Formula E ‘energy’ & ‘power’ usage explained

Following another great Formula E event in Punta del Este, Uruguay, I thought I’d just unravel a couple of things that we didn’t get a chance to talk about in our live ITV4 show on Saturday. There’ve been a few drivers suffering penalties recently for various indiscretions, either by them or their teams, that I think need a little more explanation. In Malaysia a few weeks ago, we saw Jerome d’Ambrosio, or ‘Custard’ as he’s affectionately know in the paddock, get excluded from qualifying for “exceeding power usage”.

There’s an important distinction to make here, which is the difference between ‘energy’ and ‘power’. In simple terms, energy is effectively the amount of work the battery can do, whilst power is the rate at which it’s asked do it. The drivers have a defined amount of usable energy (28kwh) in the race for each car, which is slightly less than the actual total capacity of the battery. The percentage figure you see on screen next to drivers names during the race is the percentage of usable energy each driver has left with that car, i.e. how much of his or her 28kwh. It’s up to the driver and their team to manage energy usage so as not to go over that limit. We saw Daniel Abt penalized for exceeding this figure in Beijing by a small amount. Power’s measured in kw and the drivers have a ‘power map’ or torque control switch on the steering wheel with six settings available to adjust this.


The regulations stipulate the maximum power for qualifying (200kw) and the race (150kw), with the temporary additional boost up to 180kw with Fanboost. In the race, drivers will change the power map setting to control their pace at different points and the rate at which they’re using up the 28kwh of useable energy. You can perhaps equate it to fuel flow in Formula One, where teams have a maximum of 100kgs of fuel for the race and can vary the rate at which they use it, through engine maps, but can’t go over a 100kgs/hour fuel flow limit. In qualifying however, they’re all naturally using maximum power (200kw) on their timed laps, so it begs the question, how is it possible to exceed this limit, when in theory the system can’t physically ever deliver any more power than this anyway?

The answer’s not a particularly sophisticated one, but lies in the fact that teams program their own power maps into their cars and rely on ‘the system’ to keep it from spiking over the allowed limits. The steering wheel controlled maps are essentially bits of software code to tell the Williams battery how much power to deliver through the McLaren motor control unit into the eMotor itself. Because of various system tolerances and electrical losses, when a team writes a map to request 200kw, typically they may only see something around 190-195kw as a final output. It may only seem like a small amount, but it’s a performance differentiator, so teams write maps to demand a figure that’s just over the 200kw in qualifying and just over 150kw in the race and hope that both the McLaren and Williams software will catch any spikes and prevent them from breaching the legal limits.

On a couple of occasions that hasn’t happened and the FIA data’s showed marginal, but none-the-less, excessive power usage and the result is an immediate penalty, as happened to d’Ambrosio. Teams would argue that the various systems on the car should be able to give them their maximum allowed power without the worry of going over it and you could perhaps see their point. The FIA and stewards however, would no doubt retort with the argument that if teams know there’s a risk when writing maps that demand more than their allowed maximum power, then they simply shouldn’t do it. Also back in Malaysia, Katherine Legge received a ten place grid penalty to be applied at the next event, for changing her RESS (Rechargeable Energy Storage System), or the car’s battery, after a failure. This is similar to the engine or gearbox change rules in Formula One where a component failure results in a grid drop penalty. She didn’t race in Uruguay, so rather than apply the penalty to her car, driven by Salvador Duran, it will apply to the next event she takes part in.


At the same race, Nick Heidfeld was penalised for not conducting his car swap in the designated area. That area is the team’s garage and on that occasion Nick crashed and returned to the garage on foot to take the second car.

In Punta del Este this weekend, Bruno Senna was excluded from qualifying after his team breached a regulation that states all cars taking part are under parc ferme conditions from five minutes prior to the first qualifying group getting their green light. Parc ferme conditions stipulate that no work, other than temperature management, can be carried out on the cars during this period.

Check out my video explanations of the Williams Formula E battery and McLaren electric motor.

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First published on my blog December 2014