Multiple inverters and solar panel circuits?

gp_wagp_wa Registered Users Posts: 15
As solar prices fall, I am considering more and more seriously installing a grid-tie solar system on my home. The home is all electric, and we consume about 12000 KWH's per year, so I need a rather large system to come close to breaking even; I'm thinking somewhere between 8K and 10K watts.

I want to use an inverter made in Washington state to take (better) advantage of this state's solar incentive program. Outback is the only inverter made here that I am aware of, and they don't make one rated for anywhere near the wattage I'm looking at.

Can I simply install multiple inverters in parallel? Does one need to be configured as "master" for when grid power fails? I require schooling, thanks :p

Comments

  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Multiple inverters and solar panel circuits?

    Not to answer your question, but to pose a suggestion. You might consider, before you invest in such a large system, looking for ways to reduce your electrical consumption and increase your total energy efficiency. For example, if you are "all electric" consider a demand nat gas or propane demand water heater. This will not only be substantially amore efficient use of energy, but it will reduce your Pv cost considerably. Same with space heat, cooking, dryer etc.

    The old rule of thumb about conservation still applies. For every dollar invested in conservation, it will save about $10 in Pv cost. In most houses there are a million little ways to save a little bit of energy, all adding up to a lot of energy saved. Fridges, freezers, heat pumps, up graded windows and doors, night shade insulation for windows, zone heat etc, etc etc.

    While I don't wish to discourage you from pursuing this, please consider EVERY conservation option before you invest in Pv. As I have often suggested, it makes no sense (IMHO) to put a bunch of money in Pv only to light a house full of conventional light bulbs for a simple example.

    Good luck,

    PS.

    To answer your question, and I am not well versed in Grid tie, but I don't think there is no reason you can't stack inverters in parallel. One issue you might run into is your basic panel capacity may be too small if you are going to potentially add 10kw of Pv input. There is a formula for how to size the panel relative to the Pv/utility power but I don't know it.

    Tony
  • gp_wagp_wa Registered Users Posts: 15
    Re: Multiple inverters and solar panel circuits?

    I don't plan to install natural gas. From the "green" or "carbon footprint" perspective, it's counter productive.

    It doesn't matter how large your PV system is, if you get paid by the KWH, the pay off time is roughly the same.

    We've already done some things to conserve. We heat with a high efficiency heat pump and most of our lighting is CFL. We'll be moving toward LED as prices become more reasonable.

    I was planning to install a DHW solar system, but with PV prices in the process of face planting, PV seems to make more sense now (or in the very near future). That may not be true in other states, but Washington offers almost no incentives for solar hot water; in fact, our sales tax incentive was just removed (but not for PV).

    Maybe I should tell the wimmins to quit taking so many baths :D
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Multiple inverters and solar panel circuits?

    My point is that for heating, nat. gas if available is fundamentally more efficient. Unless you either generate 100% of your electricity from your Pv, or alternatively if your grid power is 100% either hydro or RE, the curve rapidly slants toward using gas for heat generating tasks if it is available.

    I agree that gas is less "green" or "more carbon intensive" than Pv, but in most cases few people have enough Pv capacity to offset 100% if their use, especially if they are "all electric"

    From what I understand (and that is limited) the net efficiency of resistance electric heat from generator to toaster (for example) is something on the order of ~30%, taking into account generating and transmission loses. With gas, that approaches ~60%. So net net you have to use twice as much energy to heat with electricity

    Even with solar hot water, it is more than 3 times as efficient to convert sun to heat (hot water) to end use, than it is to convert sunlight to electricity, and then convert it back to heat.

    As I said before, I am not trying to discourage you from making your choices. Everyone has their own rationale for making the choices they do. For some it is money, for some it is "being green" for others it is to make a statement. All valid reasons to be sure. However, all too often I see people jumping into Pv solar because they think it sexy, when their money might be better spent on other technologies for the same net financial or green benefit. It sounds like you have done your homework.

    Tony

    Ps I have lots of friends and family in W. WA and B.C and I have done a lot of work there over the years so I am familiar with your potential climate pitfalls.
  • gp_wagp_wa Registered Users Posts: 15
    Re: Multiple inverters and solar panel circuits?
    icarus wrote: »
    Even with solar hot water, it is more than 3 times as efficient to convert sun to heat (hot water) to end use, than it is to convert sunlight to electricity, and then convert it back to heat.
    Technically, solar hot water is (of course) far more efficient than PV.

    But I don't care so much about that. Technically, sunlight is free, so efficiency is of little concern, provided I have the sun-facing square footage to produce the energy I need - which I do.

    In terms of economic efficiency, solar hot water is starting to look like a losing proposition in WA state. Not to mention the additional complexity 1) of solar hot water itself vs PV, and 2) of two distinct solar systems.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Multiple inverters and solar panel circuits?

    Not to pick nits, but, sunlight may be free the ability to capture and use that sunlight effectively is not always free.

    You might be curios to look at the following link. http://view2.fatspaniel.net/BEF/els/EndUserView.html

    It is a link to a 2kw Pv installation in Bellingham, Wa. You can look, in real time and see how much it is generating at any time, and how much it has generated in the past day/week/month/year etc. You may find it interesting to note that the system has generated ~2100 kwh in the last 12 months. 2100kwh at the average rate of $.15/kwh=$315 worth of power, for a system that probably cost ~$12-15,000 after rebates and tax credits, somewhat short of a 2.5% return. ($.15 into a $12k cost)

    To cover your current usage assuming you are in W. Wa your system will need to be about 6 times as big,, like you suggest 10-12 kw.

    Good luck,

    T
  • gp_wagp_wa Registered Users Posts: 15
    Re: Multiple inverters and solar panel circuits?

    Here's some simple math to chew on...

    If I were to produce 12000 KWH per year from solar, I would offset $996 worth of grid electricity (as of now, at $0.083/KWH). The state of Washington would pay me $0.18/KWH extra, for a grand total of $2796/year.

    Now let's say my system costs me 30K. After the federal tax credit, that's 21K. That's a 7.5 year pay back time (ignoring both electricity cost increases, and interest on the money). I think 30K might be realistic now, or in the near future for this size of a system. A year ago it would have been closer to 60K, and not on my radar.

    On the other hand, I use about 3650 KWH per year for hot water (I have a meter on the water heater, that's a real number). If I could offset that perfectly (which I can't, because I can't sell excess heat to the grid) with a DHW system, that's $303/year. If I could somehow get a solar hot water system going for $4000 (tough to do with good quality production equipment), the pay off would be over 13 years. Because of heat waste, the real pay off time would be much longer.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Multiple inverters and solar panel circuits?

    As said, it sounds like you are doing your homework. Sorry to hijack the thread.

    T
  • gp_wagp_wa Registered Users Posts: 15
    Re: Multiple inverters and solar panel circuits?
    icarus wrote: »
    As said, it sounds like you are doing your homework. Sorry to hijack the thread.

    T
    Pshaw, it's good discussion either way :D




    Somebody around has got to know about the inverters, though :confused:
  • KamalaKamala Solar Expert Posts: 452 ✭✭
    Re: Multiple inverters and solar panel circuits?

    I recall that I read somewhere :confused: that the process, acquisition of raw materials and purification thereof used to manufacture solar panels has a significant "carbon footprint" and that it may not be all that green.

    Just a thought.

    I'm not GT, but I'm pretty sure that the "high end" inverters (Xantrex/Outback) are "stackable." Even to the point of 240/120 VAC. Wait for others to weigh in tho'.

    K
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    Re: Multiple inverters and solar panel circuits?
    Kamala wrote: »
    I recall that I read somewhere :confused: that the process, acquisition of raw materials and purification thereof used to manufacture solar panels has a significant "carbon footprint" and that it may not be all that green.

    I think I've seen a couple studies that claim about a 2-4 year payback for the carbon foot print of the refining and mfg of the cells. Not bad for a product with a +20year life, unlike a CAR, which is scrapped in about 10 years, and has nothing green about it.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
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  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Multiple inverters and solar panel circuits?

    Outback produce off-grid inverters that happen to support selling excess to the grid. So if you go the outback route your system will look like: 8kW PV -> Charge controllers -> Battery bank -> GVFX inverters -> the grid.
    If you bought a proper grid tie inverter, the system would be: PV -> inverter -> the grid.

    In addition to the fact that you'd need to buy batteries and charge controllers there are also further restrictions on the design of the outback system:
    1. The input voltage limit of the outback charge controllers is 140V, so you'd have to wire your PV array to be less than that (this means thicker cabling, and potentially lost efficiency). In comparison, grid tie inverters typically support upto 500V for the PV panels.
    2. The charge controllers are limited to 80A, so if you went for a 48V battery bank, the biggest PV array you could support would be 3.8kW, so you'd need 2 charge controllers for 7.6kW. This isn't such a big deal, since you'll likely need 2 grid tie inverters anyway.
    3. The charge controller is around 98% efficient, the inverter around 95%, so your losses would be about 7%. A grid tie inverter would generally have less losses (only about 4-5%), so you'll be throwing 3% of your production away.
    4. You'll need a whole lot of extra DC breakers and cabling in a battery based system

    The advantage of the outback system is that if the grid goes down, you could power your home from the batteries.

    So, you'll have to do the sums comparing a pure GT system with an outback based battery system and include the 2-3% loss of efficiency over a 25year period to decide which system makes more economic sense.
  • moorsbmoorsb Solar Expert Posts: 38
    Re: Multiple inverters and solar panel circuits?

    As long as each invertor feeds seperate circuits, I do not think it will be a problem. The 60hz waves would have to be in sync to put the togather.
  • solar4mesolar4me Registered Users Posts: 9
    Re: Multiple inverters and solar panel circuits?

    I know someone that just had a 10.8 Kw grid tie system installed, and it uses two SMA Sunny Boy 5000 inverters. Each inverter is connected to the panel box via a 30A 240V breaker, so they are in parallel. From what I have read, this is standard practice when exceeding the capacity of a single grid tie inverter.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Multiple inverters and solar panel circuits?

    If you read the documentation from Fronius and SMA and Outback, they all speak about stacking inverters. As I suggested early on, I would guess the critical issue is likely to be breaker panel/service capacity.

    Tony
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Multiple inverters and solar panel circuits?

    The number of inverters might also depend on your electricity supplier and regional laws.

    In Germany for example, when you feed into the 3 phase distribution the phases can't be unbalanced by more than 5kW. So if you had a 5kW array you could feed into any of the phases. If you had a 10kW array then you could use 2 separate 5kW inverters connected to 2 different phases, or 4kW inverters on each of the phases.

    If you had a 20kW array you can't use 2 x 10kW inverters, because that would mean that the third phase is more than 5kW out of balance, so you could put 10kW on one phase and 5kW on each of the others.
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