Efficiency comparison - Model S v. Roadster

[ckessel]

It will be interesting to see the kilowatt-hour per mile data on the Model S when it is available, but I'll bet it is similar to the Roadster.

I'd think it'd be higher due to the larger mass of the car.

[hcsharp]

It's about 20% higher for the Model S.

[Larry Chanin]

It's about 20% higher for the Model S.

Thanks for the information. Can you tell us the source of your information?

Larry

[hcsharp]

Thanks for the information. Can you tell us the source of your information?

Larry

It's a simple calculation but I've also seen it somewhere... can't find it now. On Tesla's web site they recommend sizing your solar panel array for 300 wh per mile for the Model S. If you take the battery size compared to the range you get between 250 and 283 wh/mi depending on which pack you choose. For the Roadster the same calculation reveals 216 wh/mi. If you pick the mid sized 60kwh battery it represents a 20.6% increase over the Roadster. Tesla has stated the new charger is very slightly more efficient than the Roadster's so you can conclude the Model S consumes about 20% more power than the Roadster per mile.

In reality the difference will probably depend on driving patterns and apparently pack size. Since the S has a lower coefficient of drag, it will conceivably perform closer to Roadster numbers at highway speeds like 70 mph (115 km/hr). When you're driving in stop-and-go traffic, the light weight of the Roadster will help it perform much better than the S.

[Larry Chanin]

It's a simple calculation but I've also seen it somewhere... can't find it now. On Tesla's web site they recommend sizing your solar panel array for 300 wh per mile for the Model S. If you take the battery size compared to the range you get between 250 and 283 wh/mi depending on which pack you choose. For the Roadster the same calculation reveals 216 wh/mi. If you pick the mid sized 60kwh battery it represents a 20.6% increase over the Roadster. Tesla has stated the new charger is very slightly more efficient than the Roadster's so you can conclude the Model S consumes about 20% more power than the Roadster per mile.

In reality the difference will probably depend on driving patterns and apparently pack size. Since the S has a lower coefficient of drag, it will conceivably perform closer to Roadster numbers at highway speeds like 70 mph (115 km/hr). When you're driving in stop-and-go traffic, the light weight of the Roadster will help it perform much better than the S.

Thanks for the response.

So I gather the power draw expressed in wh/mi depends on the speed of the vehicle as well as the characteristics of the battery pack.

Here's the Roadster data that Tesla published, I was thinking we would need something similar for the Model S to make a reasonable comparison.




In addition to the better aeodynamics, the larger battery on the Model S also has a slightly more advanced chemistry than the Roadster that might mitigate the added weight. Regardless, I agree that it is likely the power draw of the Model S will be higher than the Roadster.

Larry

[Doug_G]

I'd think it'd be higher due to the larger mass of the car.

I think it would be more due to the larger aerodynamic cross-section. Even if the car is slipperier, it's' still BIGGER.

[Larry Chanin]

I think it would be more due to the larger aerodynamic cross-section. Even if the car is slipperier, it's' still BIGGER.

Hi Doug,

It seems even considering the larger cross-section the Model S is still measureably more aerodynamic than the Roadster.

I've figured the CdA by taking the dimensions and images from web site and calculated how much area the silhouette took.
W x H xpercent area of silhouette x Cd = CdA
77.3" x 56.5" x .81 x .225 = 5.53 Model S
72.9" x 44.35" x .8 x .35 = 6.29 Roadster

Again I'm not claiming that the power drain of the Model S would be less than a Roadster, just that it might not be significantly greater when considering aerodynamics and improved battery chemistry, particularly at high speeds.

Larry

[ckessel]

Battery chemistry isn't going to help energy consumption. Well, maybe a little bit indirectly by not needing as heavy a set of batteries, but that's going to be fairly small relative to total vehicle weight.

[Doug_G]

Again I'm not claiming that the power drain of the Model S would be less than a Roadster, just that it might not be significantly greater when considering aerodynamics and improved battery chemistry, particularly at high speeds.

Larry

Curious... if that's really true, there might be less of a penalty for going above 55 mph...?

[Larry Chanin]

Curious... if that's really true, there might be less of a penalty for going above 55 mph...?

Hi Doug,

I suppose we're steering off topic since presummably the study wasn't considering high speeds, but yes my thinking is that at high speeds the power drain would of course increase exponentially in both cases, but the Model S's better aerodynamics would start to narrow the power drain in comparison with the Roadster whose power drain would increase even faster than the Model S.

Larry