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Thread: Calculation time for a backup system

  1. #11
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    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.
    Comments are based on opinion...not always facts....that's why people use an alias.

  2. #12
    Site Caretaker Dave A's Avatar
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    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 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)

  4. #13
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    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
    Comments are based on opinion...not always facts....that's why people use an alias.

  5. #14
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    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.
    Comments are based on opinion...not always facts....that's why people use an alias.

  6. #15
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    @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.
    Comments are based on opinion...not always facts....that's why people use an alias.

  7. #16
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    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
    Comments are based on opinion...not always facts....that's why people use an alias.

  8. #17
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    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.
    Comments are based on opinion...not always facts....that's why people use an alias.

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