# Which will give you greater energy density?
**URL:** https://boards.straightdope.com/t/which-will-give-you-greater-energy-density/675254
**Category:** Factual Questions
**Created:** [December 2, 2013, 1:14am UTC](https://boards.straightdope.com/t/which-will-give-you-greater-energy-density/675254 "2013-12-02T01:14:33Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Qwertol](https://boards.straightdope.com/u/Qwertol)
#### Post date: [December 2, 2013, 1:14am UTC](https://boards.straightdope.com/t/which-will-give-you-greater-energy-density/675254/1 "2013-12-02T01:14:33Z")
Which setup will give you greater energy density i.e. more energy per weight, by energy I mean kinetic output of electric engine:
high performance lithium batteries + electric engine or fuel tank, heat engine, generator and electric engine?
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [December 2, 2013, 1:42am UTC](https://boards.straightdope.com/t/which-will-give-you-greater-energy-density/675254/2 "2013-12-02T01:42:57Z")
That question can only be answered correctly if the size constraints are known.
In other words, a vehicle with a gasoline powered motor/generator that is size constrained such that it only can carry 1 gallon of gasoline will probably be beaten by the all-electric car. Conversely, if there is a lot of space available, the far greater energy density of gasoline will certainly win out, even taking into consideration the size and weight of the motor/generator.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [December 2, 2013, 2:41am UTC](https://boards.straightdope.com/t/which-will-give-you-greater-energy-density/675254/3 "2013-12-02T02:41:39Z")
Do you count the oxygen the gasoline needs in your calculation?
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### Author:  [@Qwertol](https://boards.straightdope.com/u/Qwertol)
#### Post date: [December 2, 2013, 10:11am UTC](https://boards.straightdope.com/t/which-will-give-you-greater-energy-density/675254/4 "2013-12-02T10:11:20Z")
No.
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### Author:  [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)
#### Post date: [December 2, 2013, 11:40am UTC](https://boards.straightdope.com/t/which-will-give-you-greater-energy-density/675254/5 "2013-12-02T11:40:53Z")
Beowulff keyed into the main issue. If you’re looking at a tiny helicopter that fits into the palm of your hand, you will get more flight time out of a Li battery than a gasoline engine/generator system. There’s a minimum size for the engine/generator system that makes it unsuitable for small applications.
Move up to the size of a car. Once you account for the engine/generator, adding weight means adding gasoline, which beats the pants off of adding batteries in terms of energy density. Let’s say the e/g costs you 500lbs. That means you get 500lbs of battery power before a single gallon of gasoline. Once you start adding gasoline, it’s going to trip over pretty quickly to that side of the equation.