Calculation time for a backup system

**ians** · 13-Nov-21, 03:40 PM

An example:

I have an office which requires backup power for 2 hours (more like 3-4 ...but for the calculation we will use 2 hrs)

The office has 20 computers and a few other small devices like ... routers ... a switch ... a telephone system and a few other electronic devices.

Fed from 10 dedicated 20 amp circuits.

10 offices each with 2 x dedicated plug tops.

I took current readings on each circuit breaker ... adding up to 23 amps

Lets say we didnt have a current clamp ... so we had to do calculations.

20 computers @ 250w (highest value) = 5000 watts

taking into consideration a 08 pf = 6250 va

rounded to 6.3 kva

Allow for a few extra unexpected devices to be plugged in for future.

lets say an 8 KVA inverter should do the job

Now we need batteries ...

**adrianh** · 13-Nov-21, 03:57 PM

My computer has a 650 watt power supply - what about screens, printers, cellphone chargers, laptops and peripherals.

Calculate at worst case - not best case!

**ians** · 13-Nov-21, 04:19 PM

We talking normal people using a normal computer ... not a gaming computer or graphic design computer.

**ians** · 13-Nov-21, 04:21 PM

By the way I used current clamp and measured the current ... so 150 - 250 watt sounds about right.

**adrianh** · 13-Nov-21, 04:57 PM

Interesting....my piece of crap 10 year old XP desktop on my Vinyl cutter has a 300watt power supply, its screen requires 300watt. The old Pentium on the CNC runs 500watt and its screen requires 300watt. All 4 the laptops in the house are rated 330watt or more. 3 of the laptops are at least 5 years old - just i3 or i5 - nothing special.

Only the old brother printer comes in at 200watt.

**ians** · 13-Nov-21, 05:44 PM

Got hold of my mate google and did a search ... this is what I came up with ... then added a few values ... then increased and so we went from 238 watts past 600 watts ... then we need to add a screen ... lets say 400 watt?

This is the kind of challenges we face when a customer calls and requests a quote.

https://www.newegg.com/tools/power-supply-calculator/

**adrianh** · 13-Nov-21, 06:00 PM

Originally Posted by ians

Got hold of my mate google and did a search ... this is what I came up with ... then added a few values ... then increased and so we went from 238 watts past 600 watts ... then we need to add a screen ... lets say 400 watt?

This is the kind of challenges we face when a customer calls and requests a quote.

https://www.newegg.com/tools/power-supply-calculator/

The problem with these calculators is that you end up having to make serious assumptions - Different graphics cards have vastly different power requirements.

I would work on at least 800-1000watt per workstation.

**ians** · 13-Nov-21, 06:09 PM

Lets use a real life example ... running at the moment on a 5 kva inverter

I have 8 computer and 8 screens ... 2 big TV's ... a server ... 2 network switches ... an XVR with 30 cameras ... an alarm system ... and a few other small items like a router etc.

Lets say I use your 800 watt suggestion 8x800watt = 64000watts ... that would mean I need a 8 kva inverter ... drawing around 35 amps just the computers.

If I log into one of my data monitoring devices and view the charts ... right now if I live view the unit ... total load is 875 watts and nobody is in the office ... all partitions are armed ... go back 2 days to the middle of the day when everyone is at work and the system is at full load ... 1200 watts ... which means we drawing about 5.2 amps.

**adrianh** · 13-Nov-21, 06:39 PM

Dunno - Could be that the PC power supplies and things are rated for max consumption rather than general usage consumption. I am just looking at ratings on the devices.

**ians** · 13-Nov-21, 08:14 PM

Lets say we stick to 250 watts per computer and we want the system to backup the power for 3 hours ... a total of 5kw around 23 amps ...

We know for a fact that we have 23 amps on the dedicated circuits .. .so regardless of the computer power using a 25 amp guide line would be a wise choice.

I wouldnt even consider installing an inverter smaller than 8 kva.

If I want the batteries to backup the system for 3 hours (50% DOD ... lead acid ... not ideal ) and I were to install 4 x 12 VDC batteries to create 48 VDC ... I would need 947 amp/hr or 45.46 kw/hr ... R80k ... from personal experience ...we getting about 3-4 years out of lead acids deep cycles.

If I were to use lithium to backup the system for 3 hours (80% DOD) at 48 VDC ... I would need 391 amp/hr or 18.76 kw/hr ... R100 k ... havent installed a lithium battery pack yet.

That sounds like a lot of battery power.