How Much Electricity Does a Mac Use?

I've been trying to understand how I use electricity long before it became fashionable or an utter necessity. I'm not an electrician, electrical engineer, or any kind of expert in electricity. However, like many people, I do get an electricity bill every month. And with the costs associated with electricity and other forms of fossil fuel power rising, I have a great interest in controlling those costs. So, I read a bit on the subject and learned from sites like this one..."

How much electricity costs, and how they charge you

...about how electricity usage is measured and charged for in the U.S. The main thing to understand is that electric power companies measure our electricity usage in kilowatt hours (kWh). Some simple examples are: Ten 100W bulbs turned on for one hour uses 1 kWh. One 100W bulb turned on for ten hours also uses 1 kWh. According to the U.S. Energy Information Administration, the typical monthly U.S. residential electricity consumption in 2006 was 920 kWh. Of course, this varies greatly from home to home and place to place (geographically). Moreover, as fuel and other expenses rise, we also see the price per kWh rise. Where I live, for example, 1 kWh in June 1998 cost 12.6 cents. By June 2007, it had risen to 19.6 cents per kWh. And, last month (June 2008), the rate had risen to 29.2 cents/kWh. So, while the cost had risen a 55.5% from 1998 to 2007, the huge increase in the past 12 months brings the 1998 to 2008 kWh price increase in my area to 155.5%.

Most of the electricity used in homes comes from heating/cooling devices like water heaters, freezers, refrigerators, air conditioners, heaters, etc. But, I figured every little bit helped and being a tech geek decided to find out what the kWh burn rate of electronics in my home is.

So, I bought a P3 Kill A Watt electricity monitor and tested it with all kinds of devices in my home. My main goal was to understand the power used by my MacBook (Core Duo) and iMac (Core 2 Duo). You can see some of my findings in the table below.

Device	Off	Sleep	Idle	Working	Comments
Cable Modem				7
Linksys WRT54GL WiFi Access Point				4
Cordless Phone Charger	1			4	Charging state
Hand Vacuum Cleaner Charger	1			5	Charging state
Nintendo DS Charger	1			2	Charging state
iPod touch	1			4	Charging state
Sony PSP Charger	1			8	Charging state
Scientific Atlanta Explorer 8000 Cable/DVR	31			31	Off switch turns off connected TV
Microwave Oven	3			1000
Princeton 19" LCD Display	1	1		32
Western Digital 500MB USB Drive	3	3	5	13-15
MacBook Core Duo	1	2	15-18	19-49	Screen dimmed 15W Mostly in 20-25 range
iMac Core 2 Duo	2	3	55-75	76-100	Screen dimmed 55W
eMachines T6420	3	6	60-75	75-80	Windows Vista Does not include LCD power
eMachines T3256	3		120	125-155	Windows XP Does not include LCD power

My MacBook (Core Duo) and iMac (Core 2 Duo) both running in performance mode (as fast as possible) have what seems to be to be reasonable power usage considering that both unit's measurements also includes integrated LCD displays. My old (2004 model) eMachines 3256 running Windows XP with a 19" LCD display, for example, is burning upwards of 150 watts per hour when in use. The iMac (2008 model with a 20" LCD display) used around 80 to 90 watts per hour. The somewhat newer (2006) eMachines T6420 was more efficient that the older eMachines unit using about 100 watts per hour (including an external LCD). However, the iMac gained a lot in terms of efficient power use because Mac OS X can reliably suspend and resume from low-power states. My experience with Windows XP and Windows Vista is that resuming from a low-power state (sleep) is an if-fy proposition. In fact, the T6420 did not resume from sleep and had to be power-cycled to reboot. I'll guess that newer desktop Windows PCs come close to the iMac's power efficiency. The real power saver was my first generation MacBook which stayed mostly in the 20 to 25W range when under moderate normal (for me) activity including web browsing and email.

Things can be more complicated than my little table indicates though. For example, my reasonably efficient iMac has an external USB hard drive, second LCD display, printer and small powered speakers with a subwoofer. The speakers and printer are not as smart, unfortunately. This adds another 35 to 50 watts when everything is being used. Fortunately, both the external LCD and the USB hard drive go into low power modes when the iMac sleeps.

If you have all kinds of power bricks with and without devices attached to them in your home, you may have 100 watt-hours or more being used just by these energy vampires sitting around sucking power and money. If you have an iMac like mine that might be on 4 hours a day on the average (less on weekdays, more on weekends, averaging out to 4 hours), you may find somewhere between 80 and 100 kWh used per month by various gadgets in your home. In a so-called typical 920 kWh/month home, this is around 10% of total power use. I've gotten monthly power use down to around 470 kWh/month by replacing major appliances, changing out light bulbs with CFLs, and generally encouraging good energy behavior by the rest of my family (not without some complaints now and then). So, 90 kWh of that would be nearly 20% of total electricity use. So, I'm still looking around my home to find energy vampires and replacing, removing, or just using a power switch (I haven't tried to so-called smart power strips yet).

Mac OS X has simple Energy Saver settings in Systems Preferences. If you haven't looked at your settings recently, it might be a good thing to do right after you take a look at your latest electricity bill. As I said at the beginning, I do not consider myself an expert on this topic. So, if you are, please feel free to chime in to clarify or correct any information provided here.