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Choosing the right PC Power Supply Gordon Laing, October 2009
   
  Choosing PC power supplies - and saving money!

Modern PCs may feature a number of power saving technologies and increasingly efficient parts, but a typical system could still easily consume well over 200 Watts running at full steam, or more than 150 Watts even when idle – and that doesn’t even include the display.

Since most work computers are powered-up for at least one third of every day, their power consumption can represent a significant portion of your electricity bill. This of course can seriously add up if you’re running over longer periods of time or operating multiple PCs. The heat generated by PCs also means more work for air conditioning systems, which in turn results in bigger bills.

PCs which consume less power and generate lower heat are of course the answer, but there’s no need for us to abandon high-performance desktops and switch to environmentally-friendly laptops. Believe it or not, by switching one component in your desktop PC, you could potentially save power and generate less heat and noise without any compromise in performance. That component is your power supply, and in this article I’ll show you how spending around $100 USD saved me over 10% in direct power consumption, while also generating less heat and noise.








Like all PC components, the more you know about power supplies, the better equipped you’ll be when choosing the right model for your system. The most obvious specification to consider is the rating in Watts, but that’s about as far as many buyers look. In marketing terms, a 500 Watt power supply may seem better than a 350 Watt model, but dig a little deeper and you’ll see why this single figure doesn’t tell the whole story.

A PC power supply has to convert mains AC into a variety of DC outputs. A modern unit will deliver three main DC outputs – or rails – at +12v, +5v and +3.3v. In addition there’ll be a -12v and +5VSB rail, the latter allowing a PC to come out of standby.

The total output of a power supply will be divided between these rails, but in different ratios depending on the quality of the unit in question. In a modern PC the processor and graphics card both consume 12v, which, as the hungriest components, makes the +12v rail in a power supply the most important.

So you should be looking for a power supply which delivers the lion’s share of its output on the +12v rail, but lesser models may not. Check the label on the side of a power supply to see the breakdown in Amps.

Avoid power supplies which deliver more Amps to their +3.3v and +5v rails than they do to the +12v rail. This is an easy way to boost the total Wattage figure for marketing purposes, while neglecting the critical +12v rail. Ideally, the amount of Watts a power supply can deliver to the +12v rail should be around three quarters or more of its total power rating. To find out, simply multiply the number of Amps for the +12v rail by 12 and see how many Watts you end up with.

It’s also important to check the conditions under which the manufacturer’s quoted figures have been taken. Many budget models quote ratings at ambient room temperatures or those within the PC case, but the power supply itself will normally be running much hotter. Power supplies don’t perform as well at higher temperatures, so beware of figures taken at around 25 degrees Celsius and instead look for ones under more typical conditions like 50 degrees.

So how many Watts do you really need? A number of online calculators will provide an estimate given the specifications of your system and your desire to overclock; try Outer Vision’s Extreme calculator.

It’s also recommended to comfortably over-specify your power supply, not just to accommodate hungrier components in the future, but also because power supplies run at their most efficient when delivering considerably less than their maximum output. This in turn means lower power bills, and cooler PCs with less waste as heat.

If a typical modern PC consumes around 200 Watts under a hefty load, then it would seem reasonable to couple it with something like a 250 Watt power supply. That would give you enough power under load, with a little spare on top.

Unfortunately such a configuration would actually be very inefficient, consuming more power than necessary while also running hot, requiring potentially noisy cooling; indeed unless it was a high quality unit, it may not even deliver sufficient juice on the required rails to reliably power the system in the first place.

Most decent PC power supplies actually perform at their most efficient when delivering roughly half their rated output, so if your components typically draw around 200 Watts, then you’d ideally be coupling them with a power supply rated at 400 Watts or maybe higher still if you planned on upgrading in the future.

The important thing to remember is a power supply won’t constantly consume electricity at its maximum rating. It will only deliver the amount of power the PC requires at any given time, so even something as hefty as a 1000 Watt supply may only draw a fifth of that in normal operation.

 
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So a well-designed power supply with a decent rating will only consume what it needs, but will do so more efficiently than a lesser model, thereby reducing power bills along with heat and noise. Indeed the only downside in buying a higher-rated PSU is paying a little more for the unit to start with.

To put this theory into practice I upgraded the power supply in one of my office PCs. At the time of testing, this consisted of an Asus P5W DH Deluxe motherboard with a Core 2 Duo E6700 processor, 6GB of Crucial DDR-2 memory, a Gigabyte NX66256DP (Nvidia 6600) graphics card and four 300GB Seagate hard disks, three of which were in a Promise hardware RAID 5 configuration.

The existing power supply was a 365 Watt model from Enermax. This is a very respectable company, but the model itself was bought and fitted over two and a half years previously when I ran a single core Pentium 4, a lesser graphics card and just one hard disk. The power supply had been more than adequate running this original configuration, but over the years I’d upgraded the processor, graphics card and added no fewer than three extra hard disks, so it was clearly under greater pressure than before.

With Vista booted and disk activity essentially idle, I measured 170 Watts being drawn from the mains socket using a Maplin power meter; this rose to 209 Watts under 100% load when transcoding a video file. While the latter was still far from the maximum rated output of my power supply, I still wondered how a newer and more powerful model would compare.

So I powered down my PC, disconnected the old power supply and fitted a newer Corsair HX 520W model, which as its name suggests, has a maximum rated output of 520 Watts. Looking deeper at the specifications reveals triple +12 Volt rails with a combined rating of 40 Amps. The HX 520W also employs a quiet 120mm fan and a modular design which uses detachable drive and graphics power cables so there’s the minimum of clutter inside your case – just connect the cables you need and leave the others for future upgrades. For a fair comparison I ensured all the same components and drives were connected as before.

With the new power supply fitted, I re-measured the power consumption under exactly the same conditions as before. With Vista booted and the system idle, the power consumption measured 153 Watts – that’s 17 Watts less than before. Under 100% load when transcoding the same video file at the same point, the meter measured 182 Watts – that’s 27 fewer Watts. The new power supply was also more efficient even when the PC was shutdown, drawing 6 Watts compared to the 9 of its predecessor.

Switching to a more efficient power supply made a significant saving on my system: it consumed 10% less power when idle, 13% less under full load and 33% less when shutdown. By running more efficiently, the new power supply was also cooler and quieter in operation.

Remember this didn’t involve downgrading any components, throttling existing ones or tweaking any settings. The performance remained exactly the same as before with the only change being the power supply.

This is a great result for the Corsair HX 520W, although similar benefits can also be enjoyed with other quality power supplies including those from companies like Enermax and Zalman. The key as described above is to buy a model that’s both honest about its rating and comfortably over-specified for your current or future configuration.

The latter was where I’d tripped-up, thanks partly to a motherboard which lasted much longer than I’d originally anticipated. I normally rebuild my work PC every year or so, carefully matching all the new components including the power supply, but with my existing motherboard happily accommodating new technologies, I’d steadily upgraded individual parts rather than replacing the whole thing. Along with the motherboard, the other component which remained constant over this time was the power supply, which subsequently became less efficient with the steadily updated configuration.

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So this article serves not just to prove how an over-specified modern power supply can save you money, but also as a reminder for anyone who, like me, has upgraded hungry components without ensuring their power supply was still appropriate for the job. To be fair, my ageing Enermax PSU was able to cope with my current configuration, but the newer and higher-rated Corsair model could do so with less power, heat and noise. Not bad for the money, and Recommended.

The readings with the PC shutdown also remind us electrical equipment can consume power even when not switched on. The newer Corsair model may have consumed one third less power with the PC shutdown, but that’s still six Watts for effectively doing nothing. So the moral here is after shutting down at the end of the day, always switch the power off at the wall to ensure you’re not wasting energy and money. Discuss this article in the forum!


All words, images, videos and layout, copyright 2005-2014 Gordon Laing. May not be used without permission.

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