One of the things I’ve been thinking about is whether I am using something for its “highest use ” or “highest purpose” – something like electricity or space or whatever. If what I’m doing is not its highest use (general lighting rather than task lighting, for example) could I avoid using it if there is a negative cost to it (walk through a dimly lit room)? Is there something else I could do?

I recently became aware of the fact that desktop computers use a lot more power than laptops (supposedly), which use more power than tablets and phones. When I turn on my very old desktop computer for some small task, or just to stay connected to Facebook, how much electricity am I actually using? I decided to look it up.

Apple provides this information on their website, but our Mac is so old they didn’t list it anymore. The earliest one they list is 2009 so I’ll use those numbers. Power consumption for an iMac desktop is:

• 125.5 watts when idle, maximum power consumption is 215.7 watts
• Thermal output (energy lost) is 426.7 BTU/hour when idle, and maximum of 733 BTU/h.

A 2014 model does use less:

• 70 w when idle and a maximum of 176 watts in use
• 239 BTU/h when idle and 601 BTU/h at maximum use

I imagine that the energy embodied in the physical computer – the energy it took to make it – is greater than the energy loss of using my old computer so I’ll keep using it. (Maybe some smart person out there can do the math for me to figure out at what point usage becomes a greater negative that the production of a new one.  Apple provides a lot of data on their website, including the CO2 that was generated in the production of a product!)

But maybe piddley day-to-day use isn’t the highest purpose for using the desktop. Maybe our very old laptop would be better?  Again, I’ll have to take the earliest model they have listed on the website, ours might be older than that. Here’s the power consumption for a Macbook pro 2008 (our battery doesn’t work so it’s always plugged in).

• 155 watts when idle, 309 at maximum use
• 528 BTU of heat when idle, 1085 BTU at maximum

Looks like I’m doing better using my old desktop. But if I replaced it with a 2013 model — wow, that’ a big difference!

• 43 watts when idle, 205 max
• 147 BTUs when idle, 742 BTUs under maximum use

What if I switched to a tablet for lighter uses, like responding to emails, surfing the web, Facebooking? An iPad isn’t sucking energy constantly, like my other computers. In takes 3 watts to operate but the power adapter to charge it will pull 10 watts to charge and take a couple of hours. So it looks like I would be using 20 watts a day (or less) rather than 125 to 300 watts an hour.

If I consider highest use, using my desktop when I’m working and really need the big screen still makes sense but switching to a tablet for most everyday tasks would seem like the best plan. (And it might be nice not to burn my legs on that superheated old laptop any more.)

As I was looking into this, I came across this blog post with lots of ideas for computer energy savings. Check it out.