Smallest possible PSU for a HTPC with a HD 5850

[obsidian86]

380 watts at 50% is 190 watts it wont cut it

but this is a silver rated 550 watts that will give you the best results for your uses

http://www.newegg.com/Product/Product.aspx?Item=N82E16817341037

 

[Illex]

Thanks for the input obsidian86 but I don't fully agree with you.

380W should be more than sufficient for full load - barring any power spikes

The low load / idle power draw of the system I can only estimate, but I would guess that it should be well below 200W (according to most reviews around 150W, but once again they use much bigger CPUs).

If I use a 550W PSU and estimate a 150W power draw I'm using less than 30% of the available power and will spend most of the operating hours in a bad efficiency range. If I go with the 380W I get much closer to a 50% use of the PSU while having enough power available to run the 5850 on full load if required.

On the other hand, I agree with you because you suggested a "80 plus silver" PSU. Here the efficiency at 20% is the same as the "80 plus bronze" one at 50% load that I had planned on using. So while I think 550 is overkill, by going up one category to a silver certified PSU more than makes up for it - this I hadn't actually considered up to now.

... now if only I could find a gold certified PSU at 380W 😉

 

[obsidian86]

lowest gold rated is 600 watts

lowest platinum rated is 4000 watts

 

[Illex]

I had already considered the new Enermax Pro-87+ at 500W which is also gold rated... if it just wasn't so damn expensive

 

[will_chellam]

AS far as I can tell only one person on here has made the correct suggestion....

+1 to obsidian86

The power draw from the wall is essentailly the same regardles sof the size of the power supply.

What I do know is that the harder you push a powersupply to its max, the more likely it is to fail, and I bet the cost of a new mb, proc and 5850 is much more than the few %s you'll save on a more efficient supply.

 

[Illex]

AS far as I can tell only one person on here has made the correct suggestion....

+1 to obsidian86

The power draw from the wall is essentailly the same regardles sof the size of the power supply.

What I do know is that the harder you push a powersupply to its max, the more likely it is to fail, and I bet the cost of a new mb, proc and 5850 is much more than the few %s you'll save on a more efficient supply.

This is what this whole thread boils down to for me: How much power can I save by using the appropriate PSU for the job? The difference in efficiency at varying loads is exactly what's interesting to me.

Just slapping in any 600W+ PSU is the boring (but safe) solution 😉

With a quality PSU (Seasonic, Enermax etc.) I really don't expect it to fail just because of a few hours operating near full load. As I mentioned, my load profile will be stacked quite heavily to low load operation.

 

[13thmonkey]

This is what this whole thread boils down to for me: How much power can I save by using the appropriate PSU for the job? The difference in efficiency at varying loads is exactly what's interesting to me.

Just slapping in any 600W+ PSU is the boring (but safe) solution 😉

With a quality PSU (Seasonic, Enermax etc.) I really don't expect it to fail just because of a few hours operating near full load. As I mentioned, my load profile will be stacked quite heavily to low load operation.

you will save virtually nothing by getting a perfectly sized psu... psu's are most efficient between perhaps 50% and 80% of capacity, pushing up to 90% and the cooling system for the psu will get more stressed and generate greater losses, spin up the fan more (which uses power), and perhaps fail. PSU's fail quickly at full load, its not a case of 10hrs at full load is ok, but 20 is not, it'll go quickly.

I'll repeat what someone else has said, to within a few % the power at the wall is the same regardless of the psu size, and to give you a clue as to the cost, that few % will cost you maybe 10 watts per hour, so for every 100 hours that the pc is on at load you'll use an extra unit of power. You'd be far far better choosing a low power cpu and a lower powered gpu, and focusing on the mobo to choose a low power one, and low voiltage memory, fewer case fans, green HDD's. Perhaps ensuring that your tv is green, or perhaps not having a htpc, but using a dvd player hdd recorder which with less operating system overhead will be a lot cheaper to buy and run.

done some sums, at 10W extra losses due to a mis sized supply, at 5c per unit of electricity, on 24x7, will save you $4.60 per year. Is that worth the risk of killing your system, or the extra noise (in fact a 5w fan spinning at full as opposed to a near passive cooling would cost you another $2.30) so getting a bigger supply would probably only cost you $2.30. all at 5c/kwh multiply up the costs are more.

 

[Illex]

you will save virtually nothing by getting a perfectly sized psu... psu's are most efficient between perhaps 50% and 80% of capacity, pushing up to 90% and the cooling system for the psu will get more stressed and generate greater losses, spin up the fan more (which uses power), and perhaps fail. PSU's fail quickly at full load, its not a case of 10hrs at full load is ok, but 20 is not, it'll go quickly.

I'll repeat what someone else has said, to within a few % the power at the wall is the same regardless of the psu size, and to give you a clue as to the cost, that few % will cost you maybe 10 watts per hour, so for every 100 hours that the pc is on at load you'll use an extra unit of power. You'd be far far better choosing a low power cpu and a lower powered gpu, and focusing on the mobo to choose a low power one, and low voiltage memory, fewer case fans, green HDD's. Perhaps ensuring that your tv is green, or perhaps not having a htpc, but using a dvd player hdd recorder which with less operating system overhead will be a lot cheaper to buy and run.

done some sums, at 10W extra losses due to a mis sized supply, at 5c per unit of electricity, on 24x7, will save you $4.60 per year. Is that worth the risk of killing your system, or the extra noise (in fact a 5w fan spinning at full as opposed to a near passive cooling would cost you another $2.30) so getting a bigger supply would probably only cost you $2.30. all at 5c/kwh multiply up the costs are more.

Thank you for the example. I'm sure you're right about the potential savings - btw I live in Switzerland where the cost of electricity is around 4x that what you suggested... still, that won't make a huge difference.

As for the PSU, I think I will actually go with a gold rated one that's bigger than what I need. The efficiency of that operating at 20% load will still be quite a bit higher than the bronze rated Seasonic at 50% I was initially set on using.

So even thought I will be operating the PSU in a less ideal load profile it will still safe me power. Once smaller gold/silver rated PSUs (~400W) become available I might buy a new one and reuse the bigger one in a desktop PC.

 

[obsidian86]

wait for geil thortech power supplies

 

[JohnnyLucky]

Geil Thortech looks interesting - gold certified ony.

 

[Illex]

I bit the (prize) bullet and ordered the new Enermax Pro-87+ 500W (gold certified).

Thank you all for your input!

Cheers,
Illex