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Are low-power desktop CPUs worth buying?

Posted on 6th Oct 2013 at 12:43 by Antony Leather with 22 comments

Antony Leather
It's easy to forget that Intel and AMD have low-power versions of most of their current line-up of CPUs. For most of us there's little reason to consider them, with them only registering on our radar when it comes to thinking about all-in-ones, media PCs or net tops. There's a good reason for this too. While they have lower power consumption, these CPUs are very often clocked slower than their full-fat counterparts but cost noticeably more.

Take for example the Core i3-3220T. It currently retails for £95, has a clock speed of 2.8GHz and a TDP of 35W. By comparison, the Core i3-3240, which has a TDP of 55W is clocked 600MHz faster at 3.4GHz but costs just £88.

Okay, that's a relatively small price premium, but considering you're losing a fair chunk of performance, is it worth paying?

The answer if you're an always-on, large data center or server farm is of course yes - it's well known that in these scenarios upgrading for the sake of a small power saving on each component can save enough money in the long run that you easily recoup your costs. But what about a home environment? I decided to do a few rough calculations...

Are low-power desktop CPUs worth buying? test
Standard TDP CPUs are cheaper, but does it pay in the long run to buy a low power model? - Click to enlarge

Assuming a kilowatt-hour price of 15p, a 65W TDP CPU - a Sandy Bridge Core i3 2120 for example, uses around 1p an hour in electricity at full load, or around 24p a day. In comparison a 55W Ivy Bridge Core i3 CPU uses around 20p per day and a 35W TDP CPU will use 12.8p per day. Spread out over a year you're looking at £87.60, £73.00 and £46.72 respectively. Clearly these differences easily cover the extra initial outlay.

However, unless you're running a serious server at home, 100 percent load is very unlikely. Instead for an always-on HTPC or NAS PC, it's likely to be at full load no more than, say, a quarter of the time. With that taken into account we're looking at £21.90, £18.25 and £11.68 total power cost for the same three CPUs. The gap has closed significantly but you're still looking at recouping your costs within about a year.

What you probably don't need me to spell out to you at this point, though, is that for your main office/gaming/family PC, you're probably not looking at it being worth your while getting a low power chip, even for a basic family PC. If we say a family PC will on average be on for 8 hours a day, of which 2 hours will be gaming or doing some other intensive task at 100 percent load, we come out with figures of £12.78, £10.64 and £6.81. Considering the performance hit of opting for the low power model, we'd certainly not be inclined to splash the extra cash up front.

But, power saving is of course only one aspect of a low TDP chip. The other consideration is heat, and it's here that low power models make their strongest argument. By requiring less hefty coolers, they can be squeezed into much smaller cases, and even passively cooled cases.

Take for example recent fanless cases we've reviewed such as Akasa's Euler and Streacom's ST-FC5S EVO WS. A 65W CPU here proved precisely why the Euler is only rated for 35W TDP CPUs, with it becoming roasting hot during testing with the more powerful chip. The point is that opting for a low-power CPU could make things much more comfortable if heat and also noise are of more concern than performance.

Are low-power desktop CPUs worth buying? test
You could actually recoup the extra outlay of a low-power CPU in less than two years - Click to enlarge

You could also consider under-clocking your CPU - reducing the voltage and frequency to reduce power draw. This is also something I've toyed with, although the main obstacle here is that many low-end motherboards either don't allow you to tweak the CPU voltage at all, or only allow you to do so in an upwards direction. If your motherboard does allow this then it's well worth spending some time to find the best ratio between performance and power draw.

So what can we conclude? Well, ultimately, as with highly efficient PSUs, if you pay the premium for a power efficient CPU, it could pay for itself over its lifetime, but only if that PC is going to get serious amounts of usage and not be upgraded for a good couple of years. If you're just after a modest CPU for a family or office PC, though, it may be worth saving on the initial outlay and perhaps buying a more efficient PSU instead.

Unless, of course, you're talking about it being a space-saving or silent PC too. While you could underclock a cheap CPU, it's a risky bet as to whether the motherboard will allow it, and it's likely you may not be able to reduce power by enough to fit within the thermal limitations of your desired case/cooler anyway.

Have you considered a low-power CPU? Let us know in the forum.

22 Comments

Discuss in the forums Reply
jrs77 6th October 2013, 22:10 Quote
You've basically covered it all in your blog there.

I've considered low-power parts before, when the question was about running a HTPC passively, but besides that it's not worth thinking about it really.

When you use a 65/77W CPU in your normal rig, then it'll run clocked down (speedstep) anyways while browsing, writing a document or watching a film, and in that case you save just as much power as with a low-power part.
deathtaker27 6th October 2013, 22:14 Quote
I'm running a second gen i3 in my younger brothers desktop which is on about 14 hours a day, had complaints about processing power but it hardly registers on the bill which is nice

Comes down to what you need and investment up front or pay more over the long run
Glix 6th October 2013, 23:05 Quote
Gpu is missing from calculations... average mid range pcs will draw 250w (including decent monitor). That is 1/4th the power draw of vacuum/kettle/hairdryer.

Of course the real question is, is an under utilised CPU just as efficient as it's low voltage breatheren (how games acutally put the cpu to 100% usage in all areas of the cores)? The only way to know this is for bit tech to do some benchmarks on the T series and let us know the power difference and fps difference, and then do a comparison on cost.

I'll stop rambling, its late. :>
greigaitken 7th October 2013, 08:32 Quote
...but all my cpu power means slightly less heating required (except on the 30 warms days/year) so it's not wasted energy
faugusztin 7th October 2013, 09:48 Quote
The biggest issue i see is that we are comparing TDP instead of real power consumption. The problem is - TDP doesn't mean anything. For example take a look at i5-2400 vs i5-2400S vs i5-2400@i5-2400S voltage comparison :
http://www.silentpcreview.com/article1202-page3.html
Even at stock voltage and higher performance, we talk about 14W difference between a 65W TDP i5-2400S and 95W TDP i5-2400. Real power consumption difference is less than a half of the TDP difference. And because we can assume their test system used ~30W (considering the specs of the system), that means power consumption of CPU alone would be 59W at load for i5-2400S and 73W for i5-2400. Sure, there is a difference, but even the 73W value is so close to the 65W TDP that it makes no real difference.

This is the reason for only limited list of options for these power saving CPU - they are selling them only in case the original CPU was already very close to the lower TDP limit. That is why you got Pentium G620T (35W) vs Pentium G620 (65W) - the standard G620 will likely use something like 40-45W, so at expense of lowering clock they shaved off that few watts.

At least in case of Haswell they got a bit more creative and they actually did something more than that - i7-4770S is still the old style "we are close, so we lower the clock speed a bit" type, but i7-4770T with 45W TDP is actually more like the laptop style CPU - very low standard clock speed (2.5GHz) combined with very high Turbo multiplier (3.7GHz) - by high i mean +12 instead of typical +5/+6 you find in desktop CPU.

So my opinion is - if the CPU is only one TDP category lower than the original version, then it is not worth to invest in them, because the original version is probably only few watts above the TDP limit of the "power saving" version. If the CPU is two TDP categories lower, then it is worth thinking about it, as you usually compromise only at long term multicore performance, the TDP is good enough for short bursts of single or dual core performance.
Boscoe 7th October 2013, 09:53 Quote
I think you've missed something. The low power CPU will hit 100% load before the higher power counterparts meaning they won't be at 100% when the low power one is. So it will only cost you more if you were to go over that 35W CPUs full processing power capability on the higher power CPU.
tonyd223 7th October 2013, 10:42 Quote
No, not for home users.
faugusztin 7th October 2013, 10:57 Quote
Quote:
Originally Posted by Boscoe
I think you've missed something. The low power CPU will hit 100% load before the higher power counterparts meaning they won't be at 100% when the low power one is. So it will only cost you more if you were to go over that 35W CPUs full processing power capability on the higher power CPU.

I was talking about 100% use of both power saving and normal CPU. Sure, the power saving one will complete the task slower, my point was that it makes no sense to consider power saving CPU just because of the cooling, as usually you will see differences like above - 59W power use of the power saving CPU with 65W TDP, and 73W for non-power saving one. Both are very close to the claimed low power TDP, just on different sides, while the non-power saving is very far from his own claimed TDP.

So that rules out the use case scenario of "must buy power saving CPU because of passive cooling", and then comes the rest, which is again not simple, because while the higher TDP CPU consumes more power, it does the task more quickly, so it is possible that the higher power TDP CPU will actually consume less power over time compared to lower TDP CPU. In idle the power consumption is identical for same generation of CPU.

The only use-case scenario where i could see this making difference is 24/7 folding, but people usually don't buy power saving CPU for that.
GuilleAcoustic 7th October 2013, 11:04 Quote
Good article, but it's too bad that you did not put REAL results. while it's nice to know about the comsumption cost, I do not see any performance comparison nor heat under idle/load.

Let's see those test from a french site specialized in "eco friendly tech" :

- i7 3770T (4C/8T - 2.5GHz, 3.7GHz turbo - HD4000 - 45W) :

http://www.ginjfo.com/dossiers/tests-materiel/composants/processeurs/core-i7-3770t-les-performances-dun-core-i7-2600k-sans-les-watts-20120427

- i7 4765T (4C/8T - 2.0GHz, 3.0GHz turbo - HD4600 - 35W) :

http://www.ginjfo.com/dossiers/tests-materiel/composants/processeurs/test-du-core-i7-4765t-haswell-chaque-watt-au-plein-potentiel-20130807

The last link is in fact a i7-3770T Vs i7-4765T Vs i5-4670K, all without discrete GPU.

http://www.ginjfo.com/wp-content/uploads/2013/08/Corei74765T_Graph01.png

They are really capable CPUs, even for ethousiasts. Lets face it, not all of us do 3D rendering or atomic simulation at home. Now, I agree that it's a little power saving when you have an high end GPU, but for those with HD7750 like or even only the IGP it's a lot (35W vs 84W for an Haswell i7).

I'm currently running a Q6600 (4C/4T - 2.4GHz - 105W) with 2.0V DDR2 stick @800MHz (4 stick) and a Geforce9300 IGP ... the move to an i7-4765T will save 70W on the CPU side while doubling the performance (and it has a better IGP included in the TDP). The use of 2x 1.25 Lovo DDR3 will save lots of power compared to 4x 2.0V ... take a look at your own review

http://www.bit-tech.net/hardware/memory/2010/03/18/kingston-lovo-power-saving-1-25v-ddr3-ram/2

In the end, for at least 2X more performance I'd save 82W of CPU + IGP side (70W from Q6600 + 12W from Gf9300) + 20W (approx) from the memory (Kingston Lovo at 1.2V). If I add the move from mATX to mITX and replacing the 160GB HDD buy an SSD ... then it's another saving. For home / development / Sketchup and light gaming (read 2D gaming) it'll be a great rig. Two times the performance for less than half the power usage .... and if I need offload computing (3D rendering), I'd go the ARM farm route.
Panos 7th October 2013, 13:13 Quote
In UK avg kwh is 8-10p + charge, not 15p.
(have yet to find an open contract to pay by Kwh like used to be last year, all now need a standing charge).
javaman 7th October 2013, 14:31 Quote
Why not swing both ways and underclock?
@Panos in NI its 17.5p per unit. Nice to be about double the average. Who is getting power in the UK for so cheap?
John_T 7th October 2013, 15:06 Quote
Quote:
Originally Posted by Panos
In UK avg kwh is 8-10p + charge, not 15p.
(have yet to find an open contract to pay by Kwh like used to be last year, all now need a standing charge).

I pay 14.76p per daytime kWh in Southern England, (with Atlantic Electric and Gas) - that's with a standing charge on top, (26.1p per day). They were the second cheapest around when I joined. I've past the lock-in period and am ready to switch, so if you can tell me who charges 8p per kWh that would be great.

I do pay 6.9p for night-time energy, but night energy only makes up less than 18% of my usage - seeing as I generally sleep at night...
schmidtbag 7th October 2013, 15:06 Quote
While I am a computer enthusiast, I honestly could give less of a crap about getting the best performance possible, and it boggles my mind how people will only buy a CPU for it's performance rather than it's REAL selling points. When you build a system with an i3, you're doing it because you're looking for something power and heat efficient; price and performance are not the i3's selling point (price belongs to Pentium/Celeron, performance belongs to i5/i7). When you buy something with a specific intention in mind, the other downsides should not matter to you. You can't have the best of all worlds.

This is why Intel's Atom and AMD products in general suffer in the enthusiast market - people strictly look at how it performs, dismiss everything else, and decide they're bad products. Are AMD CPUs worse? Yes, but you're not looking at the selling points - AMD has a good record of backwards compatibility between generations, significantly cheaper products per-core, and in most everyday situations they get the job done fine. AMD is not by any means the best choice if you're looking for top performance or power consumption, so when you act like those are the only 2 things to consider, they look terrible.


As a side note, people today really take for granted that everything is instantaneous. Turn on your best rig from 10 years ago and see how much it can handle of today's stuff. Custom-made computers from 10 years ago were fantastic for their time and nothing you'd have been disappointed about. If you're the type who thinks AMD overall sucks and anything worse than an i3 is too slow for you, maybe you should just consider that waiting an extra second or 2 for something to work doesn't mean the platform is bad, it just means you're impatient and spoiled.
GuilleAcoustic 7th October 2013, 15:29 Quote
Quote:
Originally Posted by schmidtbag
As a side note, people today really take for granted that everything is instantaneous. Turn on your best rig from 10 years ago and see how much it can handle of today's stuff. Custom-made computers from 10 years ago were fantastic for their time and nothing you'd have been disappointed about. If you're the type who thinks AMD overall sucks and anything worse than an i3 is too slow for you, maybe you should just consider that waiting an extra second or 2 for something to work doesn't mean the platform is bad, it just means you're impatient and spoiled.

Rep jizz for that, mate ! I've discovered computing when I was 8yo. Back then, CPU where cuminating at around 7MHz, had no HDD and floppy drives transfer rate where around 240 Kbits/sec. I've learnt to wait.

I remember that my real-time programming teacher said : "Real time doesn't and will never exists. Everything take time, even if it's very quick". Now, people want everything and want it immediatly. Maybe I'm mistaken, but we are not ALL astrophysicians who need HPC at home. I can wait a little when I do 3D rendering. When I was studying 3D, CPU where only single cores (Athlon XP2000) and 3min of animation was taking a month to render 24/7 .... so if a 35W low power i7 does it in 2 hours vs 1h30 hours for the full fat 84W one, I CAN LEAVE WITH IT !

My wife's dad use a pentium G620 with no discrete GPU for family video editing and encoding. Sure it's not as quick as an i7 ... but while it's encoding, he's taking care of his garden. That's real life multi-tasking :)
faugusztin 7th October 2013, 15:42 Quote
Quote:
Originally Posted by GuilleAcoustic
Rep jizz for that, mate ! I've discovered computing when I was 8yo. Back then, CPU where cuminating at around 7MHz, had no HDD and floppy drives transfer rate where around 240 Kbits/sec. I've learnt to wait.

Floppies, they were quick. Compact Cassette on other side... 3-4 minutes for 48k program, now that needed patience. And only if the tape was read correctly and it wasn't damaged :D.
Quote:
Originally Posted by GuilleAcoustic
My wife's dad use a pentium G620 with no discrete GPU for family video editing and encoding. Sure it's not as quick as an i7 ... but while it's encoding, he's taking care of his garden. That's real life multi-tasking :)

I downgraded my laptop from 1400€ Acer Aspire S7 (Core i5 ULV, SSD) to a Acer Aspire V5-131, which has only Celeron 1017U. Sure, i upgraded it to 8GB RAM and added my good old X25-M G1 80GB, but after that, there is really not that much difference. Sure, the i5 has Turbo, Hyperthreading, bigger cache - but in reality it doesn't matter for majority of tasks. Even the compilation doesn't show too much difference because nearly everything these days does incremental compilation - so if compiling a Java class takes 0.1 vs 0.12 seconds, it makes no difference to the user.
Corky42 7th October 2013, 15:54 Quote
Quote:
Originally Posted by Panos
In UK avg kwh is 8-10p + charge, not 15p.
(have yet to find an open contract to pay by Kwh like used to be last year, all now need a standing charge).

According to this...
http://www.energysavingtrust.org.uk/Energy-Saving-Trust/Our-calculations
Standard rate Electricity = 15.32

Also backed up by...
http://blog.comparemysolar.co.uk/electricity-price-per-kwh-comparison-of-big-six-energy-companies/
Average price of 15 pence per kWh.

And a break down done by the government on averages for selected towns and cities (Table 2.2.3)
(Warning PDF)https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/65940/7341-quarterly-energy-prices-december-2012.pdf(Warning PDF)
also says the average is 15.32
KayinBlack 7th October 2013, 16:31 Quote
I'll go you all one better. I have an actual HPC setup here, a quad socket AMD board with 4 HE processors (8347HE.) Under full tilt, it takes what my wife's i5 does, before I add the GPU in. GPUs, of course, making all the difference in the world here. HE here means something. For desktops, not so sure, but when you pack a bunch of CPUs together (this is 16 real cores, mind) it certainly matters that I can cool these passively.

It's planned to heat the living room in the winter months, to save on power-we don't use the heat any more, we use our new fireplace and PC waste heat. Saves us LOTS.
schmidtbag 7th October 2013, 16:35 Quote
Quote:
Originally Posted by KayinBlack
I'll go you all one better. I have an actual HPC setup here, a quad socket AMD board with 4 HE processors (8347HE.) Under full tilt, it takes what my wife's i5 does, before I add the GPU in. GPUs, of course, making all the difference in the world here. HE here means something. For desktops, not so sure, but when you pack a bunch of CPUs together (this is 16 real cores, mind) it certainly matters that I can cool these passively.

It's planned to heat the living room in the winter months, to save on power-we don't use the heat any more, we use our new fireplace and PC waste heat. Saves us LOTS.

Haha I've done something similar. While I don't think a single computer can generate enough heat to effectively fight against the cold of winter, they are good for warming a concentrated area. My hands get cold and I once modded a desk where I cut a hole in the keyboard tray for a motherboard tray to go, and then rotated the tray so the CPU would be just below my hand. I used the heat from the CPU to warm my hand.
kent thomsen 7th October 2013, 19:39 Quote
The difference will be even less, if you take into the equation, that the low-power CPU will use less power pr second, but will keep on working for more seconds to get the job done.

I wonder if it is the same way as with other forms of energy-consumption: A certain task demands a certain amount of power to be executed, no matter what way you choose to execute it.

I.e. It will take the same amount of calories ot watt to heat a specific room to a specific temperature, regardless if the heater is a 100 W or a 500 W one.

Just my 20 cents:-)
schmidtbag 7th October 2013, 19:58 Quote
Quote:
Originally Posted by kent thomsen
The difference will be even less, if you take into the equation, that the low-power CPU will use less power pr second, but will keep on working for more seconds to get the job done.

I wonder if it is the same way as with other forms of energy-consumption: A certain task demands a certain amount of power to be executed, no matter what way you choose to execute it.

I.e. It will take the same amount of calories ot watt to heat a specific room to a specific temperature, regardless if the heater is a 100 W or a 500 W one.

Just my 20 cents:-)

Well if you're comparing something like a P4 to an i7, that's completely false. But for arguments sake, lets say you had an i3 and an i7 of the same generation and same clock rate, and suppose you have the exact same motherboard, PSU, OS, and RAM. The i3 will most likely consume more power in the end than the i7, because there are certain tasks that need to be performed every time a calculation is made and the reduction of transistors per core in the i3 reduces it's effectiveness. I remember reading that some phones include quad cores rather than dual cores intentionally for power saving purposes - the average user has no use for a quad core phone, but if tasks are evenly divided among all cores, the CPU doesn't have to raise its clock rate, which in turn saves power compared to a dual core of the same architecture with one of it's cores nearly maxed out. Think of it like this - suppose there's a 1km trail, and you're on foot. The average person could easily do this by walking and not break a sweat. But if you're hopping on 1 foot the entire way, or, running the entire way, you're going to get tired quicker. However, you'd get tired if you went on all 4s too. Efficiency is a matter of proportionate distribution of work while maintaining a certain pace. From what I've noticed, CPUs lose efficiency once you overclock them beyond a certain point (assuming you keep power management options on).
GuilleAcoustic 7th October 2013, 20:05 Quote
Quote:
Originally Posted by kent thomsen
The difference will be even less, if you take into the equation, that the low-power CPU will use less power pr second, but will keep on working for more seconds to get the job done.

I wonder if it is the same way as with other forms of energy-consumption: A certain task demands a certain amount of power to be executed, no matter what way you choose to execute it.

I.e. It will take the same amount of calories ot watt to heat a specific room to a specific temperature, regardless if the heater is a 100 W or a 500 W one.

Just my 20 cents:-)

The test I linked prove that the 35W i7-4765T is only 15% slower than the 84W i5-4670K while consuming 42% less (for the whole rig) at full load (64W vs 108W). Ok, it's 4C-8T vs 4C/4T, but it proves that HT is efficient and can increase performances even if your severly lower the wattage and frequencies.

Maybe it's better to have more cores with lower frequencies and voltages, but it really depends on what you do. With single threaded apps, you won't save anything ... but with multi-threaded apps it's a total different story.
MrJim 9th October 2013, 13:13 Quote
An important factor is idle power consumption, since it's fair to say that a home PC will spend a fair amount of time idle. A standard TDP processor will probably complete a task & return to idle more quickly than a slower-clocked low TDP processor.

As others have said, by changing power profiles in Windows it's possible to run modern CPUs at low power when extra performance isn't required.
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