I'm not exactly a natural maths whizz, a fact made abundantly clear earlier this year when I wrote that R1 invested in Apple in 2000 would now be worth around R1280. The correct figure is, in fact, R13.80. Since then I've made a habit of triple checking all my calculations. My latest column is full of such calculations, so I thought I'd better share my workings with my esteemed readers, in case I've made any other monumental cock-ups.
Let's start with total supply:
Wikipedia tells me that the world's supply of electricity in 2007 was around 19.89 trillion KwH (19,894,777,395,212 to be precise). I've rounded that up to 20 trillion, because it has almost certainly grown since then, and 20 is a much rounder and nicer number to start with.
Now we figure out the internet's usage:
If we assume, conservatively, that 5% of electricity supply is used by the net (thanks Kevin Kelly) then that means it uses 1 trillion KwH in a year. Note I've rounded the internet's footprint down (it's almost definitely closer to 6%) because I rounded the total supply number up.
...and then we figure out the cost per KwH:
Electricity prices vary widely between countries (thanks again Wikipedia) as do internet penetration rates. That makes calculating an average electricity cost quite tricky, because the internet's infrastructure is spread all over the globe.
What's more the internet penetration rates aren't really a good measure of infrastructure. For instance a disproportionate number of the world's biggest websites are hosted in the USA and China. That makes the USA and China's electricity costs much more important to than, say, ours or Argentina's. Just to get a feel for the scale, I cross referenced the top 10 internet nations against their electricity prices per KwH. You can see my workings here.
That gives me a rough weighted average of 18 US cents per KwH for the globe's top 10 internet using nations (who account for well over two thirds of all usage).
...and finally the total bill:
If the net's machinery is spread proportionally between these countries, that would make the yearly bill around $180 billion since:
1 trillion KwH X $0.18 = $180 billion
However it's likely that both America and China host disproportionately large shares of said machinery. Since they both have cheaper electricity than the average, the bill may very well be as low as $120 billion.
Then again, I may be underestimating the cost for the other third of internet using countries, many of whom have much higher costs per KwH (Denmark's is over $0.40 and Italy's is $0.37). So I wimped out and went for a range of possible costs rather than an absolute amount. This also allows for some wiggle room on the internet's usage and the world's total supply - both of which affect the final number.
So, as I said in my column, I put the current yearly bill at between $120 billion and $200 billion. But if a gun were held to my head I would go for $150 billion as a good guess.