Grid Tie/off grid system--Inverter sizing question

jacknife Registered Users Posts: 6
I am in the same boat you are in, I found some 12v lifeline 255ah batteries x Six. I think thats a lot of battery power, and 3 200 watt solar panels tied into a 1600 watt wind generator Or as some call it a P.m.A. i AM now at a point that I need to determine what size 24volt inverter
to use. Anybody that has any insight let it out. I am thinking about the Outback 3000 watt, but I am still sketchy on the math. Anybody with a formula that is in layman terms please HELP


  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
    Re: Grid Tie/off grid system--Inverter sizing question


    I moved your post to its own thread... The other thread was an offer to sell some used telecom batteries.

    In any case, I like to suggest that you know your loads first... Peak Watts / Volts*Amps (like refrigerator/pump starting loads) and average running loads are important. These numbers help you to size the rest of your system... Battery bank, solar array, inverter, etc...

    A 3kW inverter is a pretty good sized piece of gear... Both in power output and price. If your loads are small (say a standard energy start fridge/freezer), you may get away with a good quality 1,500-2,000 watt inverter quite nicely.

    But one of things that people frequently forget is that a running inverter with no AC loads at all consumes power too... A 3kW TSW inverter could easily consume 30 watts with no loads at all.
    • 30 watts * 24 hours per day = 720 Watt*Hours per day inverter load
    • 600 watts * 4 hours of sun * 0.52 system PV to AC eff = 1,248 Watt*Hours in "average conditions"
    So--just a large inverter itself with a small solar array/system could consume 50% of your solar array output if simply just left on to power random loads (fridge, pumps, lights, etc.).

    I am not saying that is what you will do in your place--Just a demonstration of what could happen if you do not size your system and configure it to meet your needs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Grid Tie/off grid system--Inverter sizing question

    Sketchy math is the best kind. Let's do some! :p

    You have six 12 Volt 255 Amp hour batteries. If configured for 24 Volts you would have 3 * 255 or 765 Amp hours @ 24 Volts. That's a maximum potential of about 9 kW hours of power, which is large for off-grid applications.

    It also isn't going to get recharged by 600 Watts of panel (and don't count on the wind turbine unless you've done a site evaluation and know you have sufficient sustained wind to realize some power from it). You might see 16 Amps from those panels on a 24 Volt system, which would be good for 160 to 320 Amp hours at best, or roughly one set of those batteries (about 3 kW hours worth).

    As for sizing the inverter, that would depend on the maximum amount of power you would use at any given time. Are you likely to have loads totaling 3000 Watts? The Outback 3524 uses 20 Watts doing nothing, btw. It can be set for "search mode" wherein it uses 6 Watts until a load is present. On the whole this is not a big savings if you need it to run a refrigerator for example as it will be turning on repeatedly throughout the day anyway.

    Eventually we can turn this sketchy math into a work of art. :p
  • jacknife
    jacknife Registered Users Posts: 6
    Re: Grid Tie/off grid system--Inverter sizing question

    Thank You.
    yes I can understand that, You are good. Now I can use a calculator after finding my load need and put things in place. You are a jewell.