# Redundant PSU Power Usage

• 7th November 2012, 03:56 PM
Mr.Ben
Redundant PSU Power Usage
This may sound like a stupid question... (It relates to the purchase of a new UPS)

What's the total power usage on a server with a redundant psu?

Say I have a server with 2 PSU's rated at 750 watt

I assume that each PSU provides 50% of the load until one fails, so 375 watts each?, bringing the total to 750w + 10% (825w), assuming that there is an overhead to having the redundancy?

Or do they both use 750w?

• 7th November 2012, 04:06 PM
VeryPC_Tom_M
I've just had a chat with someone with a little more server knowledge than me...

He agrees with your calculation of 825w, but would probably make sure that the UPS can support around 1000w, just to play on the safe side.
• 7th November 2012, 04:25 PM
Mr.Ben
In that case the UPS specced by the previous NM is very underpowered!

I calculate 7,300w approx load on a UPS that can deliver 3750w......
• 7th November 2012, 04:58 PM
glennda
Quote:

Originally Posted by Mr.Ben
This may sound like a stupid question... (It relates to the purchase of a new UPS)

What's the total power usage on a server with a redundant psu?

Say I have a server with 2 PSU's rated at 750 watt

I assume that each PSU provides 50% of the load until one fails, so 375 watts each?, bringing the total to 750w + 10% (825w), assuming that there is an overhead to having the redundancy?

Or do they both use 750w?

It depends what server it is. Some servers are active active sharing 50% some are purely active-failover. It depends what server you are running.

APC's UPS calculator is rather good at calculating server loads.
• 8th November 2012, 11:32 AM
SYNACK
This is a wildly difficult question, those values represent peek draw with full drive bays screaming with 15k drives at full tack, the hottest fastet CPUs and all expantion cards in use pulling down max voltage.

The actual useage of power is generally much lower. As others have said, the PSU sharing is also dependant on the hardware, most hp gear tends to run at 75%/25% for primary/secondary so that both have an extended lifespan but the likelihood of them both going at the same time is reduced. This is 75%/25% of actual power usage though not the peek power of the CPUs.
• 8th November 2012, 12:14 PM
twin--turbo
Put a clamp meter over both incomming supplies.

You will be nowhere near 750W - Most likly one supply will show 300-400w and the other very little.

Rob