The energy wasted by spam e-mails could power 2.4 million homes a year according to security firm McAfee. But its report offers little advice as to how to avoid this problem.

The firm’s report, produced with the help of environmental consultants ICF International, concludes that spam is responsible for the waste of 33 billion kilowatt hours of energy each year. It also produces 17 million tons of CO2, the same amount produced each year by three million cars.

However, it appears the actual production and sending of the spams make up just seven per cent or so of the energy costs (though it’s not as if spammers are likely to be carbon offsetting anyway). Instead, most of the energy is used in filtering out the unwanted messages.

Around 16% of the energy costs come from automated filtering. The rest comes from recipients, with 27% coming from having to retrieve legitimate messages from a spam folder, and just over 50% coming from reading through spams which have evaded the filters and landed in the inbox.

According to the study, this means that the energy used in automated filtering actually pays for itself in environmental terms by saving on energy wasted by recipients. Of course, more effective filtering would cut down energy costs ever further.

It’s worth noting that there’s a major caveat to the figures. It appears the energy costs at the user end were estimated by taking the time it takes for people to sift through e-mails and working how much power their computers used during this time.

The drawback with that method is that is assumes the computer would otherwise be switched off or in hibernation mode. But in reality, it’s almost certain that people who didn’t have to sort through spam would simply spend the time working on their computers instead.