Power supplies
Power supplies pt 2 - wattage and ratings
Over the last couple of months, we’ve covered simple PSU ratings and how you’d choose between anchored, semi-modular and modular designs. Now that we’ve considered the basics, we’re moving into ‘PSU201’ and some deeper concepts. Buying a PSU on watts alone is like buying a car just by its colour, except with PSUs, colours actually do mean something. This month, we talk PSU power efficiency – we investigate those ‘Gold’, ‘Silver’ and ‘Bronze’ ratings, what they mean and why they matter to your hip pocket.
Power efficiency
Back in PSU101, we said a PSU’s role is to convert the mains outlet AC voltage into a series of low-voltage DC sources or ‘rails’ to power your PC components. Now that we’re in PSU201, we can flesh that idea out and now say there is a bit more to it.
In simple terms, the AC-DC conversion process is somewhat ‘leaky’, meaning it is not 100%-perfectly efficient. Some of the energy dissipates as heat, which is neither useful nor helpful. However, while all PSUs do lose some energy as heat, the better ones lose less, particularly when the PSU is pushed hard (what we call ‘loaded down’).
Efficiency is measured as a percentage and is the ‘load’ power the PSU sends to your PC divided by the input power the PSU needs to make that load power, multiplied by 100. For example, if your PC requires 200-watts, but the PSU needs 250-watts input, its efficiency is 200 / 250 = 0.8 x 100 = 80%.
What you also need to know is that efficiency varies, depending on this electrical ‘load’ the PSU has to power (ie. your PC). What’s more, efficiency tends to drop at both ends of the load spectrum – when you’re using only a bit of the PSU’s rated power output and when you’re flogging the daylights out of it. As a result, the better PSUs maintain a higher efficiency with heavier loads (when it’s tough to do so) and lighter loads (when idle power is more noticeable).
Why does it matter?
Efficiency matters for a simple reason – money. When it comes to your power bill, you pay for the AC input power to the PSU, not the DC output power to your PC. So, the less efficient your PSU is, the more AC input power it needs and the more you pay for it.
A voluntary program called ’80 PLUS’ was created to make it easier to understand which PSUs deliver greater efficiency and it has several steps or grades from ‘ basic’ to ‘Bronze’, ‘Silver’, ‘Gold’, ‘Platinum’ and ‘Titanium’, depending on the efficiency obtained at 10%, 20%, 50% and 100% of the PSU’s rated output power. Take a look at Table 1.
In a nut-shell, the higher up the grades you go, the greater the minimum efficiency level guaranteed and for more of the load spectrum it delivers that minimum.
However, notice a couple of things. First, the highest efficiency percentage guaranteed on each grade is always at 50% of the rated output power. For example, ‘bronze’ guarantees 85% minimum across the board from 20% to 100% load, but 88% efficiency at 50% load. Similarly, ‘gold’ is 89% minimum across the range, but 92% efficiency at 50% load. Take this as another reason to consider buying a PSU nominally rated at double the output power your PC requires – you get maximum efficiency. For example, building a PC needing 230-watts, you buy 450. Putting together a 360-watt gaming rig, you buy 700. Not only does this get you maximum efficiency, but, in my personal experience, better
reliability. Electronic components in general don’t like being thrashed, so the more breathing space you can give them (and reducing the load percentage does that), the longer they tend to last.
The second point to note is that only ‘Titanium’ grade PSUs guarantee a minimum efficiency level at 10% load. This basically says most PSUs don’t get out of bed for less than 20% of maximum output power.
To put it another way, this is why you don’t buy a 1200-watt PSU for a dual-core Pentium box to read your email. PSUs struggle for efficiency at low-percentage output power, so if you have a PC that presents a 100-watt load to a PSU, that’s just 8.5% of a 1200watt PSU’s capacity. No-one’s going to guarantee you any efficiency level at 8.5%! However, 100-watts is 22% of a 450-watt PSU’s capacity, where you will get a minimum efficiency guarantee (remember, all 80 Plus efficiency guarantees cover 20% loading).
So, choosing a PSU is a balancing act between buying sufficient headroom, but not too much that the PSU just loafs along. To get decent efficiency, the PSU has got to have something to do – and that’s why buying a PSU capable of delivering twice the power you need covers most of the bases.
Basic vs Platinum
Getting back to the different grades, those differences can add up. For example, take a 400-watt PC as load and compare the actual input power to an 800-watt 80 PLUS ‘basic’ PSU and an 800-watt 80 PLUS ‘Titanium’ PSU, both running at 50% loading.
The ‘basic’ PSU needs 400 / 85% = 470-watts input, but the Titanium, only 400 / 96% = 416-watts. That’s a difference of about 55-watts or four 14-watt LED globes. If the PSU runs 24/7-continuously, the difference amounts to just over 1.3-kilowatthours (1.32kWh) per day or 480kWh for the year. At an average 32-cents-per-kWh electricity rate, that’s 480 x 0.32 = $154 per year – every year – added to your power bill just to run the ‘basic’ PSU instead of the ‘Platinum’ on the same load. Sure, most PCs don’t normally run at 400-watts day-in-day-out unless you’re crypto-mining, but even at just 200-watts, the difference is still around $80 a year, which no doubt you could spend differently.
Now of course, a Titanium PSU will cost roughly three-times the price of an 80 PLUS ‘basic’ PSU at the same power rating, so it’ll take a couple of years to get in front, but that is the extreme case. Do the sums for ‘basic’ vs ‘GOLD’ and the cost difference and break-even time should be less.
Efficiency doesn’t guarantee quality
Improving efficiency has other benefits, including reducing heat and noise. However, it doesn’t guarantee build quality, so you still need to buy a good brand first. Otherwise, if you’re gaming – especially if you’re crypto-mining – take power efficiency seriously. Next time, we look at the concept of ‘power factor’ and sort out a f ew ideas.