How bad is it to draw more power than the inverter is rated for?

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BajaPartTimer
BajaPartTimer Registered Users Posts: 5 ✭✭
in Solar Beginners Corner #1
I've inherited an off-grid solar installation with a Xantrex SW4048 inverter, which I believe is rated for 4,000 watts.

I have friends stay in the house and I try to explain to everyone the limitations of solar, but sometimes people do stupid things.
 
What happens if the draw goes above 4,000 watts? Will it damage something or does the inverter just itself off?

Does it wear out the inverter to run it near max load, say 3,000 watts for extended periods of time?
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  • Raj174
    Raj174 Solar Expert Posts: 795 ✭✭✭✭
    #2
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    Hello BajaPartTimer,

    Welcome to the forum.

    Larger inverter chargers are made to run at their rated wattage. So 3000 watt loads on your inverter for long periods of time are no problem as long as the ambient temperature isn't too high. Most have their rating spec'ed at 25 degrees C or about 77 degrees F. The specification for the inverter also states how much the inverter can be overloaded and for how long. For example, usually a 4000 watt inverter can handle a load of 4500 watts for about 30 minutes before shutting down. If you have the manual for the SW4048, look for the specifications page. Overload and surge spec's should be there.
     
     
      
    4480W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH nominal LiFePO4, Kohler Pro 5.2E genset.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Your inverter can probably manage upwards of 8,000 Watts for a few seconds without issue (many times, it is the limitations of the battery bank and DC wiring that will cause the inverter to shut down during heavy loads).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    edited August 2016 #4
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    Well....

    I had designed my new system for a larger inverter, and had the 1800 watt spare from my cabin. As it turned out I left my job of 11 years a couple days before living off my new system and had ugly feelings about the old job, so didn't invest the planned $15-1800. I regularly 'pop off' my inverter. It is protected for "over Watt" Or some such and just shuts down.

    I'll do this regularly when I'm running the hot water heater (900watts) and have the A/C and Microwave or toaster on. I'd guess I'm down to a once every couple months now, but it was pretty regular for a while. I haven't seen any long term issues. I just go out and shut it off and back on and it's fine. If it's heated up, running close to max for a while, I'll let it cool off, It will say "over heat" or something if that was the problem, but it appears to have some issue starting back up, or perhaps the abrupt stopping of the fan allows a sensor to heat up, which prevents restarting until it cools down.

    I really do like the Prosines and am a bit amazed this one has put up with me for nearly 10 years now. I guess about 3 years at the new system, I've just gotten use to shutting things off before running other. It will be some summer when I have to shut off the A/C to run the water heater when I'll break down and replace it, but maybe I'll hook up the 1100 watt exeltech (hooked up for outside use so I don't worry about things shutting down) or the cabin's other 1800 watt inverter and run them together until I replace the battery and change system voltage to 48 volt... They do have a low peak, about 50% above the 1800 watts (2900 watts I think)
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BajaPartTimer
    BajaPartTimer Registered Users Posts: 5 ✭✭
    #5
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    BB. said:
    Your inverter can probably manage upwards of 8,000 Watts for a few seconds without issue (many times, it is the limitations of the battery bank and DC wiring that will cause the inverter to shut down during heavy loads).

    -Bill
    Thanks for the explaining -- appreciated!

    As for the limitation of the battery bank, does the inverter have the ability to sense when damage is being done by overdrawing the battery bank and shutting down the system? Again, thinking of the non-thinking house guest who could run down the batteries to the point of damaging them.
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  • petertearai
    petertearai Solar Expert Posts: 471 ✭✭✭✭
    #6
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    are you there when you have guests? . If so you have some control . Get a state of charge monitor , Make up a easy to understand fact sheet about living with solar. Generally with a good inverter you wont have inverter problems . Sulphated batterys is more likly a problem from not gettting a full charge weekly , or drawing down the batterys too far .
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #7
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    BajaPartTimer said:
    As for the limitation of the battery bank, does the inverter have the ability to sense when damage is being done by overdrawing the battery bank and shutting down the system? 
    All inverters have a LVD (low voltage disconnect).  As battery voltage declines, the inverter will draw more current (amps) in order to maintain a constant power output.  At a certain voltage (often 21 volts on a 24 volt system), the inverter shuts down to protect itself from high currents.  Full featured inverters have an adjustable LVD.  The default LVD on most inverters is too low to protect the battery... it is designed to protect the inverter.  

    A rested battery (no load or charge for several hours) at 50% SOC will have a voltage of about 24.0 volts.   If you set the LVD to 24.0 volts, then you will never be able to draw the batteries below 50% SOC. 

    The problem with that approach is voltage sag... when you draw current, the voltage sags, and when you stop drawing current the voltage will rebound.  Thus, if you are at 75% SOC and turn on a load, the voltage may sag below 24.0 volts and trigger the LVD, even though the battery is nowhere near 50% SOC.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,144 ✭✭✭✭
    #8
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    24v (or 12v or 48v) @ 50% SOC is often quoted on the internet, but is grossly inaccurate for some battery types and manufacturers.

    Understanding battery bank state of charge under various loads for your particular battery setup is a valuable tool. Whether you are drawing power at a "1 hour rate", a "120 hour" rate or anywhere between, you can estimate state of charge on the fly, without the need to shut down for several hours.
    I always have more questions than answers. That's the nature of life.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited August 2016 #9
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    Something checking something more reliable than voltage makes sense to me.  Given the cost of batteries, the price of this seems reasonable.

    https://www.solar-electric.com/bogart-engineering-tm-2030-a-battery-monitor.html?gclid=CKH3-ZLEtM4CFQ8vaQodi9cKKQ

    I am available for custom hardware/firmware development

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    edited August 2016 #10
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    The inverter only can sense input voltage... Roughly 11.5/22.0/44.0 volts (12/24/48 volt battery banks) is a good starting point for LVD setting.

    More or less, the maximum continuous load (maximum 2x surge is "built in") from a flooded cell lead acid deep cycle battery bank would be around:
    • 400 AH @ 12 volt battery bank = ~1,000 Watt maximum continuous AC inverter (or even "max cost effective" solar array)
    • 200 AH @ 24 volts = ~1,000 Watt max AC inverter/solar array
    • 100 AH @ 48 volts = ~1,000 Watt max AC inverter/solar array
    If you try a larger inverter on a small battery bank, the battery voltage will probably sag/collapse at some point.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited August 2016 #11
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    Heat (a common result of extended high loads) always reduces the life of electrolytic capacitors.    But the numbers depend on the design and components used.  And what the designer considers a reasonable lifetime.

    I am available for custom hardware/firmware development

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  • Marc Kurth
    Marc Kurth Solar Expert Posts: 1,144 ✭✭✭✭
    #12
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    @BB, Yes Sir. You have a good way of expressing the concept to help people learn.

    Here is a chart indicating battery voltage vs. load - for Concorde AGM's ONLY. (Fullriver DC series is close) I fully realize that most people aren't using them, but perhaps folks will share charts/curves for their own batteries.

    In essence, you can predict voltage sag with reasonable accuracy once you get a feel for your own profile. You will also readily see changes in your battery bank that can help you flag problems in their earlier stages.

    I have tested this chart (within my test equipment capacity) on large and small batteries, and it is surprisingly accurate. Note that they are correct in saying that older batteries will a bit read lower than new.

    http://centexbatteries.com/V-vs-load.jpg

    Every customer gets instructions from me about this. Too many have relied solely on their battery monitors and damaged their battery banks because of calibration errors/problems - so I spend the time to teach this basic concept as a "sanity check" to their monitoring equipment.

    Any customer of mine will repeat verbally: "My volt meter and ammeter are my friends."  :)

    Marc
    I always have more questions than answers. That's the nature of life.
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