The range of an electric vehicle is dictated primarily by the size of the battery pack and the driving efficiency. How efficiently you drive is obviously down to driving style and speed, but with EVs, other factors like temperature and heater use impacts on this too.
To understand how much range you're going to get from a vehicle, you need to appreciate that your energy usage (i.e. driving efficiency) has a huge impact on the distance you're going to be able to travel and there's many factors that effect your energy usage at any one time.
So when looking at the claimed range of a vehicle, you need to understand how this is achieved and all the factors that can effect this.
In an EV, the battery pack is the fuel tank. Battery capacity is measured in kWh (kilo watt hours) and the fuel is obviously electricity. The larger the battery, the more electricity it can store and the further the vehicle can drive.
As a very rough and simplistic guide, if you assume an average economy of 4 mpkWh (miles per kilo watt hour), you'll get the following range from these battery sizes:
22kWh = 88 miles
30kWh = 120 miles
40kWh = 160 miles
64kWh = 256 miles
and so on.
The rate at which you use your fuel (i.e. electricity) obviously dictates the range, just like it does in an ICE vehicle. But with EVs, there's other factors we have to consider as these can have a large effect on the efficiency. Many are the same with ICE vehicles, but because the fuel tank stores more energy and therefore the range is longer on an ICE vehicle, we tend to ignore all the factors that effect the efficiency.
The way you drive has the largest impact on the economy - just like it does with an ICE vehicle. Your speed, acceleration, braking all effect the economy. An EV will be far less efficient at motorway speeds due to wind resistance and the constant energy drain from the battery without any slowing down for traffic lights, junctions and roundabouts to allow the regen braking to put some charge back into the battery. EVs are at their most efficient when driving in urban environments and rural B-roads simply because the average speed is much lower and there's plenty of slowing down for corners, junctions, traffic etc. This is as long as the car is driven properly and the driver anticipates these and allows the vehicle to slow down using the motor (regen braking effect) which charges the battery, rather than using the mechanical brakes (which just throws away that kinetic energy into friction and heat and unnecessarily wears down your brakes).
You have probably heard that EVs are more efficient in the summer than in the winter. This is mostly due to two reasons. Firstly batteries are less efficient when cold, so in colder winter temperatures, the battery doesn't perform so well. Secondly, heating an EV can use huge amounts of energy which will result in a second drain on the battery pack and therefore less range. Depending on the vehicle design, the range can be anything from 15-30% less in the winter.
There are features in EVs which help to mitigate the losses in cold temperatures. Many EVs now have heat pump heater systems which are more efficient than a resistive heater. Some will have heated seats and a heated steering wheel which uses far less energy than the heater and is a more efficient way of keeping warm. Most EVs have pre-conditioning which allows the car to heat (or cool) while connected to the charger before you set off. Some newer EVs have heaters in the battery packs to keep them at their optimum temperature.
Other factors which affect economy
The power of the electric motor in the vehicle has an impact on the economy. A bigger, more powerful motor will use more electricity.
The efficiency of the motor and the other components in the powertrain can very between models. The size, weight and aerodynamics of the vehicle will all have an impact on its efficiency too. For example, the first generation Hyundai Ioniq is the most efficient EV due to its low drag efficiency and a very efficient powertrain. These do around 1.5mpkWh more than other EVs of the time and even though it only has a 28kWh battery, they can do the same range as a 40kWh Leaf.
Driving in the rain can reduce range a little due to the extra resistance of water on the road, particularly if there's lots of puddles. Driving in strong wind will have an impact of efficiency. But of course this is no different to an ICE vehicle.
What economy should I expect?
As a general rule, if you drive your EV efficiently, you should get around 4.5mpkWh in the summer. Without air conditioning on and very careful driving, some may get up to 5.5mpkWh. I can get up to 7mpkWh in my Hyundai Ioniq! In winter, reduce this by around 20%, so expect around 3.5mpkWh.
So using these figures of 4.5mpkWh in the summer and 3.5mpkWh in the winter, you would expect the following ranges from these battery sizes:
22kWh = 99 miles in summer, 77 miles in winter
30kWh = 135 miles in summer, 105 miles in winter
40kWh = 180 miles in summer, 140 miles in winter
64kWh = 288 miles in summer, 224 miles in winter
But of course, this all does depend on the vehicle and your driving style.
Manufacturer's claimed range vs real world range
Manufacturers use a testing procedure to prove the fuel consumption and emissions of their vehicles. We're used to the tested economy figures on ICE vehicles being unrealistic in the real world and many cheated the diesel emission tests, so these haven't been realistic either! But the same applies to EVs, where the tested range stated by the manufacturer doesn't quite match what you'd achieve in real world driving. What confuses the matter even further is that there are three different test cycles which give different results. These test are:
EPA (Environmental Protection Agency) used in the United States
NEDC (New European Driving Cycle)
WLTP (Worldwide Harmonised Light Vehicle Test Procedure)
These EPA test cycle includes more highway driving, whereas the NEDC tends to favour city driving. In the real world, EPA range figures are generally easily achievable with UK driving, whereas the NEDC was over optimistic by quite a huge margin. The newer WLTP test cycle started in 2017 and all vehicles in Europe must use from Sept 2019 is more realistic as it is a more thorough test.
For example, the Nissan Leaf 40kWh is rated at:
EPA 151 miles
NEDC 235 miles
WLTP 168 miles
In the UK, the real world range would be around 162 miles in the summer and around 126 miles in the winter.