Human energy consumption: How dim is your lightbulb?

Ever looked at your electricity bill and wondered "gee, how many kilowatt hours does my body use?" Wonder no longer. Here's how to figure it out.

An adult body needs, on average, 12 kilocalories per pound to maintain it's weight. If you want to add a pound of fat, you need an extra 3600 kilocalories. My "ideal weight" for my height according to most charts I've looked at is 195 pounds. So, my body needs 12 x 195 lbs = 2,340 kilocalories to maintain it's weight. Now lets convert that to watts.

2,340 kilocalories per day / 24 hours = 97.5 kilocalories per hour / 3600 seconds per hour = 0.0271 kilocalories per second x 4,184 joules / kilocalorie = 113.32 joules/second or 113.32 watts. So I'm roughly equal to the power consumption of a slightly bright light bulb (100 watts is a little above average).

If I draw 113.32 watts for an entire hour, that's .113 kilowatt hours. Over a 24 hour span, that's 2.712 kilowatt hours per day. Over an entire year, that's 989.88 kilowatt hours. At an average price of say, $0.11 per kilowatt hour for electricity - that means my body would consume $108.89 of electricity per year if I could somehow plug myself into the wall.


Interestingly, the brain is on average 2% of the body's mass but consumes 20% of the body's energy. In my case, my brain should weigh 3.9 lbs and uses 197.97 kilowatts of energy per year.

The actual power output of the human body is MUCH lower than the amount of energy it consumes. A human being can put out something like 100 watts of power on average (say, by driving a stationary bicycle) over a long span of time. Lets look at the maximum efficiency of a human.

150 lbs = 87 watts = 2.1 kilowatt hours per day energy consumed
100 watts per hour x 10 hours a day = 1 kilowatt hour

1.0 / 2.1 = 47.6% efficient

A horse is supposedly about 8 times more powerful than a human. That's why 1 horsepower is equal to 745.7 watts. Consider how incredibly powerful your car is: my car has 173 horsepower at peak at the crank. To the wheels, on average, it probably puts down something like 65 horsepower or 48,740 watts of power. In other words, I would have to work 487 seconds (8 minutes) to equal the power output of 1 second on average from my car.

A brief analysis of Public Accounting

For some reason, I got interested in the relative size of public accounting firms and decided to do a little research.  I found Public Accounting Report's - Top 100 For 2006 here (PDF warning, ~600k!) from August of 2006.

Using the data from page 2, I ran a few numbers on the top 25 firms ranked by revenue.  I knew that it was big, but I never realized the incredible magnitude difference between the Big 4 Accounting Firms and everyone else in the market.

Average Revenue:
Big 4: $6,167MM
Next 10: $473MM
Bottom 11: $109MM

Average number of partners:
Big 4: 2,079
Next 10: 253
Bottom 11: 61

Average number of professionals:
Big 4: 18,245
Next 10: 1,809
Bottom 11: 482

Average number of offices:
Big 4: 96
Next 10: 38
Bottom 11: 10

Average professionals per office:
Big 4: 189
Next 10: 52
Bottom 11: 62

Average SEC Clients:
Big 4: 1,596
Next 10: 111
Bottom 11: 27

In fact, if you combined the 5th through the 25th ranked firm, you still wouldn't equal one of the average Big 4 firms.  Instead, you just have a lot more partners/professionals/offices producing lower revenue.

Revenue: $5,930MM vs $6,167MM for the Big 4 average
Partners: 3,204 vs 2,079  for the Big 4 average
Professionals: 23,390 vs 18,245 for the Big 4 average
Offices: 491 vs 96 for the Big 4 average
SEC Clients: 1,407 vs 1,596 for the Big 4 average

As you can see, there are some incredible economies of scale for the Big 4 firms.  It appears that much of it is related to the number of SEC clients.  The Big 4 have a combined total of 6,383 vs only 1,407 for the next 21 biggest CPA firms.

Compared to the average for "The Next 10" the Big 4 has:

  • 13 times as much revenue
  • 8 times as many partners
  • 10 times as many professionals
  • 2.5 times as many offices
  • 14 times as many SEC clients
Of additional interest was the total number of CPA's in the United States.   The AICPA claims to have 330,000 members and states that 3 out of every 4 CPAs are members.  This means there are somewhere around 440,000 CPAs in the United States.

If you add together the number of Partners with the number of Professionals (assuming that they are almost entirely CPAs) of the Big 4, you get 81,297. So, roughly 18.5% of all CPAs are employed by the Big 4. Another 26,594 are employed by the next 21 firms ranked by revenue or about 6%. The total for the Top 25 is 107,891 employees. The AICPA states that 133,379 of it's members are employed in public accounting. Using the same 3/4 ratio of members/actual, this means there are a total of 177,839 CPAs total in public accounting.  Of this number, 61% work for the top 25 firms and 45% work for the Big 4 alone.