  • Make good use of sur­plus solar pow­er.
  • Keep an eye on the bat­tery tem­per­a­ture when fast charg­ing.
  • Use pre­con­di­tion­ing to reduce charg­ing stop times.
  • Take elec­tric­i­ty costs into account when com­par­ing options.
  • Have old­er elec­tri­cal instal­la­tions pro­fes­sion­al­ly inspect­ed.


The max­i­mum charg­ing pow­er should be suit­ed to the vehi­cle. A high­er pow­er out­put is not always pos­si­ble or advis­able.

What car manufacturers can do to tackle charging losses

Man­u­fac­tur­ers also have ways of fur­ther reduc­ing charg­ing loss­es. The effi­cien­cy of the on-board charg­er is par­tic­u­lar­ly impor­tant in this regard. AC charg­ing accounts for a large pro­por­tion of dai­ly charg­ing oper­a­tions. This area there­fore offers con­sid­er­able poten­tial for sav­ings.

The 12-volt elec­tri­cal sys­tem should also con­sume as lit­tle ener­gy as pos­si­ble whilst charg­ing. Con­trol units that remain active unnec­es­sar­i­ly increase loss­es. Fur­ther­more, man­u­fac­tur­ers should com­mu­ni­cate actu­al charg­ing loss­es trans­par­ent­ly. This would enable con­sumers to charge more effi­cient­ly and com­pare costs more effec­tive­ly.

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Picture of Erwin Kartnaller

Erwin Kartnaller

Erwin Kartnaller has been a fixture in Swiss trade journalism covering commercial vehicles and mobility for over four decades. After 16 years working for daily newspapers and family magazines, he moved into trade media and wrote for virtually every commercial vehicle magazine published in Switzerland, with very few exceptions. For nine and a half years, he was editor-in-chief of the Swiss Commercial Vehicle Association ASTAG's magazine, where his precise language and clear stance had a lasting impact on industry communication. This was followed by positions in the automotive industry, including at Eurotax autoinformation and aboutFLEET. He also wrote specialist articles for Auto&Wirtschaft and Auto Bild Schweiz. With his platform www.ekacom.com (http://www.ekacom.com/), he continued to wield his “sharp pen” – an independent voice for clear words and pointed commentary. In his last professional role as editor-in-chief of Polyscope, he immersed himself in the world of electronics – a field that is increasingly shaping mobility. Today, after his active professional life, Erwin Kartnaller is once again devoting himself entirely to his passion on the mobility-360.ch platform: mobility in all its facets.
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