The guardian article mentioned by THelper to me is suspect.
It cites McCaffey that claims that 62 trillion spam messages take 33 billion kWh of power.
That's 2000 messages per kWh.
Hmm. Let's suppose that the average message is a megabyte. Typical internet connection at the consumer level is aobut 5 Mbit/s So a megabyte takes about 1.3 seconds to send. Ignore the computer for now. The household router uses 8 watts. As you go up the internet food chain, both the power and speed go up. My bet is that the larger ones are more efficient, but lets pretend they aren't. Let's also say that the message takes 2 seconds. There are 3600 seconds in an hour, so a message takes 2/3600 * 8 watt hours. This time we will round the other way 1/2000 * 8 watt hours or 1/250 of a watt hour.
From me to gmail.com is 15 hops. Assume this is typical. So 30 hops gets my email to my destination. So 30 * 1/250 = 1/8 of a watt hour. So a kWh will handle 8000 messages.
Further: Lets take another look at that average message size. I currently have 30,000 messages in Gmail, using about 3 GB. So 10000 messages per GB or about 1/7 of a megabyte each. So our kWh could transport 80,000 messages.
Ok. Look at the electricity to make the message. I'm in front of my mac pro. It takes me 10 minutes to write 500 words to a friend. My computer with it's 3 monitors uses 200 watts. So 10/60 * 200/1000 = 1/30 kWh. Writing it is by FAR the big energy cost.
But: How many do I write? According to gmail, 2000 last year, so about 6 a day. (If I wrote it, I keep it.)Not counting spam, I receive about 60 a day. Writing still dominates. I use a fifth of a kWh to write. Reading most messages takes under 30 seconds. If we assume 1 minute each, then it works out that reading and writing are about equal. Both are monsters compared to moving the data around the net.
So since the average message is written once and read once, we have 2/5 of a kWh per message. So that works out to only 2.5 messages per kWh. Ouch.
Something to think about.
(I started to write this expecting to blow the numbers out of the water as being way too large. Now I see them as being too small.)