Wednesday, July 30, 2008

A Horse in the Sun

Horsepower (hp) is a unit of (naturally) power, or energy delivered per unit time. It is most commonly used to rate the power delivered by internal combustion engines (automobiles, lawn mowers). For a precise definition and history of this unit, this entry in Wikipedia is a good read. There is some variability in the precise conversion between horsepower and other units, but it is approximately equal to 745 watts.

I find this value rather interesting, as this is roughly the incident power delivered, per square meter, of radiation from the sun -- and also roughly the area available on a horse for collecting said power. And of course, harvesting that power is a rather inefficient process -- about 10-20% for solar cells.

Something to think about when you see an automobile rated at 200 hp: it would take 2000 square meters of (cheap) solar panels to deliver the equivalent power. That is nearly half of a football field. Try towing that behind your car.

Sobering thought.


westside said...


This is a very interesting point.

How did you get 745W/m^2 for incident solar energy?

In this article they mention on the sunlit side its closed to 680W/m^2's_energy_budget

I'd appreciate any insight into your thinking. My guess is that depending on location on the earth and solar ephemeris you could determine the amount more precisely.

This is also interesting in the context of biofuels which are typically less than 1% efficient today (when considering only photosynthesis, not actual production and refinement)!

Anonymous said...

That 200 hp is the maximum power that the engine will deliver running at high rpm with the pedal to the floor. The time-average power for driving is much less, and is the appropriate measure for comparison if you want to assess the prospects for powering cars from solar energy. I don't think anyone believes you can carry or two enough solar cells for a reasonable vehicle to power itself from solar energy generated on the fly (even in the desert at noon). But we will not require a football field full of solar cells to provide sufficient electricity or hydrogen for a single personal vehicle in normal use.

Dad said...

Very interesting. I do like that sort of visualisation \ comparison.. Although, I read about an ev plane that can fly non stop from the sun it collects and make it through the night... And for most of the day, my car just sits in the sun doing nothing...