# Calculating "How much power do I need"



## corbin (Apr 6, 2010)

Howdy! I'm researching how much power I will need for my '69 bug conversion.

I've read this from the Wiki:

http://www.diyelectriccar.com/forums/showthread.php?t=15508

But I'm wondering how accurate the formula is for calculating the power requirement. Specifically in this formula:

Power in Watts = ((Mass in kg) (9.8m/s²) (Velocity in m/s) (Rolling Resistance)) + ((0.6465) (Coefficient of Drag) (Area in m²) (Velocity^3))

Why does it take the density of air and divide by 2? (0.6565

Here is what I calculated for a few speeds:

Speed (MPH)	kWatts
...
50 mph	8 kW
55 mph	10 kW
60 mph	13 kW
65 mph	16 kW
70 mph	20 kW
...

Does this seem right? The basic values:
2412 lbs gross, Cd=0.48, Area=1.80 m^2, rolling resistance: 0.00810.

Next, for a given "top speed" you are supposed to choose a voltage that will maintain a given top speed:

http://www.diyelectriccar.com/forums/showthread.php?t=11709

So, I'm not sure how to correlate voltage to a given top speed. What factors into the voltage and a given speed?

Thanks!

-corbin

PS: I'm hoping to post spreadsheets and data when I'm done researching on the wiki (and my blog), so others can benefit from my time.


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## EVfun (Mar 14, 2010)

About right, perhaps a tad optimistic. My Buggy is based on a 40hp Beetle. It's lighter but less aerodynamic. Actual power required to go 60 mph is 18 kW (150 amps from a 120 volt pack.)


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## exile (Oct 25, 2008)

Well, formal is correct but, energy losses in transmission, motor and controller are not included. 

Exempla:
Motor 90%
Controller 85%
Transmission 97%

60 mph -> 13 kW / (0,90*0,85*0,97) ~ 17,5kW

I hope it help


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## tomofreno (Mar 3, 2009)

> Speed (MPH) kWatts
> ...
> 50 mph 8 kW
> 55 mph 10 kW
> ...


 That is for constant speed (and needs efficiencies taken into account as pointed out), but you will need much more power to accelerate, depending on what acceleration rate you desire. You can estimate that using the spreadsheet at the link in post #1 in the "DC Motor Charts..." thread in the "EV Performance" forum here (2nd thread on the list today).


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## corbin (Apr 6, 2010)

Thanks for all the advice and info! I'll look into the spreadsheet in DC Motor charts, and input the efficiencies into my own.

I'm also going to do some calculations based on the large hill I have to conquer coming home.

corbin


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