Note - the plot is a little hard to read after resizing. The left vertical axis is efficiency in miles / kWh. The right vertical axis is temperature in degress Fahrenheit. Data points are taken monthly and this represents a little over 2 years worth of data.
I have a 2013 SV and live in Seattle. I drive about 1300 miles a month. For some time I've been thinking about what the main factors are that drive efficiency. I've noted that in the summer I peak out at 4.5 or 4.6 miles / kWh, but bottom out at 3.4 miles/ kWh in winter (all efficiency figures measured "from the wall"). This difference has always seemed rather extreme to me, particularly in light of Seattle's benign climate.
The efficiency data is calculated from the energy meter on my EVSE (adding any kWh from network charging) and dividing by miles driven in the month. For this plot, I plotted the mean ambient monthly temperature from a weather station not far from my house and compared it to the seasonal variation. The correlation is extremely high.
Since my annual usage pattern is quite uniform (commuting + errands), this data suggests to me that efficiency in miles kWh, across the 40 degree - 70 degree F range, is nearly proportional to ambient temperature. Furthermore, the slope (.043) suggests that a 10 degree temperature differential is equivalent to .43 additional miles per kWh. From a range perspective 10 degrees is equivalent to 8.6 additional miles of range (assuming 20 kWh usable battery capacity). The above statement assumes equivalent driving habits and usage patterns to mine so your mileage may vary, but I thought it was an interesting result.
