What is the most efficient way to handle occasional high loads

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  • Peter_VPeter_V Posts: 224Solar Expert ✭✭✭
    Photowhit said:
    Peter_V said:

    Back in Feburary there was a 10 day period where we used more than we produced on all but 2 days.  So 1kwh a day would have added up to an additional 10kwh deficit. 
    Peter, I believe that was with a grid connected system? 

    I'm NOT trying to be a jerk, but off grid systems have some additional loses. Lead acid batteries use about 15-20% more energy to recharge than if the energy was used directly, So stored energy incurs, the charge controller losses, similar to your grid tied inverter, as well as the losses charging the battery and then the losses inverting the DC to AC current.

    Looks like your planning on adding some additional capacity, so perhaps you had this in mind, but I thought I'd bring it up directly.
    My code assumes 25% loss for PbA and 10% for LiIon, Currently I'm assuming 15% loss in the battery charger.  I currently measure production from the AC side of the micro-inverters, so their loss is already accounted for.  
    Right now I'm trying to get a better idea of the losses in the off-grid inverter.  They like to publish the 'maximum' efficiency which isn't very useful.
    A few inverters include efficiency graphs, most of them drop like a stone when the power production falls below 1000 watts.
  • EstragonEstragon Posts: 2,451Registered Users ✭✭✭✭
    Inverters (at least PSW - MSW have other issues) generally have peak efficiency at close to full load, and much lower at lower loads.

    With AC chargers, power factor correction can be a significant issue. Some can be as low as ,55 or so, others more like .9 +. Low PF = heat in genset instead of amps into battery.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • Dave AngeliniDave Angelini Posts: 3,766Solar Expert ✭✭✭✭
    Peter_V said:


    And yeah the trackers are nice for spreading out the power production, yesterday I had almost 9.5 hours of production.
    However, with panels being so cheap these days, if I was building an array today I would got with two arrays facing east and west.  For the same money I'd get the same extended power output but much more power output on over-cast days.
    What kind of tracker? I am north of you and yesterday I was getting useable power at 6:30am and ending at 7:30pm.
    It sounds like you are in the suburbs getting shading?

    For lithium I use 95% at 1KW load on an XW+ and 96% charging from their high voltage mppt. Lithium batteries are typically 99%.
    For most anyone with a large system the loss at small loads is in the noise of the system and is pretty much ignored offgrid because you can't sell the excess anyway. Your winter loads are ridiculous BTW for Offgrid. Your winter weather is not that good!

    I have a client who has a 4 bed 3 bath house on 100 acres. They built it 10 years ago and the listed price of $400K is what the paid for the land 10 years ago. They will lose $300K. 

     If one needs to get their money back someday they need to be careful with this kind of thing. If not, why not it is good for the economy :)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • Peter_VPeter_V Posts: 224Solar Expert ✭✭✭
    edited April 21 #35
    Peter_V said:


    And yeah the trackers are nice for spreading out the power production, yesterday I had almost 9.5 hours of production.
    However, with panels being so cheap these days, if I was building an array today I would got with two arrays facing east and west.  For the same money I'd get the same extended power output but much more power output on over-cast days.
    What kind of tracker? I am north of you and yesterday I was getting useable power at 6:30am and ending at 7:30pm.
    It sounds like you are in the suburbs getting shading?

    For lithium I use 95% at 1KW load on an XW+ and 96% charging from their high voltage mppt. Lithium batteries are typically 99%.
    For most anyone with a large system the loss at small loads is in the noise of the system and is pretty much ignored offgrid because you can't sell the excess anyway. Your winter loads are ridiculous BTW for Offgrid. Your winter weather is not that good!

    I have a client who has a 4 bed 3 bath house on 100 acres. They built it 10 years ago and the listed price of $400K is what the paid for the land 10 years ago. They will lose $300K. 

     If one needs to get their money back someday they need to be careful with this kind of thing. If not, why not it is good for the economy :)
    Zomeworks trackers.  They have no electronics, instead they work based on heat and gravity, so they take an hour or so to "wake up" and flip over in the morning.  I also have a little bit of shading in the evening(after 6pm) from my roof.
    The AC rating on my inverters is 2,850 watts, yesterday the array produced 26.4kwh (AC)., the day before it was overcast for a bit in the morning and I only produced ~25kwh.

    Since I'm thinking of using used EV batteries, I'm assuming higher losses.  Thee daily losses might be small, but cumulative losses over multiple days of less than optimum weather is a concern, at least to me.  The difference in prices between different brands is not significant for the most part so I see no reason not to try to find the most efficient inverter.  Especially since I'm only planning to use a battery bank large enough to handle the loads that I can't time shift to when the sun is shining.
    I.e. I won't charge the Volt from the house bank, nor will I run the dryer or oven.  I only turn on my server lab (used for work) during the day anyway.

    My winter loads are what they are, why do you consider them ridiculous?

    My current array has already paid for itself and then some, so I'm not concerned with getting my money back since I already have.  The only additional costs will be the new inverter and batteries,
    With the new rate structure they are implementing my monthly electric bill (with my current array) will be around $65-$75, roughly $840 a year.
    The monthly savings from not having an electric bill should eventually pay for the new inverter and the costs for replacing the batteries sometime down the road.

    I have 2 years before the new rates are fully implemented so right now I'm just trying to find the best way to do this.  At that point I'll have over 3 years worth of data collected on use and production from my current setup which should be enough to make reasonable estimates of future productions, etc.
  • Peter_VPeter_V Posts: 224Solar Expert ✭✭✭
    Estragon said:
    Inverters (at least PSW - MSW have other issues) generally have peak efficiency at close to full load, and much lower at lower loads.

    With AC chargers, power factor correction can be a significant issue. Some can be as low as ,55 or so, others more like .9 +. Low PF = heat in genset instead of amps into battery.
    I'm thinking of building a DC genset.  Three phase permanent magnet generator feeding a 3 phase rectifier with an LC filter.  

    If it's properly designed the power factor won't be an issue and I can control the charging current by varying the engine speed.
  • EstragonEstragon Posts: 2,451Registered Users ✭✭✭✭
    The caveat on AC chargers was for commercially available products. As most residential and small commercial grid connected don't pay for PF losses, some chargers designed for that market don't bother with PF correction. Off-grid we're the power utility, so we care about PF corrected chargers.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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