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How to measure excess power

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  • How to measure excess power

    Hi All,
    Have 8 x 300w mono panels running our small house totally off grid. More than enough power for what we have been using. I am keen to work out what sort of spare capacity we have but. If I check the charge controller around midday to see how much its putting out its normally already charged the batteries and in trickle charge mode.
    what I was thinking is finding a way to dump the excess power to say a water heater and measure how much power goes into that circuit. That would tell me how much excess power we have.

    Looking at building a bigger house at some point that will no doubt need more power so would be nice to work out how much spare head room we have now.

    any one got any other ideas ?
    cheers

  • #2
    Many of us like a free web tool called PVWatts. This tool does an excellent job of estimating you how much energy you will get in a specific situation, with all of the assumptions you can give it. It takes into account your panel size, panel efficiency, direction the panels point, angle the panels are to horizontal, your location, and even your weather patterns (cloud cover, etc). If you run this tool, you can get a very good sense of the amount of power that your panels could generate in your situation.

    The biggest weakness of this tool is that it's limited to the data you give it. For example, it's hard to tell it about trees shading your panels or about dust buildup from lack of rain or cleaning. It makes some assumptions about the efficiency of your electronics. If in doubt, make some guesses or let it default.

    https://pvwatts.nrel.gov/pvwatts.php

    You may get more accurate information pumping energy into water and measuring the water over an entire year to average out seasonal differences, but PVWatts will give you a fairly good answer in 15 minutes, for free.
    7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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    • #3
      Originally posted by Helimadness View Post
      Hi All,
      ......
      what I was thinking is finding a way to dump the excess power to say a water heater and measure how much power goes into that circuit. That would tell me how much excess power we have.........
      I agree with @bob-n that PV Watts is a good starting point. It can be especially helpful as you design your new home because you can change the variables of tilt and azimuth to evaluate the cost effectiveness of design tweaks.

      I have a similar situation with my hybrid inverter with trying to understand the excess capacity. The challenge for me has been to find a way to modulate the load to match the maximum output of the panels to find out exactly how much spare capacity I have. I have not had much success getting consistent detail without spending the whole day turning on and off loads and logging output of the panels. The issue is every day is different and eventually the gross numbers modeled from PV Watts has turned out to be sufficient for my purposes.
      9 kW solar. Driving EVs since 2012

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      • #4
        Originally posted by Ampster
        I have a similar situation with my hybrid inverter with trying to understand the excess capacity. The challenge for me has been to find a way to modulate the load to match the maximum output of the panels to find out exactly how much spare capacity I have. I have not had much success getting consistent detail without spending the whole day turning on and off loads and logging output of the panels. The issue is every day is different and eventually the gross numbers modeled from PV Watts has turned out to be sufficient for my purposes.
        Perhaps you could benefit from a readout, of the percentage of available solar
        power your array is putting out. That idea has not resulted in any hardware here
        just yet. But the latest thought is throw up a unloaded panel and observe its Voc
        which is zero current/power. If I may assume the maximum power point voltage
        of an array is 80% (make your minor correction) of Voc, a measure of the
        difference could give a measure of the available power actually being used. It
        is not entirely linear, but corrections could be made.

        This idea makes a couple of assumptions.
        1. that your control is of a smooth MPPT nature,
        2. that power drawn is decreased by allowing array voltage to increase,
        3. temp differece between the working panels ad the Voc reference are small, and
        4. voltage drop in the required connections can be arranged to not cause much error.

        Bruce Roe



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