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Thread: Inverters

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    Inverters

    Can anyone advise me on Inverters please ? Would like to get one for our Jhb office and home. For the office we need to run 8 computers (if possible) when there is a power failure. At home I would like to know if there is one that will run basics such as TV, fridge, freezer and a few lights. How long do these things last and what sort of cost are they ?
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    Hi there
    Think you are after UPS (uninterrupted power supplies). Might want to put on the Electrical Contracting Industry part of the part of the forum. Your sure to get more answers.
    There as quite a few factor to take into account. The larger the load and the longer you want to stay on for the bigger the battery bank the bigger the cost.

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    I would like sport 4 to 5 hours of use - 4 computers and 4 laptops.
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    UPS are generally used to provide short-term power (giving you enough time to save and switch off) and protection against voltage spikes. Some UPS are smart devices and can be set to automatically and correctly shutdown/hibernate/sleep your pc, when there is an input failure, thus preventing damage to your equipment, inverters cannot do this . As mentioned, both UPS and Inverter needs a correctly seized battery bank depending on how long you want to keep the device/s running. UPS and Inverters almost work in the same way, both make use battery's and both need a way of keeping the batteries charged and both need to convert DC to AC. For the PCs at work you would need to work out the total power (watts) needed per PC and either size you UPS or Inverter for each individual PC or find a big enough unit to handle all the PC's together. Maybe using a generator would be more practical/cheaper, I'm not sure on the various prices

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    A deep cycle battery with a inverter could work for your Flat Screen and DSTV. I got the Prices of Deep cycle batteries the other day ranging from R1500 - R2000 for a 12V but gel ("maintenance free") batteries are more expensive. The deep cycle batteries will have a Amp/Hour rate and using this you will be able to workout your viewing time. A car battery is not suitable for this application

    Also bare in mind that the batteries in the UPS or using with an inverter also have a limited life span and would need to be replaced later, but then a generator would also need to be serviced from time to time.

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    If you work on your computer using 200 watts plus 70 watts for the LCD (more if its and old school moniter). Plus your lap top chargers which are minimal, say 30 watts. You need to run a total of 1200 watts or 1.2Kw. You always upsize the inverter by about 25% give you space so you could use a 1.5kVA inverter, maybe even go 2kVA. You need to run for 5 hours so for 5 hours you will consume a total of 5kWhrs (1kw for 5 hours) to get that out of batteries you will need 416 Amp hours worth of battery (if they are 12 volts) (5000watts / 12volts) BUT double it as you really don't want to ever flatten your batteries beyond half way. As they say above you need deep cycle batteries, not a normal old car battery. It would work but not for very long. This is a rough guide line. There are other factor to take in consideration but you can maybe get quotes based on 1.5kVA or 2VA UPS with 8 x 100 amp hour deep cycle batteries to get an idea of what the price would be. The supplier would give you more advice. Hope it helps.....

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    There's two completely different issues you need to assess. Firstly is the load because as pointed out this determines the size in KVA of your UPS/inverter. Secondly is the required running time because this determines the size of the battery bank you need.

    I would suggest not oversizing the system by more than 10% or so because it starts getting very expensive. If you want it to carry you through load shedding then 4-5 hours is more than double (probably actually triple) what you need. This amount of redundancy is expensive.

    Also forget the fridge and freezer, they'll stay cold for an hour or two without power and motors are best avoided because their start currents and the highly inductive nature of their load makes them very taxing on any back-up power system.

    Stick to running a couple of laptops/notebooks only because they're built to be low consumption, not PC's etc. A few lights are fine as long as they're LED's and obvously your router. Even with a small/medium LED/LCD TV I'd estimate your load would be < 1kW.

    Look at the Victron inverter/chargers. They're pure sinewave output and they're also scaleable so you can always add a second one to increase your power if necessary at a later date. You can even connect three single phase units together to give you a 3-phase output and they're capable of having multiple inputs so you can connect solar/wind turbines as well as batteries to them. They have a built-in charger as well. I've installed several of these over the last decade and they're very good. Not cheap but you certainly get what you pay for in this case.
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    For running computers, laptops and the 'better quality' LED lighting, you do not need a pure sine wave convertor, and a square wave is more than adequate. After all, these devices convert the AC into a DC signal and then using switch mode supplies, convert the DC to the voltage required, so effectively the cost of the sine wave is totally lost in these applications.

    Pure sine wave convertors are suitable to run motors and microwaves, as they place less strain on this equipment, which requires a sine wave to be efficient.

    As Andy says, first work out your total consumption, add 50% for inefficiencies and to allow the inverters to run at 50% of their rated value, as the full rating figures on most inverters is for short periods, next calculate the number of hours you wish to run on battery power, multiply by 2.5, as you want to not drain your battery's to less than 60% of capacity, to allow over 800 cycles or more out of them, going lower reduces the cycles in logarithmic fashion. Sit back calculate, and then recover from the shock , and work out what you can afford. Get a genny as a back up for the extra time you need, it will be cheaper.

    If you are not sure of the final figures, place the numbers here and we will do the final calculations for you.
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    Guys thanks for all your input.
    I have no idea of the numbers
    For the office - we would def need 3 x desktop computers + 3 x laptops + 1 x router (at a squeeze) - the power outages are becoming more frequent and it is starting to cost money for downtime. Our girls need access to their computers

    For the house we perhaps only need to be able to run a few lights which could be normal bulbs & perhaps the tv and dstv if it can be included

    Our power outages are sometimes up to 4 hours so it would be more important for work to be able to operate the computers/laptops for this period.
    I suppose home would not be too much of a crisis if we only have a small inverter
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