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Default How Much Power Do I Need?

This thread will show you how to calculate the power required at the wheels to travel at a particular speed in your car on flat ground with no breeze. If you find this article difficult to understand you may wish to read Power (kW) and Energy (kWh). You can use this eqution and its results to work out your range, size your battery pack, size your motor, work out your top speed or a whole range of things.

1. The POWER requirements of your car at a particular speed is:
Power in Watts = ((Mass in kg) (9.8m/sē) (Velocity in m/s) (Rolling Resistance)) + ((0.6465) (Coefficient of Drag) (Area in mē) (Velocity^3))

The 9.8m/s is acceleration due to gravity, the 0.6465 is 0.5 times the density of air in kg/m^3. If you enter the required numbers it will give you your power consumption in watts. Once you have worked that out you can use it to work out the following.

2. The ENERGY required to maintain the speed in Step 1. for a certain period of time is given by:
Energy in Watt-Hours = (Power in Watts)(Time in Hours)

For example the power required to travel 60mph (converted to m/s) might be 20kW, driving for ninety minutes would mean you use (20 000)(1.5) = 30kWh of energy. Assuming no efficiency losses (in the drive train or batteries) you would get 90 miles of range with a 30kwh pack. You could use this value for Sizing your Battery Pack, since the energy stored in your battery pack will be its voltage times its amp-hour rating but please allow for your drive train inefficiency and the characteristics of your batteries.

3. The EFFICIENCY of you vehicle at the speed from Step 1. is given by:
Efficiency in Wh/mile = ((Power in Watts)(1 hour))/(velocity in mph)*

So if it takes 20kW to travel 60mph then you will get (20 000W)(1hr)/60 = 333Wh/mile. Please note that is the efficiency after the wheel, please take into account the energy losses of the electrical and mechanical drive train in order to better reflect your real world efficiency.

A great read on automotive power:

* This equation is incorrect. "Efficiency in Wh/mile = ((Power in Watts)(1 hour))/(velocity in mph)" It should read " Efficiency in Wh/mile = (Power in Watts)/(speed in mph)". Of course the Power in Watts is that which is required for that particular speed. The term "speed" should be used because velocity typically infers a vector quantity.

Contributors: major, mattW, corbin
Created by mattW, 06-18-2008 at 10:39 AM
Last edited by major, 09-12-2012 at 01:19 PM
4 Comments , 52380 Views
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Old 06-15-2010, 07:06 AM
BLSTIC BLSTIC is offline
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Default Re: How Much Power Do I Need?

It's worth noting that an EF Falcon (a 1550kg Australian Ford family car) consumes 13kw at the wheels to drive at 105km/h. The car has a cd of 0.31, and is normally powered by a 4 litre engine. It gets 7.5l/100km in this state, and is about 18% efficient.

That figure was actually measured on a dyno, it is not a calculated figure.

It's also worth noting that unless you want to spend an hour getting to your cruise speed you need more power. A figure of around 35kw/1000kg has been proven to be adequate for safe highway overtaking and hill climbing
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Last edited by BLSTIC; 06-16-2010 at 08:04 PM.
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Old 07-04-2010, 09:46 AM
Franky.EV Franky.EV is offline
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Default Re: How Much Power Do I Need?

I've manage to put all these Formulas in a oooCalc spreadsheet :

I also take into account, grade, power to run the ancillaries, gearbox, etc. to get as close of possible of the need of the ev'ers.

All blue text in the spreadsheet are parameters you can modify.
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Old 09-12-2012, 12:38 PM
dralorg dralorg is offline
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Default Re: How Much Power Do I Need?

Hi, please can you tell me what's the name of the Book you've linked?

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Old 09-17-2012, 11:04 PM
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winzeracer winzeracer is offline
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Default Re: How Much Power Do I Need?

Originally Posted by dralorg View Post
Hi, please can you tell me what's the name of the Book you've linked?

Looks like Corbin was the one to paste that link you can PM him here

Or if you have specific questions on this matter ask them in the Technical Discussions area. Welcome to the forum!
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