# Interest group forums > Electrical Contracting Industry Forum > Electrical Load Shedding Forum >  Calculation time for a backup system

## ians

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 ...

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## adrianh

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

Calculate at worst case - not best case!

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## ians

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

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## ians

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

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## adrianh

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.

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## 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/

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## adrianh

> 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.

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## ians

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.

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## adrianh

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.

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## ians

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.

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## ians

Something to note ... as electricians we always calculate stuff at higher voltages ... like 230 or 400 or 660 ... 11KV etc

But when dealing with batteries ... inverters and stuff ... and its 2 am in the morning your brain is not thinking clear ...  dont mix the voltage/current and power.

for example ... when looking at how long a battery will last. 

Lets say your power is 1000 watts ... the voltage is 230 VAC and the current is 4.35 amps.

When you working out how long the battery will last ... make sure you work out the DC current 

dont make the mistake of thinking 4.35 amps (@230 VAC) should last 20 hours on a 100 amp/hr battery

Actually the power is 1200 watts / the battery voltage ... lets say 48 VDC for this example = 25 amps

If you have 100 amp/hr capacity and lets say you are using flood lead acid batteries ... best you check the manufacturers spec.

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## Dave A

I find the best way is to stick to power in watts or kilowatts rather than think amp hours when working out the energy storage required from the battery system. Once you've finalised you energy storage requirement, you can break back to the configuration of the battery storage in volts and amp hours. And at that point take care to look at not only the depth of discharge, but the *peak DC draw that will be put on each battery*.

I agree you should have an 8kW inverter at least with a measured base draw of 23 amp on the 230V side. Someone's going to plug in a kettle  :Stick Out Tongue:  Also in an office environment, take a careful look at the printers. Laser printers have a fuser unit that will ramp up the current draw when a page is printed.

You pose some interesting cost figures on lead acid vs LiFePo. With a cost factor that close, after you consider time to recharge, tolerance to depth of discharge and ultimate lifetime of the options, to my mind going LiFePo is a no-brainer. It becomes a decision as to which LiFePo system you are going to buy...

Given what is going on with load shedding/blackouts, I would also plan around at least double the expected 2 hours cover you are talking about - to get to 50% DOD while the office is active. What's that old engineering adage? When in doubt, double up.

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adrianh (14-Nov-21)

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## ians

An example 

AC mains 

load = 1000 watts 

Voltage = 230 VAC 

Current = 4.35 amps

DC battery 

Load = 1000 watts 

voltage = 48 VDC 

Current = 20.83 amps 

Using a flood lead acid deep cycle battery for 4 hours standby 

The battery capacity would need to be a min. of 264 amp/hr 

Generally deep cycle batteries are 100-105 amp/hr 

4 x 12 VDC batteries in series ... make up 48 VDC ... 105 amp/hr capacity 

3 x 48 VDC in parallel would make up 315 amp/hr ... giving you a little room for losses. 

There is a catch ... for this type of setup where we have a constant 1000 watt load ... it is easy to design a backup.

If we look at a another project I am busy with ... we have information like the average kw/h per day (35 kw/h) ... but not everything is connected to the essential side of the inverter and there is solar panels on the roof.

To complicate this even more ... the load on the essential side is higher after hours than it is during the day

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## ians

Load shedding stopped as of Friday afternoon ... by Monday everyone has forgotten and more on to other worries.

If only it was that simple ... life will go back to normal and  suddenly boom we will notice the lights off again ... no warning ... suddenly we will be back at stage 4 and so the panic will begin AGAIN ... builders will be sold out of generators in 2 days again ... everyone will be contacting installers for backup power.

I have noticed another trend ... people asking why they are not saving on electricity after installing systems ranging from R50 k - R xxx k 

They installed solar panels and battery backup yet the electricity bill is still about the same.

There are a few reasons 

the system is designed purely as a backup for essential loads ... the essential loads represent a small portion on the daily power consumption. 

The system is a hybrid system and not setup properly ... too much power exporting to the grid and they are using a digital meter ( because they registered the system) 

the installer didnt fit the CT or smart meter required to restrict the export. 

They are using too much power in the evening and the battery capacity is not big enough.

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## ians

@dave The office environment makes it easier to predict the load ... with red plugs ( I only use blue plugs for inverter power) ... it should prevent people from plugging in the kettle ... but we all know it is not the case ... I was looking at a project across the road from your office yesterday (I would have popped in for coffee but you were closed) ... every red plug had a 3 or 5 way extender plug with with red plug tops ... I can only assume they had an earth leakage tripping issue so they went out and bought a pile of extender plugs and solved the tripping problem.

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## ians

One of the reasons I started this exercise ... I was  trying to figue out why it seemed like we went from what seemed like 12hours of backup time on one of the inverter installations.

I have fitted all the units with data logger and remote viewing ... I just need to find a decent smart meter ... with remote viewing ... It is a pity Schneider have such crap support in SA ... they have some really cool systems ... another really cool system is the span smart DB ... also no available in SA  :Frown:

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## ians

This is my problem ... I will sit and research ... go through piles of scrap paper until I figue it all out ... gonna need to go put all the coffee cups in the dishwasher and throw away the pizza boxes and red bull cans ...maybe try get some sleep today.

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