Information from various forums on the internet suggests an average energy usage of around 367 Whr /mile for the Tesla, as reported by drivers in the US; there is of course no information on pattern or style of driving this relates to. This is similar to the EPA figure of 380 quoted above, which implies a range of 224 miles, dropping to a working figure of 204 miles if we allow for a 20 mile reserve. Both figures seem consistent with the data from the Tesla website. Some posts on the internet suggest that under some circumstances the range may be as low as around 145 miles.
It is also possible that driving in the UK may well involve a different mix of urban and motorway driving driving, and a different set of climate conditions. The problem with unofficial data from the internet is of course that it is from a self-selected group and can be expected to show bias. Since the figures for the Tesla are close, it seems reasonable to use the EPA combined figure as the basis for evaluation – it is generated from a defined and repeatable testing regime and can be compared fairly with similar data for other similar vehicles. The 380 Whr/mile figure is taken to be a reasonable estimate of real world performance.
It does not necessarily follow that the combined figure is the right basis to compare all vehicles, since it is quite possible that the characteristics of EV’s just happen to make this valid for that type of vehicle. Evidence points to overall, real world, ICE car fuel figures being lower than indicated by the standard tests. Notwithstanding their limitations, it is the case that the official figures represent the only repeatable, easily available basis for comparing the energy usage of different cars.
380 Whr/mile can be expressed as the inverse (distance per unit fuel) as 2.63 mile/kWhr.
The Nissan Leaf (the largest selling EV in the UK and US?) is probably limited to a useful range of less than 130 miles, even with the new 30 kWhr battery and under favourable conditions. The official US EPA energy use figure for this model (2016 Leaf, 30 kWhr) for combined driving is 3.3 mile/kWhr, which implies a typical range of not more than 90 miles. The limited information available on the internet tends to paint a much rosier picture of both the energy usage and the range. Perhaps this illustrates that anecdotal information on EV power usage should be treated with scepticism.
The actual energy requirement is higher than might at first appear, for the simple reason that charging the battery is not 100% efficient, due to the need to convert from AC to DC and the losses inherent in converting electrical energy to chemical within the battery. There will also be losses when the battery is discharged. It is assumed here that the energy figures relate to the available power from the battery, and that there is a nett loss of 15% between the power input (into the charging system) and the power available from the battery.
The batteries in the Tesla may lose around 1% of their charge per day. This is not insignificant. By itself it would increase energy consumption by around 10% for the average mileage driver. This is assumed to be rolled into the 15% loss figure given above.
For the Tesla the figure of 2.63mile/kWhr drops to 2.24 mile per kWhr input at the charging point.
This figure is for a new EV. The treatment a battery gets will affect its life and its capacity, and rechargeable batteries tend to degrade with each charging cycle. The loss of capacity in Tesla cars seems to be of the order of 0.025% per charge, or perhaps 1% per year at normal mileages. Other sources suggest perhaps 1.2% per 10,000 miles. Not a lot, but it doesn’t help the range (a 5% reduction is a loss of 12 miles), and there are reports of occasional much larger loss rates. It is unclear if the battery efficiency also deteriorates with time.
The figure of 204 miles for the range is low when compared to conventionally powered cars, particularly if there is a risk that on any given journey even that figure might not be attained.
Tesla offer special ‘supercharge’ stations that can recharge the battery fully in about 40 minutes; charging at home is a lot slower, a full charge will take the best part of 4 hours at 22 kW. But 22 kW is equal to 90 amps at 240V, way above anything a normal domestic circuit can deliver. A typical domestic power circuit will be limited to about 16 amps, or 3.8 kW ; at this rate it would take around 26 hours to recharge a flat battery system. To get reasonable recharging rates at home will require a separate power supply direct from the consumer unit using at least a 6 mm2 cable. Even a 10 mm2 cable will only be rated for a current of 45 amps, giving a charging rate of around 11 kW (recharge in 7-8hrs). So not just any old socket in the home will do; a dedicated heavy duty power supply specifically intended for recharging will be required to get a reasonable recharge rate.
I drive an automatic diesel Volvo V70. Quite different of course from the Tesla, but it will nevertheless serve as a real world comparison, as a practical, large, well made, ICE car. The official fuel figure for this model is 55.3 mpg (combined). As is well known, the official tests over state the real fuel economy since they are performed under optimal conditions and with non-essential electrical users like a/c switched off. Ref. 13 gives the average reported (real-world) fuel consumption figure for this car model as 44.4 mpg, consistent with the fuel economy figures I get. Using 45 mpg rather than 55.3 represents an uplift factor on CO2 emissions of 23% .
The V70 fuel tank takes 70 litres (15.4 UK gallons), so taking 45 mpg (9.91 mpl) as a reasonable figure for real world overall fuel economy this gives a maximum range of 694 miles, and a nominal working range of 650 miles. If the car was manual then the range would be marginally higher, and the fuel economy better.
Judging from the US EPA figures the Tesla energy usage is relatively independent of the cycle used for testing, whereas internal combustion engines show a marked difference. The implication is that electric vehicles are most suited to low speed, intermittent urban driving, where ICE's can suffer a big drop off in overall efficiency.
EV02-0116
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