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.

LEAF Instrumentation

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.

  • Driving at 60 mph on level freeway uses about 250 Wh / mi.
  • Driving uphill requires about 1.5 kWh per 1,000 feet elevation gain. You'll get back about 1 kWh on the downhill side.
  • Rain increases energy use. We use 20 Wh/mi as a rule of thumb.

    It seldom rains hard in Seattle; it's usually more of a mist or steady drizzle, so energy use in heavy rain will be higher.

  • Winter driving in Seattle (temperatures in the 40's) requires about 15% more energy.

    We typically see increased energy use of 10%-20% during winter.

  • Using climate control (air conditioning or heat) will increase energy use.

    Using just the fan has minimal impact.

  • Faster speeds will increase energy use per mile, escalating rapidly when traveling at freeway speeds.

    For example, increasing from 60 mph to 70 mph will reduce your range by about 20%.

    See JB's blog on range vs. speed in a Tesla Roadster for a great explanation of this.

  • Slower speeds will decrease energy use per mile, but can of course result in higher energy use in climate control as it will be on longer.
  • Wind will have an impact on your range, either good or bad depending on direction.
  • In a LEAF, there are about 20-21 kWh available for use from a full charge.

    The battery in the LEAF is 24 kWh, but the car restricts its use to the middle of the battery's range, avoiding both fully charging and fully discharging the pack to prolong the life of the batteries.

    Charging to full at Level 2 with a cool battery (5 temperature bars) tends to get to a bit over 22 kWh. Our two experiences charging to 100% with DCQC resulted in just over 21 kWh (263 gids at 7 temperature bars, 84 degrees ambient temperature; 266 at 6 bars, 61 degrees).

    Our rule of thumb is to avoid the bottom 5% of the battery, or 1.2 kWh. The turtle appears at 0.64 kWh, with minimal range left at this point. The very low battery warning is shown at 1.92 kWh, which alerts us that we'll need to charge within 2-3 miles.

Example

Here is an example for driving from Skykomish, WA, to Stevens Pass:

  • The distance from Skykomish (1000' elevation) to Stevens Pass (4050') is 16 miles.
  • At 250 Wh / mi, it will require 4 kWh for the 16-mile trip.
  • The elevation change of 3050', assuming 1.5 kWh / 1000', will add 4.58 kWh to our energy needs.
  • In nice weather, that would mean a total of 8.58 kWh for the trip (38.4% of a LEAF's pack).
  • In rainy weather, adding 20 Wh / mi would increase the total to 8.9 kWh (39.9%).
  • In cold weather, using 15% additional energy would bring the total to 10.23 kWh (45.6%).

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.

External Resources

We like these two sites for elevation information:

  • http://www.jurassictest.ch/GR/ – This site shows only metric units, but the elevation graph has a nice interactive feature where it shows corresponding map locations as you drag along the graph. This site also does the energy use calculation for various vehicles, including LEAF, Roadster, and iMiEV.
  • http://www.gpsvisualizer.com/elevation – Paste a Google Map URL here, change units to English, and click the "Draw elevation profile" button.

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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