Battery pack size, or for that matter gasoline tank size, is the primary determiner of driving range. It's a simple equation: Divide the energy stored on the vehicle by the energy consumption per mile, and you have the driving range:
range = energy stored / energy consumed per mile
In an electric car the quantity of stored energy is measured in kiloWatt-hours. One car might have a 24 kiloWatt-hour pack, and another have an 85 kiloWatt-hour pack, or well over 3 times the amount of energy. Assuming the two cars consume energy at the same rate, the second car will have 3.5 times the driving range.
Here's two concrete ways to correlate energy storage, energy consumption, and range.
range = gallons gasoline / gallons consumed per mile range = kiloWatt-hours / kWh consumed per mile
In practical reality it is far more complex than that. For example the car with the bigger battery pack probably weighs more, requiring more energy to drive down the road, and therefore achieving less efficiency in terms of kiloWatt-hours to drive a given distance. Or the same car driven up a mountain will consume more energy than when driven on flat land. Or a car driving into a steady wind will consume more energy.
For example, that 85 kWh car weighs a lot more than the 24 kWh car, and achieved a 265 mile range to the 84 miles range estimated for the 24 kWh car. The higher weight means a higher energy consumption per mile simply because of the weight. Therefore it doesn't get 3.5x the range of the 24 kWh car, but more like 3.1x.
Many people are asking when there will be an electric car with 400 miles range. The answer is a matter of balancing the kiloWatt-hours of battery, against the kiloWatt-hours consumed per mile, against the battery pack weight, and other factors like cost. An electric car can be built with enough energy storage to drive 500 miles, for example, but would it be too heavy to be efficient, or would it cost too much?
The official range estimate was measured in a laboratory based on standardized testing protocols, and not in the real world. Remember that it is an estimate, and nothing is better than your experience with your car on the roads you typically drive.
Even so, battery pack size is a great first order estimate of range. For example, for several months before GM released details of the Chevy Bolt we knew it would have a 200 mile (or so) range. Making the assumption GM meant the EPA range would be 200 miles, that meant the car would likely have a 60 kiloWatt-hour battery pack. How did we derive this guess? Simple, the 60 kiloWatt-hour Tesla Model S had a 208 mile EPA range. It's a rough extrapolation that 60 kWh gives about 200 miles range. But what really counts is the officially sanctioned EPA estimate, not the number tossed around by the automakers marketing department.
Another complication is driving habits - the hotrodder tends to consume more energy than the sedate driver.
Driving habits, energy consumption, and driving range
It's not all about how big your battery pack is, but what you do with it.
It doesn't matter what kind of car you have, it carries in its gas tank or battery pack a given quantity of energy. Your driving habits directly impact the rate of energy consumption. You can drive fast and hard, using up that energy more quickly, giving you less driving range, than if you hypermiled your way around town.
The story of the Tortoise and Hare is apropos. One day the Tortoise challenged the Hare to a race. The Hare, thinking he can outrun anybody, especially a pokey old creature like the Tortoise, gleefully accepted the challenge. You no doubt heard this story as a child, and know that the Tortoise ended up winning while the Hare could barely huff and puff his way across the finish line.
How does this apply to electric car drivers? Or, for that matter, to gasoline car drivers?
The Hare is like the hotrodder, squealing their tires at every chance, slamming the brakes hard at every stop, driving over the speed limit, etc. It's well known driving this way consumes energy like there's no tomorrow.
Optimizing your driving range means adopting some hypermiler techniques, rather than hotrodding your way around town. More aggressive driving habits means consuming more energy per mile, and therefore getting fewer miles of range. Such people, while they might enjoy the speed, especially the thrill of 100% torque at 0 RPM, will have to stop to recharge more often. Just like the Hare had to stop and recharge, and get beaten by the Tortoise.
Here's some numbers pulled out of thin air:
24,000 Watt-hours / 300 Wh per mile = 80 miles range 24,000 Watt-hours / 400 Wh per mile = 60 miles range
Take a 24 kiloWatt-hour car, drive it aggressively (400 Wh/mile versus 300) and your total range is slashed.
There's no magic to this, just simple physics. Consume more energy to drive, and your fixed quantity of energy will disappear more quickly. A hotrodding gasoline car driver faces the same fate.
Innate energy consumption
It's not just your driving habits, but the car's design, that affects energy consumption. To demonstrate this let's look at the energy efficiency of a few cars:
|2014 BMW i3 BEV||270 Wh/mile||124 MPGe||81 miles||22 kWh|
|2014 Chevy Spark EV||280 Wh/mile||119 MPGe||82 miles||19 kWh|
|2014 Honda Fit EV||290 Wh/mile||118 MPGe||82 miles||20 kWh|
|2015 VW e-Golf||290 Wh/mile||116 MPGe||83 miles||24 kWh|
|2015 Nissan Leaf||300 Wh/mile||114 MPGe||84 miles||24 kWh|
|2015 Kia Soul EV||320 Wh/mile||105 MPGe||93 miles||27 kWh|
|2014 Ford Focus Electric||320 Wh/mile||105 MPGe||76 miles||23 kWh|
|2015 Tesla Model S 85D||340 Wh/mile||100 MPGe||270 miles||85 kWh|
|2014 Tesla Model S 60||350 Wh/mile||95 MPGe||208 miles||60 kWh|
|2014 Tesla Model S 85||380 Wh/mile||89 MPGe||265 miles||85 kWh|
The BMW i3 has a smaller than usual battery pack, and most of the car's structure is made from carbon fiber. Because carbon fiber has an extremely high strength to weight ratio, the BMW i3 weighs very little but has an ultra-rigid structure to keep passengers safe. As a result it has the lowest energy consumption per mile of any electric car. How? A light weight vehicle takes less energy than a heavy vehicle.
Both the BMW i3 and Chevy Spark EV have a smaller-than-typical battery pack, but achieve the same range. The trick is that both are light-weight, the i3 because of carbon fiber and the Spark EV because it's simply a small car.
By contrast, while the Tesla Model S uses lots of light-weight aluminum, the ultra big battery pack is simply heavy. The weight negatively impacts energy efficiency, and it has the highest energy consumption of any electric car.
Note the direct correlation between high energy consumption and the lower the MPGe value.