24/7 CPU usage cost

How much do you factor the cost of electricity into your computer purchases? If you run your computer for 24 hours/day 7 days/week, and you have the CPU for 3 years or more, you could save a considerable amount of money with a CPU that uses less electricity.

I realized this could be true when I saw the power consumption figures for the new $200 Intel Core i5 750 CPU.

The rule of thumb is $10 per 10 watts of electricity per year. This is based on a kWh cost of 11.5 cents, which is slightly higher than the national average in 2008-2009.  Hawaii pays 25 cents or more, and Kentucky pays around 6 cents.  So obviously, you should find out what your kWh cost is and factor that into this article.

So for 3 years, that’s a $30 electricity savings for a measly 10 watts.

Now look at this graph:
Anandtech CPU Power consumption

I leave my main computer on 24/7.  It just takes too much time for it to boot and restore everything I’ve been working on, plus I give it tasks to do overnight, such as virus scans and downloads.  However, only during a virus scan is it above 15% CPU usage for any consistent amount of time.  So basically, I run an idle CPU 95% of the time.

The next closest CPU on that list that isn’t more expensive is the AMD Phenom II 965 for around $180.  It idles at 35W more than the i5 750, which works out to more than $100 in electricity savings over the next 3 years.  Under 100% load, it uses about 42W more than the i5 750, though unless you are running SETI@Home, Folding@Home, or distributed.net, you probably aren’t seeing anything near 100% load on a significant basis.

From X-bitlab’s review, the Athlon II X4 is about 15w higher at idle and equivalent under full load.

Core i5 clock for clock comparison

Core i7 HT on:  10-15% slower  (if the apps/load can be distributed across >4 cores — apparently Win7 does a much better job with this)
Core i7 HT off:  equivalent
Core 2 Quad (Q9400): 10-20% faster
Phenom II X4 (estimate):  tough to say, but it looks to be around 15-40% faster — I still haven’t figured this one out exactly, but the 965 (3.4) beats the i5 750 (2.66 + Turboboost) in some tests and loses in others
Athlon II X4: 30-60% faster
The Core i7 can overclock about 10-15% higher than the Phenom II and about 20-25% higher than the Athlon II X4

AnandTech benchmarks
Xbit-labs benchmarks

CPU/RAM/Motherboard Prices

The Core i5 motherboards and the DDR3 memory it uses are a little more expensive than the Phenom’s.  The difference is around $30-60 for a motherboard and 4GB of memory.  For the sake of this article, we’ll call it $40.

Core i5 750 (2.66 GHz): $399
Phenom II X4 965 (3.4 GHz):  $345
Athlon II X4 620 (2.4 GHz):  $260

Final Analysis

Core i5 750 is roughly equivalent in performance to the Phenom II X4 965.  Factoring the electricity cost savings, the i5 is actually about $40 less than, and it saves money the longer you have it.

Comparing to the Athlon II X4 is a little more traditional as the power savings are only in the 15w range, so we have to lean on the performance difference, the Core i5 is 40% faster and the final price after electricity works out to about $100 over 3 years.  So then you have to decide if 40% performance boost is worth $100.

If you’re considering a quadcore, I have a hard time imagining why you’d consider less than the i5 750.

(Note: The hotter it is where you live, the more the electricity usage matters because your A/C has to work harder to account for the extra waste heat of the CPU.  The colder it is where you live, the less it matters because you probably have to spend money to heat the room anyway.)

Anandtech CPU Power consumptions