Predicting Energy Use
Last Updated July 23, 2012
We have collected and analyzed data from a variety of driving situations and have compiled the information below as a guide for estimating how much energy is likely to be required for a trip in an electric vehicle.
While this level of analysis isn't necessary for regular daily driving well within the range of the vehicle, it can be useful when planning a longer trip, especially when elevation changes are involved.
You will be able to use a certain number of kilowatt-hours (kWh) from your electric vehicle's battery pack. (The exact value will vary based on vehicle model.) The car will spend or gain kWh as you make your trip. The values below are the estimates we use to predict those effects. Note that the pack kWh is not something that can be directly measured. The car determines this value through sophisticated measurements and calculations, which result in periodic adjustments that are seen as "jumps" in the energy estimate.
We recommend using conservative values, assuming that everything will take just a little more than you expect, planning not to use the bottom 5-10% of the charge, and of course, watching your gauges to make sure you truly have the energy you need.
For the LEAF, it's important to understand that the display showing miles remaining is an estimate based on very recent driving. This means that when you are climbing a long, steep hill, it is likely to start predicting that you won't make it to your destination down the other side. Even worse, as you descend, it will produce an overly-optimistic estimate that will erode quickly when you reach flat (or uphill) terrain. We ignore the car's display of remaining range, as it is erroneous and misleading.
The SOC bars provide a coarse but reliable way to monitor the energy remaining in the battery.
We have an after-market state-of-charge (SOC) meter that enables higher resolution monitoring of battery state than the factory instrumentation. This meter shows the SOC as a percentage, and also in a unit called a "gid" (named by the owner community in honor of the meter's creator, Gary Giddings). Each gid represents 80 Wh of energy in the battery.
By giving the driver better information about the state of charge, especially in low charge situations, having an SOC meter adds about 10% to the usable range of the LEAF. Without a meter, conservative drivers will avoid going below 1 bar, essentially abandoning the bottom 15-20% of the battery. More adventurous drivers recognize that there is substantial energy available below the last bar and may try to use it without any information about how much is left, thus risking running out of juice. With state-of-charge information at the percent level, drivers have the information they need to confidently use more of the battery's capacity and extend the usable range of the car.
Calculating Energy Use
These are the values we use to predict how much energy we'll need for a trip. We have observed that the Nissan LEAF, Tesla Roadster, and Mitsubishi iMiEV have very similar energy profiles, so these values should work well for any of them.
Note that these values are a just a guideline! They have worked well for us, but you may find that you need slightly different values based on your driving style.
Here is an example for driving from Skykomish, WA, to Stevens Pass:
So, if you're driving up to Stevens Pass in a LEAF on a cold, rainy day, you'll want to stop to charge in Skykomish if you're below about 55% (assuming getting to the pass with 10% remaining). This would be 155 gids, or making sure that you have at least 7 bars.
Using a Spreadsheet
This spreadsheet (XLS, 23k) provides an example for calculating energy use, including accounting for hills and rain. It shows the kWh required for several drive segments, and for LEAF drivers includes the corresponding values for gids, SOC, and bars.
We like these two sites for elevation information:
You can use the wind map at http://hint.fm/wind/ to see the current wind conditions. Double-click to zoom.
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