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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthortony
    • CommentTimeJul 11th 2018
    Can we install a 6kW inverter to serve 2.1kW of panels?

    As I see it if there were 6kW of panels in winter they would only generate 2 kW a lot of the time
    • CommentAuthorbxman
    • CommentTimeJul 11th 2018 edited
    You would not want to a 6kW inverter to serve 2.1kW of panels . the efficiency of inverters is poor on a low % of their potential output so it is better to add additional panels in winter.
    Have 2 3kW inverters matched to 3kW of panels and combine both strings onto a single inverter in winter would be my advise .

    • CommentAuthortony
    • CommentTimeJul 12th 2018
    • CommentTimeJul 12th 2018
    That winter-summer switching is an idea - is it OK with your certification? Is it worth it - how big is the real effect of lo-load inefficiency?
    • CommentAuthorbarney
    • CommentTimeJul 12th 2018
    A typical inverter at low load (say 5%) of rated output is about 75% efficient

    This rises quickly so typically at about 10% load, the invertor is about 85% efficient

    At 15% load, the invertor is about 95% efficient

    Peak efficiency is around 30% loaded - say around 97% efficient

    It will tail off steadily towards full load where is should be about 90% efficient

    The invertor efficiency is = Power ac/Power dc

    Power ac = AC Power out

    Power dc = DC Power in

    dc power is also a function of panel dc voltage

    Depending on the scale of the system, that gross mismatch between Panel capacity and invertor capacity could be a significant chunk of energy lost


    • CommentTimeJul 12th 2018
    Gd info - thanks.

    What about
    Posted By: fostertomThat winter-summer switching- is it OK with your certification?
    • CommentAuthorbarney
    • CommentTimeJul 12th 2018
    No idea about the certification Tom, I don't go anywhere near MCS

    In terms of designing up a simple DC switching panel so you can put low winter output of all panels onto a smaller invertor and back again to two or more invertors for high summer output is entirely feasible - but not without challenges. Switching DC is a bit of a bugger to avoid lots of contact arcing and potentially a bit risky due to welded contacts without DC compatible equipment

    Keep in mind that at the end of the day, it gets dark - so there is a period at the start and end of each day where you are at the low efficiency point anyway.

    It should be easy to analyse the effects when initially selecting the panels and the invertor so that you end up with the best mix of panel size and grouping to invertor size contrasted from summer to winter - I bet it's rarely done however, in most cases, I think the owner just sucks it up.

    As I mentioned, you have to be really underloaded to get down to a poor efficiency point - in the vast majority of cases it's rarely less than 80% so is anyone really bothered ?

    There is probably a greater impact in fixing the orientation and angle of tilt on the average system


    stick through some inverter software see what results you get
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