With more than 100 Roadsters delivered to customers so far and more on the road each week, it’s natural for some customers to run “experiments†on them. Because we have such an entrepreneurial and highly technical customer base, many of these experiments are quite detailed and attempt to answer questions that we have in some cases never discussed publicly.
ne of the most common and tricky experiments is testing how far the Roadster will go on a single charge in various driving conditions. For the latest powertrain configuration (powertrain 1.5), we have demonstrated dynamometer test results of 244 miles range in a complicated combination of highway and city drive cycles defined by the EPA.
However, this is just one data point; real-world range can vary substantially depending on driving style, environmental conditions, and usage of accessories such as the electric cabin air conditioning system and the electric cabin heating system. The cycles defined by the EPA attempt to make a representative average of these different factors and combine it into one number. Many customers, potential customers and curious observers want the details behind this average and how results may change depending on conditions.
We need a designed set of experiments to understand how efficiently or how far a Roadster could drive under different conditions. This can quickly become a huge amount of testing as there are so many variables and conditions, but fortunately we have spent quite a bit of time internally building a very accurate computer model of how the Roadster will behave under different driving scenarios. We’ve validated the model by testing at a reduced number of points  enough to give us good confidence in the results. The details of this model are quite interesting and could be a whole separate blog, but for now let’s just use it as a tool to help us understand Roadster efficiency and range.
The simplest experiment to consider, and one that gives great insight into the whole vehicle performance, is how efficiency and range vary as a function of driving speed. This is assuming that speed is held constant (i.e. cruise control) at each point. For any test or model run, there are many inputs that need to be specifically stated to make the results meaningful. So here are some of the critical inputs that we have assumed:
• Single driver ~180lbs
• Soft top or Hard top on vehicle (with windows up)
• No air conditioning usage
• No heat usage
• No headlights or cabin air blower (large 12V loads)
• Tires inflated to recommended efficiency setting 30/40 front/rear psi
The outputs of this model run are in battery energy usage per mile. In the case of an EV this is typically expressed in terms of Wh/mile. A 100W light blub running for 1 hour will use 100 Wh of electricity. Since 1 Watt is also just 1 Joule/second you can easily convert Wh into Joules by multiplying by (60 [seconds])*(60 [minutes]). It is also important to note that we are discussing DC Wh/mile or energy coming OUT of the battery pack inside the vehicle. This is very relevant to range but does not consider the losses associated with the onboard battery charger or some of the slight round-trip energy losses in the battery itself....
