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  1. #1
    Apprentice (new member) West OZ Mark's Avatar
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    Question Inverter Power supply

    I have a property that is run on a generator system for a 3x1 house and we are going to get either solar power or mains power conected in the next 12 - 18 months due to the cost.

    During the day the only things I have running on a regular basis is a fridge, freezer, T.v and Fax/Phone running off the generator and it seems a bit of an overkill.

    I have priced up a 2kw inverter and deep cycle battery bank system but have got a price back for $7k.

    My question is can I set up a cheeper sytem to run just a basic system for running the mentioned appliances? I have seen 2kw inverters 12v-240v for about $300.

    If I can set something up for around the $2-$3k I would be happy and I could stop my generator running from 14-16 hrs per day to 4 hrs per day.

    My generator is a 5.5kva diesel with a 12volt DC output that I could use for recharging the battery.

    Any sugestion on design setup appreciated
    Thank....Mark
    When I die, I want to go peacefully like my Grandfather did, in his sleep---not screeming, like the passengers in his car.

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    Quote Originally Posted by West OZ Mark View Post
    I have a property that is run on a generator system for a 3x1 house and we are going to get either solar power or mains power conected in the next 12 - 18 months due to the cost.

    During the day the only things I have running on a regular basis is a fridge, freezer, T.v and Fax/Phone running off the generator and it seems a bit of an overkill.

    I have priced up a 2kw inverter and deep cycle battery bank system but have got a price back for $7k.

    My question is can I set up a cheeper sytem to run just a basic system for running the mentioned appliances? I have seen 2kw inverters 12v-240v for about $300.

    If I can set something up for around the $2-$3k I would be happy and I could stop my generator running from 14-16 hrs per day to 4 hrs per day.

    My generator is a 5.5kva diesel with a 12volt DC output that I could use for recharging the battery.

    Any sugestion on design setup appreciated
    Thank....Mark
    Sorry Mark but this is going to be a long post.

    The first thing you need to do is determine the total load that the inverter will need to supply. You have mentioned that you have a fridge & a freezer. The inverter must be sized to accommodate the starting current for each of these devices, plus the extra load of the T.V. & phone/fax etc. You may need to look on the compressor of both the fridge & freezer to get the info you need. If possible, try to find the L.R.A. (locked rotor current) of each compressor. This info may be in any operation manuals (if you still have them). If the L.R.A. is not indicated, use the F.L.A. (full load current) & multiply it by 4.

    Don't forget to include any lighting.

    Inverters typically have 2 ratings (like a generator)...a "continuous" rating & a "surge" rating. You may also notice that generators & inverters are sized in VA (Volt Amps) & usually not kilowatts...there is a good reason for this. Let's do a sample calculation;

    Formulas - W (power) = E (volts) x I (current) x P.F. (Power Factor). We need to find the current (I) so;

    I = W divided by (E x P.F.)

    Power Factor is used if you can't find the FLA or LRA of the fridge/freezer & only the wattage is known. Note - I have assumed that the fridge & the freezer have a P.F. of 0.8.

    Continuous Load calculation.
    4 x 60 Watt incandescent lights - - - - - - - - - - 60 divided by 240 = 0.25 Amps.
    1 x 300 Watt fridge - - - - - - - - - - - - - - - - - - - 300 divided by (240 x 0.8) = 1.5 Amps.
    1 x 300 Watt freezer - - - - - - - - - - - - - - - - - - 300 divided by (240 x 0.8) = 1.5 Amps.
    1 x 300 Watt TV - - - - - - - - - - - - - - - - - - - - - 300 divided by 240 = 1.25 Amps.
    1 x 100W "printer" fax/phone - - - - - - - - - - - - 100 divided by 240 = 0.4 Amps.

    Total continuous Watts as listed - 1060 Watts.
    Total continuous current as calculated - approx 5 Amps.

    Surge Load calculation.
    The only items that will have a surge current are the fridge, freezer & TV.

    1 x 300 Watt fridge - - - - - - - - - 1.5 Amps x 4 = 6 Amps.
    Fridge surge - - - - - - - - - - - - - - 6 Amps - 1.5 Amps = 4.5 Amps.

    1 x 300 Watt freezer - - - - - - - - 1.5 Amps x 4 = 6 Amps.
    Freezer surge - - - - - - - - - - - - - 6 Amps - 1.5 Amps = 4.5 Amps.

    1 x 300 Watt TV - - - - - - - - - - - TV surge is approx 0.5 Amps.

    Total surge current - 9.5 Amps.

    Your inverter will need to supply a continuous current of 5 Amps.
    Your inverter will need to supply a total surge current of 5 Amps + 9.5 Amps = 15 Amps.
    Converting these figures to "Power" (VA);
    15 Amps x 240v = 3 600 VA (Watts) surge.
    5 Amps x 240v = 1 200 VA (Watts) continuous.


    Since you will not find an inverter with these exact "continuous" & "surge" ratings, you must buy an inverter that can accommodate the surge rating. ie you will need an inverter with a surge rating of no less than 3 600 VA for the above situation.

    Other things to consider.
    1] Many inverters use PWM (Pulse Width Modulation) to produce the alternating waveform. Electronic equipment does not like this kind of waveform & can be damaged by it. If possible, buy a "True Sine Wave" inverter...more expensive but better.

    2] Generators naturally produce a "True Sine Wave", so you may consider keeping your genset & not buying an inverter.

    3] The use of Deep Cycle batteries is recommended. No matter what type of batteries you choose to use, it his highly recommended that they not be discharged any more than about 50% of their total charge (DOD). If the inverter cannot give you info on instantaneous battery charge, you can use a Hydrometer to get this info. This maximum Depth Of Discharge (DOD) will ensure a much longer life expectancy from the batteries than if they were more deeply discharged.

    4] You will need to calculate the amount of batteries needed against the size of the inverter (3 600 VA or 15 Amps) & the time you wish to run your equipment.

    5] It is advisable that the inverter have a "built in" battery charger & other monitoring facilities.

    5] It is advisable that the inverter have a "built in" battery equalisation process. (used only for battery banks).

    6] The inverter supply MUST be totally separate from the normal house supply (this goes for generators too). This can be achieved by having separate power & lighting circuits or with the use of a "transfer switch". If separate circuits are used, it is recommended that the equipment NOT be earthed, unless required by the inverter system. Your electrician should know about this.

    Other notes.
    My calculations are based on the fridge & freezer starting currents being 4 times their run currents. In some cases, the starting currents of these devices are smaller than 4 times their run currents.
    The sizes of the fridge/freezer used in the calculation are "typical" but you must still find the sizes of YOUR fridge/freezer.
    A licensed electrician should make the necessary connections &/or changes to any circuitry/wiring.
    You could use your generator to charge the batteries but I wouldn't recommend it as the inverters battery charger should have all the necessary battery monitoring devices & will be of a specific type to suit the batteries (usually "constant current" charging on the better inverters).
    I advise that you do some reading on battery maintenance if you are going to use an inverter.

  3. #3
    wood welder
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    Another option may be to run a dual conversion UPS (uninterruptible power system), the type that is used in computer rooms. The dual conversion inverter output is sinewave so it's friendly enough. They are designed to power high end servers.

    a 2 kva Liebert costs about $1500. You can also add as many battery banks as you want
    to increase your generator downtime, but, the longer you run it down, the longer the battery banks take to charge.

    There are a few sizes available.http://www.emersonnetworkpower-partn...u/default.aspx

  4. #4
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    Hi Mark,

    Having been in the reverse situation to yourself in the last couple of years I can understand your quandary.

    I agree with most that has been said in the previous posts but offer the following observations.

    1. Dont even think about one of those small ($300) invertors running on 12 volts as they are not made for continuous use.
    2. The gen set at 5.5 kva is probably 2 to 3 times bigger than it needs to be for most of the loads you are running hence you are no doubt burning up a lot of fuel unneccessarily.
    3. If you size a battery bank correctly you could conceiveably get away with running your gen set for around 4 hours per day and the batteries would provide the power for the rest of the day.
    4. Surprisingly, a fridge / freezer will draw quite low current when running my inverter shows around 100 watts run current when it is on. Don't forget the fridge will only be on when the thermostat goes high and this can cycle maybe 12 times per hour for around 2 minutes depending on the usual conditions.
    5. If you run a workshop as well as the system you have described then you will need to look more carefully at the invertor and battery bank size or run the gen set when you are operating your power equipment.

    IMHO, suggest you set up a decent invertor of around 3kva capacity and have it hooked up to the gen set and a decent battery bank the higher voltage for this the better. The invertor will also control the battery charging function very effectively and give you overall a much more energy efficient system with your gen set. Once you set up the inverter and battery bank then an addition of either solar or wind generation becomes a viable option.

    My 2 bobs worth
    Peter

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    I just realised that I made mistakes in my calculations. Corrections as follows;

    Quote Originally Posted by elkangorito View Post
    Sorry Mark but this is going to be a long post.
    Continuous Load calculation.
    4 x 60 Watt incandescent lights - - - - - - - - - - 240 divided by 240 = 1 Amp.
    1 x 300 Watt fridge - - - - - - - - - - - - - - - - - - - 300 divided by (240 x 0.8) = 1.5 Amps.
    1 x 300 Watt freezer - - - - - - - - - - - - - - - - - - 300 divided by (240 x 0.8) = 1.5 Amps.
    1 x 300 Watt TV - - - - - - - - - - - - - - - - - - - - - 300 divided by 240 = 1.25 Amps.
    1 x 100W "printer" fax/phone - - - - - - - - - - - - 100 divided by 240 = 0.4 Amps.

    Total continuous Watts as listed - 1240 Watts.
    Total continuous current as calculated - approx 6 Amps.

    Surge Load calculation.
    The only items that will have a surge current are the fridge, freezer & TV.

    1 x 300 Watt fridge - - - - - - - - - 1.5 Amps x 4 = 6 Amps.
    Fridge surge - - - - - - - - - - - - - - 6 Amps - 1.5 Amps = 4.5 Amps.

    1 x 300 Watt freezer - - - - - - - - 1.5 Amps x 4 = 6 Amps.
    Freezer surge - - - - - - - - - - - - - 6 Amps - 1.5 Amps = 4.5 Amps.

    1 x 300 Watt TV - - - - - - - - - - - TV surge is approx 0.5 Amps.

    Total surge current - 9.5 Amps.

    Your inverter will need to supply a continuous current of 6 Amps.
    Your inverter will need to supply a total surge current of 6 Amps + 9.5 Amps = 16 Amps.
    Converting these figures to "Power" (VA);
    16 Amps x 240v = 3 840 VA (Watts) surge.
    6 Amps x 240v = 1 440 VA (Watts) continuous.


    Since you will not find an inverter with these exact "continuous" & "surge" ratings, you must buy an inverter that can accommodate the surge rating. ie you will need an inverter with a surge rating of no less than 3 840 VA for the above situation.

  6. #6
    Apprentice (new member) West OZ Mark's Avatar
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    Thanks for that info people

    Elkangorito thanks for that. Gives me some where to start from. Definately sounds plausible.

    Again Thanks....Mark
    When I die, I want to go peacefully like my Grandfather did, in his sleep---not screeming, like the passengers in his car.

  7. #7
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    Another option you may like to consider is to buy a small 1 to 2 KW generator similar to the Honda i series

    They will run quite slow on what they call eco mode, you would then have a dual system which you run according to the load.

    This may not be your best system as they are petrol units not diesel

    doug

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    Mark,

    Although some people may make things seem simple, what you wish to undertake is NOT simple. You MUST know your load & therefore it MUST be calculated properly. If you over-calculate, no problem but if you under-calculate, you risk the failure of expensive equipment. You can't "guess" the electrical load of your fridge or freezer. The load must be exactly known. In this case, over estimation is better than under estimation.

    It is not recommended that you use an "online" (dual conversion) UPS. The reason for this is that these devices usually filter the the incoming supply. In other words, you will be paying for this filtering, should you require it. The use of a generator circumvents the requirement of any filtering.
    UPS units are more expensive than inverter units (per same size) , complete with batteries. Also, UPS units, like inverter units, have a limit to the number of batteries that they can charge. For example, you may be able to buy an inverter that will sustain a battery "back up" of 8 hours but if a larger "back up" time is required, a more expensive inverter will be required. This is because the "built in" battery charger has a limit as to how many batteries it can charge.
    UPS units & inverters serve two totally different purposes. You do not need or want a UPS unit to solve your problem. UPS units are primarily used for "critical" equipment. "Critical" usually means "data", so unless you wish you absolutely protect your data on your home computer, a UPS unit is NOT recommended. On the other hand, a generator can be used to supply such "critical" equipment with "clean" power but a transfer switch must be used with the appropriate logic control.

    Please ask questions. What you have asked may have seemed simple but really, it is not. In your case, do not believe "simplistic" answers unless backed up by fact/calculation.

  9. #9
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    Even with the extra filtering of power and extra features on a UPS, they are still a LOT cheaper than the inverter banks sold here in Australia.

    The $7k inverter bank Mark was quoted may be very good but that amount will buy a 6kva UPS unit with external battery bank or you could get a 4.5kva unit for less. Also there is no limit to the number of battery modules that can be added, but the charge time will increase.

    As Elkangorito suggests, horses for courses but UPS units are actually a lot cheaper than people think.

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    Quote Originally Posted by Timmo View Post
    Even with the extra filtering of power and extra features on a UPS, they are still a LOT cheaper than the inverter banks sold here in Australia.

    The $7k inverter bank Mark was quoted may be very good but that amount will buy a 6kva UPS unit with external battery bank or you could get a 4.5kva unit for less. Also there is no limit to the number of battery modules that can be added, but the charge time will increase.

    As Elkangorito suggests, horses for courses but UPS units are actually a lot cheaper than people think.

    Thanks for that correction Timmo . Obviously market forces & competition have made small UPS's cheaper. The demand for inverters in Australia is not that great (yet).

  11. #11
    Golden Member GraemeCook's Avatar
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    Mark

    You have received some very sound advice, but may I suggest that you also do some long term budgeting.

    Using Elkangorito's methodology, and estimating your future electrical demand - we all use much more equipment than you have listed - you should be able to estimate how many solar panels that you will require and their cost. If you later go solar then it will be added to your battery system and you will continue to need the generator for cloudy days.

    If you later connect to mains then the generator and battery system may become redundant and the resale value will be much less than the new cost. The short term costs could be high.

    Perhaps the best and cheapest option may be to bring forward the date of connection to the power grid.

    Unless you live in a very remote area with extremely high grid connection costs mains power is almost always the cheapest option.

    Cheers

    Graeme

    PS: Also there is a major inefficiency in battery systems in that when you put power into and out of a battery you lose about twenty percent. Elkangorito may be able to give a more elegant and accurate explanation.

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    Quote Originally Posted by GraemeCook View Post
    PS: Also there is a major inefficiency in battery systems in that when you put power into and out of a battery you lose about twenty percent. Elkangorito may be able to give a more elegant and accurate explanation.
    Would you believe that I've only noticed this now? Sorry for the delay.

    What Graeme said is true. The use of batteries introduces losses. These losses equate to the time it takes for the electrical charging process to complete (the Law of Conservation of Energy says that "you don't get something for nothing."). Depending upon the quality & type of batteries used, this "loss" can be as high as 30% but generally, 20% is the normal expected energy loss.

    Thanks Graeme .

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