# Converting Watt-hr to Watts

1. Sep 8, 2009

### tacojohn

1. The problem statement, all variables and given/known data

An automobile gets 20 miles per gallon when traveling at 60 miles per hour. With the energy content of gasoline at 36,000 watt-hr per gallon, convert this amount of power into watts.

2. Relevant equations

W h (watt hours) | ([length]^2 [mass])/[time]^2 | energy
W (watts) | ([length]^2 [mass])/[time]^3 | power

Energy = Power (60) * time (3) = 60 x 3 = 180 Wh

3. The attempt at a solution

If watt-hrs and watts are not compatible how am I supposed to convert these?

2. Sep 8, 2009

### kuruman

You are not expected to convert the 36,000 watt-hr to watts. As you noted, it cannot be done. The power that you need to find is the energy per unit time that this car consumes as it travels at 60 miles per hour.

3. Sep 8, 2009

### lanedance

Energy is ususally measured in Joules (J)
Power is the rate of change of energy - ie. how much energy you are using in a given time. Usually measuered in Watts = Joules per second (W = J/s)

so work out how much fuel the car consumes per time unit, then convert that to how much energy per time unit & you have power - pow

Last edited: Sep 8, 2009
4. Sep 8, 2009

Correct-o.