Generator sizing for new system

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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #32
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    Re: Generator sizing for new system
    vtmaps wrote: »
    That's 23 amps at 240 volts AC. You would need a transformer to step it down to the 120 volts (at up to 46 amps) that your inverter needs.

    I think your inverter can only take up to 30 amps at 120 volts. If you go to two inverters and stack them in series, they could handle the 240 volt output. Stacking them in series is not as efficient as parallel stacking. The transformer may be a better solution if you want to do parallel stacking.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #33
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    Re: Generator sizing for new system
    rbell wrote: »
    Ok so this one is not a good choice then or it is and I just need the transformer? I the inverter series the right choice? That explains why my current Coleman 6500 which is now broke always set off the breaker.

    Inverter generators have two great features
    1) excellent (the best) waveform, however some conventional generators are plenty good enough
    2) they are more efficient at low power than conventional generators, but at full power they are less efficient than conventional.

    If you can manage to run your generator near its maximum power most of the time, you will probably be better off with a conventional generator.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #34
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    Re: Generator sizing for new system

    Ok I am really confused now. I called to cancel the order and he said the honda was too small for what I need if I need 40 amps at 120 volts. And the cost will be a lot higher by the time I buy the honda and the transformer and end up with less than half the running kw.
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #35
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    Re: Generator sizing for new system

    I assume what I would need is an Outback Power Systems PSX-240
    6 kVA 120/240 VAC 60 Hz Transformer. To go with the generator? Thanks again for all your help. I am sending everything to my home in Baja so I need to be this right.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #36
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    Re: Generator sizing for new system

    First thing you need to do is decide if you need the higher Watt inverter capacity (i.e. two inverters).

    Now, if you have two inverters stacked for 240 VAC output you can run 240 VAC input from the gen and use both chargers on the same battery bank for maximum charging capacity. If you need that much current.

    If you go with the 48 Volt single or double inverters the AC output is nearly the same as the 24 Volt versions (3 kW for a 3048 vs. 3.5 kW for a 3524) but the DC current to supply it is cut roughly in half. The Amp hour capacity of the battery bank goes down (for the same stored Wattage) and so does the needed charging current.

    The PSX-240 can be used with any combination or Voltage inverters, and how it is used makes a difference in what you get from it. For example 240 VAC output from a single 120 VAC inverter or leg balancing on 240 VAC from two stacked inverters or leg balancing on a 240 VAC generator to a single 120 VAC inverter. There are about a dozen different wiring diagrams for this stuff, and it all depends on what you need on input and output in terms of both Voltage and current.

    I'm not surprised you're confused because it is really, really complicated.

    Outback has a whole lot of wiring diagrams available on their website showing the various options: http://www.outbackpower.com/index.php/wiring-diagrams

    That may or may not help. But again the first thing you must determine is whether or not you need to go up from the 3.5 kW inverter you have now. The second thing is whether or not you should switch to 48 Volts to reduce the current on the DC side.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #37
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    Re: Generator sizing for new system

    I guess you are looking at gasoline as your "preferred" choice of fuel?

    That is not an idle question... Fuel choice is a very important choice and can change over time (some people are looking at abadoning propane because of high delivery and tank rental costs, plus propane costs can be volatile).

    Fuel choices are also affected by storage and operational temperature considerations. Propane and Diesel can be a pain in sub zero (F) weather (so I hear). Diesel and Gasoline both have storage issues. Gasoline (especially summer grades) has alcohol/other "oxygenates" that can kill smaller engines. In California, fixed engines are now being pushed to meet pollution control regulations (diesels are getting rare/expensive for off road/off grid use).

    Gensets are (usually) more efficient if loaded >50% of rated output... And few "inexpensive" generators can really supply a constant (heavy) load for hours on end (i.e., battery charging) at 80% or more of name plate.

    Chris Olson here has a lot of experience with different fuels and, because of his power usage/configuration, he can load generators up to 100% of their rated output for hours/days at a time.

    It really depends on your load profile in the end... As vtMaps said, if your needs are mostly battery charging, look at 10-13% of charging current (20 hour battery bank capacity) and laying that up against a generator running at >50% of nameplate power (the full "acceptable" battery charging current range would be 5% to ~25% of battery AH capacity--operating at the edges can have issues).

    You may even end up with two generators... A smaller unit for standard seasonal battery charging. And a second, larger unit, for running the shop/emergency backup for smaller genset.

    A Honda eu2000i or eu3000i (inverter-generator type) is going to be much less fuel efficient vs a 10 kW diesel generator (on a $$/kWH of fuel costs). However, if you run the smaller genset at 50-80% of rated output vs running the 10 kW at 10% of rated output--The smaller generator will still have much less overall fuel costs (1/2 to 1/5 the total gallons of fuel per season).

    And this is why it "gets complicated".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #38
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    Re: Generator sizing for new system

    I think you need to start from the beginning. Apparently, you need to use generator because your panels do not produce enough on cloudy days. There are two ways to look at this.

    1. You can re-charge your batteries from the generator and then wait until the bad period is over. To do that you need a generator to charge batteries and support loads at the same time. Hence the recommendation to size generator twice the size of the inverter. Part of the generator production goes to charging. Since the charging is done through the inverter, this part needs to be the size of the inverter to fully utilize invertor's capacity. While inverter is charging, generator needs to support all loads, which would normally be supported by inverter. Therefore, load support requires the capacity equal to the size of the inverter. Both parts taken together (charging + load support) require generator capacity twice the size of the inverter.
    Your inverter should be big enough to charge batteries. You need at least 170A@24V = 4kW of inverter. You can add a second Outback, but I don't know if you can use both Outbacks at the same time to charge batteries from the same generator. If not, you need a new inverter.

    2. You can use the generator to support some loads some of the time, while the rest is run by solar. There's a thread here on how to do that. This requires much smaller generator and is more efficient, because most of the generator's energy goes directly to loads. This saves at least 30% of energy, because otherwise this energy would have to be rectified, stored in the batteries, retrieved back, and then inverted. Each of these steps involves losses. Therefore, direct support is much more efficient end requires 30% less fuel. However, it is tricky to time generator's run to your loads.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #39
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    Re: Generator sizing for new system

    Another thought is to buy 2 smaller Hondas and Slave the output together. It gives you redundant back up and the output of one larger one. With 2 Inverter-generators you can do whatever want. When I first did it, I thought this will be a pain, but after using them it's easier than one large one. Put them on one fuel tank and let them go. Generator output like solar can be longer and slower when your getting 7 hrs out of a gallon of gas and still supporting all your loads and charging.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #40
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    Re: Generator sizing for new system
    rbell wrote: »
    Ok I am really confused now. I called to cancel the order and he said the honda was too small for what I need if I need 40 amps at 120 volts. And the cost will be a lot higher by the time I buy the honda and the transformer and end up with less than half the running kw.

    Like I said earlier, at 120 volts you can only get 30 amps out of one outlet on the eu6500. Also, if you have one inverter, you can't use more than 30 amps at 120 volts. The honda eu6500 can put out more than 30 amps at 120 volts, but you would have to use 2 or more of its outlets. The honda can put out its full power through one outlet at 240 volts. You can step the full power down with transformer to 120 volts, but your inverter can't handle that much power at 120 volts.

    Cariboocoot is correct: before making any decision on the generator you must decide if you are sticking with one inverter or buying another.

    If you go with 2 inverters you must decide whether to run them in series (240 volts) or parallel (120 volts). Parallel is more efficient when you are not maxing out the system... one inverter can sleep while the other is working. When you need more power, the 2nd inverter wakes up and contributes. If you are in series, both inverters are active even if you're drawing only 100 watts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #41
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    Re: Generator sizing for new system
    vtmaps wrote: »

    If you go with 2 inverters you must decide whether to run them in series (240 volts) or parallel (120 volts). Parallel is more efficient when you are not maxing out the system... one inverter can sleep while the other is working. When you need more power, the 2nd inverter wakes up and contributes. If you are in series, both inverters are active even if you're drawing only 100 watts.

    --vtMaps

    To add to that, you would use the series connection only to get 240 VAC: two 3kW inverters stacked in series gives 3kW @ 240 VAC, but stacked in parallel gives 6kW @ 120 VAC. If you need 240 VAC for something, the parallel inverters can be run through a PSX-240 to get the Voltage up (at half current). This is the most efficient for occasional 240 VAC loads. The alternate is to use the transformer on series-stacked inverters (240 VAC) to balance the 120 VAC loads between them.

    If your brain isn't whirling like a Dervish now I would be surprised. :p
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #42
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    Re: Generator sizing for new system

    Ok so since I bought all 12 volt batteries which were well over 3000$ each so I am going to stick with 24 volts because I have 4 batteries now and if I want to expand I would have to get four more for 48v and Only 2 for 24. Plus all my equipment is new and set up for 24 volts. So I will get another 24 volt inverter when I see if my loads are over 4kw. Regardless adding the second 24 volt inverter will make charging more efficient and give 6000 watts for load its in series. So based on staying with 24 and either one or two inverters what size generator is optimal?
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #43
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    Re: Generator sizing for new system
    rbell wrote: »
    Ok so since I bought all 12 volt batteries which were well over 3000$ each so I am going to stick with 24 volts because I have 4 batteries now and if I want to expand I would have to get four more for 48v and Only 2 for 24.

    You do not need to buy any more batteries. You can wire your 4 batteries for 12V (4 parallel), 24V (2 x 2) or 48V (4 in series).
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #44
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    Re: Generator sizing for new system
    rbell wrote: »
    Ok so since I bought all 12 volt batteries which were well over 3000$ each so I am going to stick with 24 volts because I have 4 batteries now and if I want to expand I would have to get four more for 48v and Only 2 for 24. Plus all my equipment is new and set up for 24 volts. So I will get another 24 volt inverter when I see if my loads are over 4kw. Regardless adding the second 24 volt inverter will make charging more efficient and give 6000 watts for load its in series. So based on staying with 24 and either one or two inverters what size generator is optimal?

    Um, you already have four 12 Volt batteries?
    Two in series = 24 Volts, four in series = 48. You would not actually need to buy any batteries. The stored power is the same whether the four are configured as two parallel strings of two in series (24 Volt) or all four in series (48 Volt).

    Not sure if I pointed to this here before or not: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    But yes, the big issue around which everything hinges at the moment is whether or not you need inverter capacity above the 3.5 kW unit you have already.

    The generator size will hinge on maximum charging capacity desired plus average loads expected during charging. Keep in mind that when power is not needed for loads (which are rarely constant) it is available for charging, and when power is needed for loads the FX inverter will reduce charging current then bring it back up as needed. They are very good at that.

    The FX3524's charger is capable of 82 Amps maximum. Two together can manage 164 Amps. To supply that with a generator requires 2.5/5.0 kW minimum respectively. If you round that up to compensate for the inevitable PF issues and losses and supply typical loads you would need at least a 4 kW gen for the one inverter and probably 7 for two of them. With 1690 Amp hours of battery @ 24 Volts you really would want to be able to manage 160 Amps of charging. This is one reason why having two of the inverters would be desirable.
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #45
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    Re: Generator sizing for new system

    You are correct I don't need more batteries now but if I find I don't have enough I will need to add more so I would need to get minimum 4 at 12 volts.
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #46
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    Re: Generator sizing for new system

    Ok thanks again. It sounds like what I need is 7-8 kw at the most so that rules out honda for me. I really prefer propane anyway as its delivered to me every week if I need it. The other important factor is it must be fairly quiet.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #47
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    Re: Generator sizing for new system

    new to this thread, so may have missed something.
    Is that 7 - 8 kw an instantaneous or continuous load, as there is a difference.
    Just for comparison, 7 KWh is ~= to 1/2 of what my full sized house uses in an entire day, so it looks like there is something amiss on the consumption side. Estimated? Based on manufacturers rating of motors?

    hth

    fwiw the more parallel battery banks you have the harder it is to get a 'balanced' charge among them.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
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    Cotek ST1500W 24V Inverter,OmniCharge 3024,
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    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #48
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    Re: Generator sizing for new system
    westbranch wrote: »
    new to this thread, so may have missed something.
    Is that 7 - 8 kw an instantaneous or continuous load, as there is a difference.
    Just for comparison, 7 KWh is ~= to 1/2 of what my full sized house uses in an entire day, so it looks like there is something amiss on the consumption side. Estimated? Based on manufacturers rating of motors?

    hth

    fwiw the more parallel battery banks you have the harder it is to get a 'balanced' charge among them.

    It's generator size based on being able to supply a full 160 Amps @ 24 Volts charging (3.8 kW) plus PF correction plus averaged loads.

    We're it me I'd risk the load demand against the charging demand a bit and get a Honda EU6500i. :D
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #49
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    Re: Generator sizing for new system

    So today I found a neighbour that will buy my FP-1 Outback inveter for a fair price. I called a supplier and I can get an FP2-10 loaded with two vfx3048 inverters and all my components for 2k off becaue its the model before the current one and does not have the two mx80'chargers built in which i dont need anyway as i have three of those. So updating to 48 volts seems logical. I know I will just have to re-wire the batteries from 24-48 volts. I was wondering about my solar panels. I have 16x100 and 16x240. Will they need to be wired differently to go from 24 volts to 48? So now going to 48 volts I assume your recommendation for a gen set of around 7kw is still good? I must say this is a great forum you have all been so helpful!
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #50
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    Re: Generator sizing for new system

    I would be happy to if I could get the honda but the ei6500 is only 5500 watts? I even thought about another users suggestion and go with two honda 3000watts which would be great as I could use one for camping when needed but outback said I can use two generators with the two inverters because of phasing issues. I am sure someone here will say that's not correct ?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #51
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    Re: Generator sizing for new system

    Array question: what are the specs and configuration now? The array Vmp needs to be >70 to work properly with a 48 Volt system. Other than that you're just trading Volts for Amps.

    The gen issue is a matter of balance: you will not need 160 Amps (or now 80 Amps but the same Watts which is what matters with the gen) all the time and certainly not through the whole charge procedure. Way that against supplying the loads and how much will actually have to be supplied during charging.

    For example I charge my 232 Amp hour 24 Volt bank at 25 Amps off the gen. That's roughly 600 Watts. Average load use is <200 Watts so I only approach 50% of the EU2000i's capacity (which is derated up here @ 3200 feet). I do not need the gen to be able to supply 25 Amps charging for 4 hours straight or to power the 850 Watt water pump for example. If I did I'd need 2X the generator capacity.

    It's amazing what you can get away with if you're willing to adapt a bit. :D
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #52
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    Re: Generator sizing for new system

    I am afraid I don't know what the specs are other than I have 16 100's that are over 6 years old and the 235's I have are new. I have to look at the invoice to see what type they are. I assume the panels will work and its just a matter of how they are wired together correct? Because if I have to change out my panels too then I really will have to think about wether its worth going up to 48 volts.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #53
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    Re: Generator sizing for new system

    Two different arrays made of two different panel types need to be on two controllers.

    The main difference in arrays between a 24 Volt system and a 48 Volt system will be the Vmp needs to double. So if the existing arrays are wired with a Vmp <70 then they will need to be reconfigured. Without complete panel specs I can't say how or if it is even possible.
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #54
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    Re: Generator sizing for new system

    Are you saying that I might now be able to use my existing panels or they just might need to be hooked up differently to go to 48 volts? Thanks
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #55
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    Re: Generator sizing for new system
    rbell wrote: »
    Are you saying that I might now be able to use my existing panels or they just might need to be hooked up differently to go to 48 volts? Thanks

    He/we are saying that we need to know the Vmp, Voc, and the Isc of both types of panels, and we need to know the configurations. The configuration is the number of strings. i.e. 16 panels could be 8 strings of 2 panels per string, 4 strings of 4 panels per string, 2 strings of 8 panels per string, or even 16 strings of 1 panel per string. If you can't find these numbers, at least tell us how many fuses/circuit breakers are in each combiner box, and the voltage (from the array) coming into each controller... from that we can make some good guesses to the panel specs.

    It would also help us to know how far the panels are from the controller and what gauge cable runs from the combiner to the controller.

    All of these things are related in the optimal design of a system.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • rbell
    rbell Solar Expert Posts: 25 ✭
    #56
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    Re: Generator sizing for new system

    Ok thanks. I will not be able to get that info until next month when I go back down to Baja. I will only be able to get the deal on the fp-2 now though as there is only one left. If there is a chance my panels would have to be replaced I don't want to continue with upgrading to 48 volts and stick with 24 and just get another inverter and stack them.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #57
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    Re: Generator sizing for new system

    If you've got an even number of panels running a 24 Volt system, chances are very good they will work on a 48 Volt system. Most likely you will have to re-wire the panel strings to double the number in series in order to up the Voltage. The wiring from the array to the controller should not be a concern because if it can handle the power at the lower Voltage/higher current of the 24 Volt system then it should also be able to handle the same amount of power at higher Voltage/lower current.

    There will probably be a need to change out the fuses/breakers as they may be rated for too high a current or too low a Voltage. It is not unmanageable though.
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