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Solar charging to large UPS and sizing etc

  • 30-09-2020 9:29pm
    Moderators, Motoring & Transport Moderators Posts: 6,365 Mod ✭✭✭✭ Irish Steve

    So, for all sorts of reasons, there are changes likely here, and it's a case of trying to decide if it's worth going forward with what may seem like an off the wall idea.

    A long time ago, because of computer hardware I had here, which had to be capable of running even with interuptions to the power, I got myself a 16Kva 240V single phase UPS.

    Long story short, things changed very shortly afterwards, and it wasn't commissioned at the time. As made, it worked with lead acid batteries, a whole clatter of them, from memory, there were 34 of them, and it came with some very large batteries in a separate case so that it could back up a server for a number of hours. The DC voltage is 408v, and in order to charge that size of a reserve bank, the input to the unit is 3 phase, which fortunately I have available. The original batteries were recycled a number of years ago, as they were no longer suitable for use due to age.

    The house is a large dormer bungalow, with a 70 Ft south east facing roof, so good for sunlight, and I am wondering if it's worth looking at putting this UPS back into service as the main supply for the entire house, and having solar panels on the roof to charge the batteries if there's enough power available to do so.

    The standard batteries for this unit are only 7Ah, little small alarm size, but it has clearly been modified to be capable of charging the much larger batteries that it came with, which were somewhere around 100 Ah, maybe slightly larger.

    The power usage in the house now (no large computer any more) averages about 12KWh per day, based on 12 months, and we've the usual appliances like kettle, back up immersion heater, but nothing massive, no electric showers.

    For the same reason as the UPS, there's also a standby diesel generator that is set up to automatically cut in if ESB goes AWOL, so I don't need massive storage to run things as such, and we also have night saver power option.

    So, the thoughts go along the lines of suitable solar panels on the roof to extract as much power as can be sensibly captured during the day, and use solar/battery during the day, and then if necessary, top up/ recharge the batteries using night saver power overnight, with the backup being that if the batteries are depleted, we can either use "normal" rate power during the day, and if all else goes pear shaped, the generator (3 Ph 30 Kva) will provide all the power we need to run everything.

    Is this viable, or am I better off to just sell off the UPS and look at a lower power solar option as and when they become a better price option?

    Another thought, the UPS was designed for Lead Acid cells, but clearly with thinks like LiPo batteries now more common, would it be worth my while looking to use something like that technology for the storage? Another option that may be viable would be for me to get some suitable deep discharge fork lift style/ size/ capacity cells for the storage, I have the space to put them in without serious issue, and have worked with such cells a few years ago for a while, so I'm aware of the complications and risks of large lead acid cells.

    Thoughts, ideas, comments good or bad would be appreciated, as there are clearly people active in this forum with a lot more experience in this area than I have.

    Shore, if it was easy, everybody would be doin it.😁


  • Posts: 0 [Deleted User]

    I can do what you are talking about doing Steve. I'm synchronising inverter sets to run the house. Mostly as a test load to be honest, I ran out of dump loads after 4kW.
    It's not viable.
    I run lead and li-ion. They are as good as eachother. One is lightweight the other is very big for not much money.

    Just get solar that's the best deal. Diminishing returns after that with all these fashionable battery cycling devices.

  • Moderators, Motoring & Transport Moderators Posts: 6,365 Mod ✭✭✭✭ Irish Steve

    Thanks for the thoughts, I guess what I was sort of envisaging is that I have the existing UPS, which runs at 406 Volts DC, producing 230V AC out, and I was hoping that there would be an easy way to interface solar panels into that unit, as with a 16KvA max output, it would be capable of running the entire house without any other supports, so if I can find a way to put solar into the batteries for that unit, my hope was to significantly reduce my inputs from the grid, and if it went well, I might be able to only take grid input during off peak hours to ensure the batteries were sufficiently charged to meet the demand.

    I'm aware that there are losses from running a unit like this, but hopefully, there will be long term advantages in being able to use Solar for a significant part of the demand, and yes, I'm also going to look at ways to use solar for hot water, as that would be a help also, as it will reduce the requirement for the heating system to be used.

    I'm also aware that given time, and the likely demand for things like EV charging, I'm not going to be able to go completely off grid, but from what I can see of this, it makes sense to retain the 3 phase supply, which I had to have at one time to run the old 300Mb disc drives on a computer here (6Kw per drive for 30 seconds on start up, they ATE power), and the same thought goes for the standby generator, it's paid for, and definitely has big advantages when ESB goes AWOL.

    Shore, if it was easy, everybody would be doin it.😁

  • Posts: 0 [Deleted User]

    [pardon the defective shift key]

    i'm testing this at the moment. she's a 3kwh 4kva synchronising hybrid. round trip 79p/c closer 70p/c normal operating conditions and that's mostly big resistors.


    the efficiency isn't there steve you can't beat less is more even before the system costs.

    that yolk you have sounds like a behemoth. what's the quiescent 150w?

    if you want back-up then economics is a redundant argument but 6kva is enough for most.

    solar can pay for itself very quickly and hence finance more solar. this battery thing is a product of local markets and lack of subsidy. none of the grown ups are trying to make more power by increasing the system losses.

    i'm all for backup i just have a loada battery inverters i can plug into eachother for more power or energy, i can run the house about half a day of our 20kwh daily with everything i have onsite [about 40p/c of my total owned battery capacity]. no diesels, no petrols i reload mine with sunlight.