Grid tie system, with off grid battery.

smalltrucksmalltruck Registered Users Posts: 11 ✭✭
I am looking for some answers.  First the setup.

 I have a grid tie SolarEdge system that was installed by a company and everything is running fine.  I also have a small off grid system that I set up to run my outdoor wood boiler with my old panels.  The older panels 800watt system with MPPT controller charge and 660AH 12volt battery bank which runs an off-grid inverter to power my wood boiler.  
So here is the question my off-grid portion makes more power then the wood boiler needs.  Can I set a small plug and play grid tie inverter to come on a 13.5 or so volts to use the otherwise wasted power.  And secondly will it create any problems with my existing SolarEdge HD-Wave inverter? 
Any great advise will be welcome.
Thanks all,

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,167 admin
    For an off grid/full time solar power system, recommend 10%-13%+ rate of charge:
    • 800 Watt array * 0.77 panel+controller deratings * 1/14.5 volts nominal charging voltage = 42.5 Amps charging current
    • 660 AH * 0.10 rate of charge nominal suggested = 66 Amp recommended
    • 660 AH * 0.05 minimum rate of charge suggested = 33 Amp minimum
    Your present array is a bit on the "small side" for charging your present battery bank. Adding additional loads, be very careful that they do not cause you to "deficit charge" your battery bank, or over discharge your bank.

    Assuming you are around Flint Mi, fixed array, south, titled for max winter harvest (wood boiler):
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Flint
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 32° angle from vertical:
    (Optimal winter settings)

    JanFebMarAprMayJun
    3.01
     
    3.58
     
    4.07
     
    4.23
     
    4.27
     
    4.43
     
    JulAugSepOctNovDec
    4.59
     
    4.40
     
    4.38
     
    3.68
     
    2.68
     
    2.69
     
    An 800 Watt array in December, average daily sun is 2.69 hours:
    • 800 Watts * 0.52 typical end to end off grid solar system eff * 2.69 hours of sun = 1,119 Watt*hours of 120 VAC power average per day for December
    That is not a lot of energy... So this ends up monitoring your battery bank "happyness" level (state of charge, does the battery bank get fully charged (over 90%) at least once or twice a week, average max discharge of 25% to a max of 50% discharge, and get recharged the next day or two)...

    If your second Inverter is used much during the winter, I would be very careful on energy usage. If you are doing this in summer, probably not as big of issue.

    Comments on my guesses/energy usage/location/etc.?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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