fronius sizing question

Although I have installed a couple of dozen off the grid systems I've just done one grid-tie system. One of my clients got a post in his neighborhood and we upgraded his xantrex fx to a gtfx. We put 2 115 amp/hour batteries in series on it just to make it run and it's working fantastic. A non client in my village was impressed and decided he wanted one. Since he's already on the grid I thought of Fronius. So I sent them a rough configuration and don't agree with nor understand their answer.

This guy has a small welding shop along with his house. He consumes about 1KW a day for his shop and about 4KW for his house. They are separately metered. The shop is 240 single phase - the house 120. We have a lot of sun here and I thought I'd put in a 1.5KW array maybe even 1.2. He doesn't have much money and here it's net metering. There's no point in producing more than he can use. The trick to this one is that he uses an old miller arc welder that is 220 and rated at 100 amps. Obviously he doesn't use that much and depending upon what he is welding the continuous use could be (is) much less. But one time I put a clamp-on over the wires of a friends arc welder of a similar size and age and it pulled about 80 amp and startup was 110 or something like that. It was awhile ago so could be wrong but remember the 80+. Maybe he was running 110. Anyway given the same story Fronius recommended an array of 2.2 or something like that and either their IG 2000 or 3000. When I questioned them about it they got hazy and said load and conditions and etc. Can anyone comment on this? I've always learned correctly or not that basically you look at your load and figure your inverter close to what you might pull continuously. Perhaps there's something I'm seriously missing in the basics or I don't understand what a watt rating for a grid-tie inverter means. Does it just pass everything above 2000 amps through from the grid and the inverter rating means not much? I don't get it.

Thanks.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: fronius sizing question

    Well I'm confused about this: "xantrex fx to a gtfx". "FX" and "GTFX" are Outback model designations, not Xantrex.

    As far as a GT inverter size is concerned, the loads don't really matter. The utility will supply any additional power they need; the inverter will simply lessen the demand as it is able. The array should be sized to feed the inverter, so that it can supply its maximum from PV. Without utility buy-back, this need becomes debatable. A standard GT inverter will only put out its max when the sun is shining bright. If that is not when the loads are on and there is no utility purchase of surplus then there's not much advantage to having the system.

    The best way to mitigate this situation is with a hybrid GT inverter like the Outback or Xantrex units, which charge batteries to supply power as needed. They can sell to grid if that's available, or not if it isn't. They will also draw from grid to keep the batteries up when solar is not available.

    On the whole it doesn't sound like a wise investment in this situation. If you were to go with a standard GT set-up I'd size it for the average load mid-day to negate that predictable demand.
  • dellsworthdellsworth Registered Users Posts: 20
    Re: fronius sizing question

    sorry. I meant outback for the original inverter. He has a xantrex charge controller.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: fronius sizing question

    That makes more sense - except for having a Xantrex charge controller on an Outback system. ;)

    For a standard GT system you'd expect to have an array that is slightly larger than the inverter's Wattage rating, which is its maximum output. That way even with panel derating you could produce all the inverter is capable of when the sun shines bright. Example:

    3000 Watt inverter + 77% typical efficiency derating for panels = 3896 Watts for the array.
    In actuality, all the companies have array string tools for calculating the right configurations to be sure minimum Voltages are met and maximum Voltages are not exceeded while producing enough power to make sure the inverter is not under-utilized. No sense in powering a 3kW inverter from a 1kW array and only using a 1/3 (less than) of the inverter's output capacity.

    But if you can't sell to the grid it's a waste anyway, because the power needs to be used when it is produced. Without the utility buy-back there is no storage of surplus power.

    If you want to set up something that will off-set the approximately 1kW mid-day usage, consider microinverters. Roughly 200 Watts each, one panel per, very easy to install, and more resistant to shading problems due to the individual nature of the set-up. You still need to do the whole permit & approval dance though.
  • dellsworthdellsworth Registered Users Posts: 20
    Re: fronius sizing question

    I was looking at Fronius. Not Outback. I confused the issue by talking about my only grid-tie experience which is with Outback. Sorry. Although the utility doesn't "buy" back you can "sell" to them to offset your consumption. It's net-metering. It works nicely with the Outback system. All the daytime extra goes to the grid deducting from their nightly consumption.

    So does your response hold true for Fronius. It doesn't matter what size inverter since the load comes from the grid?
  • dellsworthdellsworth Registered Users Posts: 20
    Re: fronius sizing question

    On the system I have experience with - the Outback system - the client has a 1KW array on a tracker and a 2.5KW GTFX inverter. They are currently "selling" about 1KW a day above consumption. I'm used to thinking in terms of off-grid systems where my inverter may be supplied from my batteries and array sizing just needs to keep the batteries up to snuff relatively to demand over multiple days. I guess I assumed the grid works as a big battery. My confusion has been inverter sizing. If it simply does pass thru when the demand is higher than it can supply then I can see the point of just putting in an inverter relative to the array size.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: fronius sizing question

    The standard grid-tie inverters produce whatever power they can from what's available from the panels at the moment. This is fed to the service panel. Any amount not consumed by the household loads back-feeds to the grid.

    Now we run into the semantics issue: what's the difference between the utility "buying" your surplus power and you "selling" it to them? :confused: Are they taking it and not offering anything in return? There's a whole lot of different ways utilities treat grid-tie, ranging from "not allowed" to buying it for more than they charge to resell it. I get the impression here that they will credit your bill against future utility use?

    BTW, a 2.5 kW GT inverter should have a 2.5 kW array on it for maximum utilization. The tracker will give you longer hours of usable production, but will not increase the peak power. It should also have at least 250 Amp hours of battery on it, but that's probably not an issue here as it is unlikely to have less.
  • dellsworthdellsworth Registered Users Posts: 20
    Re: fronius sizing question

    You've been very helpful and I understand much clearer how the inverter works with grid-tie and demand. You are correct that here grid-tie means your demand (from them) is offset by your sale (to them). You get 3 months of "credit" against consumption. Unfortunately here we will have no clouds until about May. I'm sure most of you would view this as fortunately and of course we do too but you lose the excess you put into the grid. So I feel my question is answered on how to configure a Fronius inverter for the welding client.

    It's interesting what you say about the array and the inverter. I always assumed (talking OFF the grid systems) that the inverter drew it's power from the batteries and you COULD hit peak power. Remember that here we have a system that was originally configured for OFF grid use and we simply converted the inverter to GT when the utility showed up. So I can see the point that now the array doesn't match the inverter size. But it works and was cheap. The client is happy and selling. The battery bank of course used to be quite large but it was more than ready to be changed out when the utility switch over occurred. I figured all we cared about is that they hold roughly enough for a nights consumption (if the grid went down) and the main thing being that the GTFX function since it needs the batteries to operate. Probably I was wrong to do it that way but having no experience it was a cheap price to pay for learning (the one thing cheaper in Mexico than in the states for solar systems is reasonably good deep cycle batteries - delivered no extra cost!). As I said I simply put in 2 115 amp/hour batteries in series (obviously it's a 24V system) and it's been working very well. Actually a long time ago I asked outback what the minimum was for it to operate and they told me 100 amp/hours.

    Again thanks for your help. Those 200watt micro inverters look interesting and I suppose they have the advantage I can haul them to Mexico in my backpack 8)
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: fronius sizing question

    You are correct: when you've got batteries the equations are much different because they can store the power produced over time to be used as needed. For example an array producing 1 kW over 4 hours results in 4 kW hours of power. Stored in batteries, you could theoretically use 2 kW for 2 hours or any combination of Watts & hours that works out to the 4 kW hour total.

    It works the same with the straight (no batteries) grid tie if the utility co-operates and will "store" any surplus power you produce. If they don't, the only thing you get is mitigating power use at the time the panels & inverter are producing. Sounds like this particular company will credit the account for surplus, but nothing more. Sometimes they do this and "reset to zero" periodically, negating any gains beyond that point. In that situation the best you can hope for is an annual bill equivalent to $0.00; you never come out better.

    When they do "buy" from you, having the array meet or slightly exceed the inverter size keeps production potential at its peak, which is best for recouping your investment.

    Another thing about the microinverters is that they can be oriented in different directions to produce "all day long", sort of like using a tracker: some point towards morning light and produce power 'til the sun angle shifts to "mid-day panels" and then to "afternoon panels". Lower peak Wattage for the same amount of panels, but producing longer through the day. This can be advantageous if the main goal is to off-set household use rather than put out maximum power at peak sun time.
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: fronius sizing question

    Thoughts on using fronuis. They output 240 vac. How are you going to feed the 120 volt meter at house where most of the electric is used. I have an ig-2000 and can get 15kwh on a real good day in pennsylvania, bieng fed with 14 Kyocera 158 watt panel. manual says recomended imput of 1500 to 2500 wp. mpp voltage range starts 150 volt to 400volt. Can your customer feed the 240 v meter and receive billing credit for both of his meters? If he could you could use the minumum amount of panels listed on fronuis string chart and get all the power needed. As An example I looked up the kyocera 185-gx-lpu panels our host has on sale for $300.00 each. Minumum of 9 panels rated 1655 watt dc to inverter. This might be even more power than needed for the amount of electric you say customer uses. solarvic
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: fronius sizing question

    :confused:
    North American services are 240 VAC split-phase. There's no trouble connecting the Fronius or any other standard grid-tie inverter to them.
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: fronius sizing question

    I know that north america is 240 split phase. He said the house was metered at 120 v. So will the 240 volt gti inverter feed into a 120 volt meter? He could feed to the 240 v meter ok but said they only use 1 kwh per day on that meter. So what I ask is if he could feed the 240 volt meter and get credit on his bill for both meters even though he would probably be feeding one meter. If he could do this the model ig-2000 inverter is plenty big enough and the price for them has dropped a lot. solarvic
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