I found another data point: an experience of @pmiddeld for +4 °C with temperature maintenance heating power level 0.5 kW, reported here: http://www.ioniqforum.com/forum/35281-post81.html
. The graph was updated only by adding this data point, which turns out to fit well to the previously determined parameter values.
Maybe some more explanation is useful. An important entity for the above posts is how much heat is flowing out of the car each minute or hour, which is called the heat loss. This heat loss depends proportionally on the temperature difference between inside and outside, according to the so-called heat loss rate
, which is a parameter that indicates heat loss for just 1 °C difference in temperature. The heat loss rate was now estimated at 80 Watt per °C difference in temperature between outside and inside. For example, when the outside temperature is +4 °C and the interior is at 22 °C, then this temperature difference is 18 °C, so the heat loss is 80*18 Watt = 1440 Watt. You can interpret this in the sense that the car heats the outside air as if it was a 1440 Watt heater.
To compensate for this heat loss, and thus maintaining the interior temperature at the same level, you need to add the same amount of heat by the car's heater. Fortunately, under these conditions a heat pump can produce 1440 Watt by only using a fraction of that amount of energy, in this case about 500 Watt. To know how much in kWh this will take from the battery you can just multiply this number by the duration of the trip in hours. For example, for a two hour trip it will cost your battery 2*500 = 1000 Wh = 1 kWh. This reduces the available full charge of 28 kWh by 1/28 = 0.036, which is almost 4%. You can expect a reduction in range by the same percentage due to having the heater on.