Inverter -- one or many?

khottonkhotton Solar Expert Posts: 38
I am seeking to design an off-grid solar PV system. Because of the high cost of nearly all components needed (batteries and panels in particular) I am trying to maximize system efficiency along with minimizing my power consumption. My goal is to get by on a 1kW peak PV array in central Mexico 21-deg Lattitude semi-arid climate, 6800-ft elevation.

A 24VDC system with 1kW of PV (8 - Kryocera 130's), a single Morningstar TriStar-45 charge controller and 8 "golf-cart" AGM batteries (440Ah at 24VDC). These components were selected to be the most cost effective options for this power level.

As for inverters, I plan on using a Exeltech 1100W 24V for just the electrical outlets in the kitchen and bathroom (house is two bedroom, single bathroom). This inverter would be just enough for my biggest load (800W cooking power compact microwave oven).

I was thinking of using a smaller inverter (Exeltech 600W 24V) for the remaining electrical outlet in the house. I chose this because I only envision powering small loads (laptop computer, small tv, boombox stereo, a few CFL's in lamps) with this inverter.

Finally, a still smaller (Exeltech 250W 24V) inverter for the remaining ceiling mounted CFL's and perhaps two ceiling fans. Living with PV solar electricity off-grid will mean minimizing usage and so I don't feel I will be powering more than 75W of lights + 100W of ceiling fans at any one time.

Does this approach make sense? I am trying to size and utilize each inverter near is peak efficiency zone (25% to 75% of rating). I was thinking at night I could turn off the two larger inverters (if using the ceiling fan) or all three if not. Or another posibility is turn the large one off during the times of the day when I am not using power in the kitchen or bath.

Would this approach be more power efficienct than a single large inverter (Outback FX2524T) considering the efficiency of at times operating maybe only one CFL? Also what about the "inverter ON, no load" power consumption: 10W for 1100W inverter, 8W for 600W, 6W for 250W). Yes I can turn them off when not being used, but is this better than one large inverter that I would need to leave on most of the time?

PS: I am also planning on a SunDanzer 8 ft3 refrigerator at 24V and no freezer.

Please advise; your comments and recommendations are invited.

Kevin

Comments

  • System2System2 Posts: 6,290 admin
    Re: Inverter -- one or many?

    For several years, I've been using an old 150 watt, msw Statpower inverter, on 24/7 for running the CF lights stc. It's idle draw is only about 20 ma. The lowest I've found. For other loads, I use larger, remote switched 1000 watt and 1500 watt units. This has served me well. Like you mentioned, the larger inverters are power pigs while on standby. In fact, as I key this in, the table is lit with a 4 watt CF in a table lamp, in which I installed a cheap 75 watt msw inverter. Been using it for 3 or 4 years now with NO problems yet. This lamp has a switch in the 12 volt line supplying it, so the inverter is only ever on when light is used.
  • mike95490mike95490 Solar Expert Posts: 8,250 ✭✭✭✭✭
    Re: Inverter -- one or many?

    Look into the 24VDC CFL and cold cathode bulbs coming on the market.

    Also, would the inverters sync with each other, or are you planning on dedicating a inverter per power leg ?
    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
    gen: http://tinyurl.com/LMR-Lister ,

  • westbranchwestbranch Solar Expert Posts: 5,138 ✭✭✭✭
    Re: Inverter -- one or many?

    Hi Kevin, without knowing the size of your place it is hard to make good comments, but have you considered 'native' 24 v dc for your lights and doing without the inverter?Possibly less loss due to an idle inverter but there are line losses to consider.
    also, if you can find them 24 v LEDs.

    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • khottonkhotton Solar Expert Posts: 38
    Re: Inverter -- one or many?

    Thanks for the suggestions. I was planning to wire each inverter to a separate circuit so that there would be no sync issues. I was avoiding 24VDC lights because they seem so rare and expensive. My design philosophy is Reliability/Simplicity/Availability/Cost. By far the most common and cheapest lights to use are CFL's for 120VAC. I had considered a step-down transformer (like the Samlex 24VDC to 12VDC) but worry about its efficiency/reliability, to use with 12VDC CFL's (but even these are more expensive are less available then 120VAC CFL's).

    House is only 1100 square feet.

    Kevin
  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Inverter -- one or many?

    I would be surprised at a 1 kw inverter handling a 800 watt microwave. I have a 800 watt unit which pulls over 1500 watts when its operating unless I have a faulty one ? It draws so much power we very rarely use it ! I have a 24 volt inverter and when we use the microwave it it regualry takes 60 amps from the battery (XBM) which roughly indicates 6 amps 240 ac which is 1500 watts.
  • khottonkhotton Solar Expert Posts: 38
    Re: Inverter -- one or many?

    It sounds like you have a very inefficient microwave. I currently (on grid power) use a 1100W cooking power microwave and on full-power (measuring with a Kilowatt meter) it draws about 1550W (1100W/1550W = 0.71 efficiency). I would expect an 800W microwave to draw closer to 800/0.7 = 1143W. Exeltech 1100W inverter.

    Kevin
  • nigtomdawnigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Inverter -- one or many?

    I havent gone in to too much detail on my figures but yours seem more accurate and seem to prove my point somewhat. Running this on your figures shows about a 4 % overload of your main inverters rated value, leaving little or nothing for anything else. Only trying will give the final verdict. But I suspect you may have problems.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Inverter -- one or many?

    Kevin,

    I think you’re off on an interesting little project!

    I’d suggest you map out a couple of different scenarios to see how much difference thee might be. For example, powering everything from a single FX2524T, or most short-term/high power things from an FX2524T and small long-term loads from a small inverter, or your three-inverter approach.

    The FX2524 inverter’s tare loss of ~20 W is not insignificant. But, the “typical” efficiency spec is 92% and I found mine to be 89% efficient when powering a 367 W load. The Exeltechs aren’t this good. And, if you can turn off all units overnight in the three inverter approach, then you can also turn off the FX2524. Also, like the Exeltech inverters, the FX2524T can be on/off controlled via a hardwire remote.

    The FX2524 has some other advantages. One is that it can handle anything you’re likely to throw at it, i.e., the microwave plus anything else. It also has a built in battery charger, which might come in handy if you decide you need a backup generator.

    So, take a good look at your likely operating scenario and then see what works best for you.

    Regards,
    Jim / crewzer
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: Inverter -- one or many?

    crewzer,

    where did you find the 92% number? ... I seem to remember the FX is about 91% PEAK, not average efficiency .. but this is from memory but this should be much better than any of the cheap mod-sine inverters
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Inverter -- one or many?

    It's from this spec sheet: http://www.outbackpower.com/pdfs_spec/FX_Domestic.pdf

    HTH,
    Jim / crewzer
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Inverter -- one or many?

    I also found this report to be both interesting and confusing: http://www.consumerenergycenter.org/erprebate/inverter_tests/summaries/GridPoint%20Connect%20C36.pdf I was under the impression that the higher DC input voltage led to higher efficiency in a "low" DC to "high" AC inverter... :confused: I wonder if some of the data is "upside down"?

    Regards,
    Jim / crewzer
  • boBboB Solar Expert Posts: 958 ✭✭✭✭
    Re: Inverter -- one or many?
    crewzer wrote: »
    I also found this report to be both interesting and confusing: efficiency in a "low" DC to "high" AC inverter... :confused: I wonder if some of the data is "upside down"?
    Regards,
    Jim / crewzer

    Hi Jimzer... Since this is a CEC efficiency graph, I think what it means is that when selling back to the grid, the "system efficiency" is highest when keeping the voltage across the battery low. This stops energy from being wasted in the battery because if you apply the same voltage across that battery as its open circuit rest voltage, it won't draw any current. So, they normally suggest to apply just a ~slightly~ higher voltage to the battery than its rest voltage, to keep it alive, but lower the energy wasted.

    In that case, 60 Volts should be much worse than 51 volts.

    And, as it turns out, not floating the battery all the time appears to add some life to it too. Kind of like the medical community recently saying that eating less may let us live longer. What an analogy, huh ?

    boB
    K7IQ
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Inverter -- one or many?

    Very interesting, Gran-boB...;)

    Thanks,
    Jim / crewzer
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