Connecting new 15kw inverter /charger with 80a MPPT to existing solar system

Stan55 Registered Users Posts: 3
Hi All my existing solar system is a Sunnyboy 6-7 years old.

I have a 15kw 240 split phase 48vdc inverter/charger, with a built-in 80a MPPT controllerwith a 640Ah battery bank and I have installed a 3 pole 100amp auto-transfer switch making utility primary and inverter A/C output as my secondary with (separate neutrals) source.
I have also an existing breaker at the main house panel with a 100 amp supplying a subpanel 50 ft inside the house. This inverter is installed near the Main house panel in a garage…also just 5ft away from the inverter is the existing solar system.
with 2 grid tied inverters a small 6 space 240v subpanel to back feed the main house panel.

Now that I have board you with that info…I have a Q….. I can seem to find an answer too…I am sure it’s out here somewhere.

I want to tap into the existing system to do what is called (islanding)  I now know..
to tell the solar system to keep running/charging during a power loss.
Can someone tell me if this is even possible with out changing the grid tied inverters to a (battery ready) type inverter?
Or can I just simply tie into the sub-panel at the inverters where a label say’s (do NOT tap Here)?
I am sorry for the long winded Q but I want to make sure who (if anyone ) does respond has all the info needed to make an informed answer or reply.


  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Sorry.   Simple GT inverters cannot island. you need batteries and a hybrid inverter.
      There is a inverter, that can generate 120VAC for one outlet, that you have to manually switch on, and if a cloud passes over the panels, it resets and you have to push a button again, and it only works when the sun is out.   Great for keeping a cell phone & laptop charged, won't keep ice cream cold overnight.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • BB.
    BB. Super Moderators, Administrators Posts: 33,522 admin

    There are several ways of slicing this system up... First need to know your daily loads (peak Watts "power" and Watt*Hours "energy") usage per day...

    One way is to put the entire home on backup solar (and genset?) power... Or, just to power a "protected" subpanel for your "critical" loads... A fridge, LED lighting, washing machine, solar friendly water/well pump, LED TV, laptop computer, cell/tablet charging, etc... That is (I would suggest) around 3,300 WH (3.3 kWH per day) for a "near normal" small and/or very energy efficient home... Use propane/wood/etc. for heating, hot water, cooking...

    If you have a 48 volt @ 640 AH flooded cell battery bank... Then a "optimal" daily energy usage would be (from the battery bank)... 2 days of storage, 50% max planned discharge for a full time off grid home:
    • 48 volts * 640 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 = 6,528 WH per day (i.e., two dark and stormy days, discharge to 50% state of charge for better battery life).
    Next need to figure out your solar array. Say San Diego California, fixed array facing south:

    San Diego
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
    (For best year-round performance)


    Relatively sunny region... Say you want to run 6,528 WH per day on an average December day...
    • 6,528 WH per day * 1/0.52 off grid AC system efficiency * 1/4.89 hours of sun (December average) = 2,567 Watt "Dec" break even array...
    For full time off grid, break your loads into two types... Your "base loads" (those that mush run 24x7, and your optional loads (washing machine, irrigation pump, TV, induction cooker, etc...)--Or loads that you can turn off during bad weather (or run a genset when needed). You should plan on only using 50-65% of your "predicted" solar harvest for base loads... And use the rest when the sun is shining and you have a couple days to fully recharge during poor weather....
    • 6,528 WH per day December average harvest * 0.65 base load fudge factor = 4,243 WH per day base load fudge factor
    • 6,528 WH per day December average harvest * 0.50 base load fudge factor = 3,264 WH per day base load fudge factor (more conservative)
    Obviously, I don't know much about your system... But this is roughly how the bits and pieces I know at the moment would work out..,

    Also, your 15 kWatt hybrid/offgrid AC inverter is way too large for a 640 AH @ 48 volt battery bank... A much better fit would be 3,200 to 6,400 Watt AC inverter. Your present FLA battery bank would be hard pressed to supply 15+ kWatts of power "reliably".

    And if you look at your average power... Say most of it is 5 hours per evening (everyone home, cooking, lights, etc.):
    • 6,528 WH average battery load / 5 hours per night = 1,307 Watt average load
    Much closer to a 3,200 Watt AC inverter than a 15 kWatt inverter. For example at full load:
    • 640 AH * 48 volts * 0.85 AC inverter eff = 26,112 WH total bank capacity
    • 26,112 WH battery bank / 15,000 Watt fully loaded AC inverter = 1.7 Hours from 100% to 0% bank state of charge
    Not very useful--Less than two hours of energy--And the battery bank may not even last a fraction of that (depending on type of batteries, age, health, state of charge, etc.).

    With solar--Conservation is your #1 project... Off Grid Solar energy is expensive. The less energy you need, the smaller and less expensive your system will be... Especially if this is for backup power (use GT or Hybrid inverters to feed solar power back to the home/utility via net metering normally--Backup power only when power failures occur).

    Probably way too much information (at least for your original question)... But as you an see, the wiring to support your "protected loads" with a 3.2 kWH inverter would be ~30 amp @ 120 VAC circuit... Not 50+ amps at 240 VAC...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Stan55
    Stan55 Registered Users Posts: 3
    Wow ,Thanks you both.
    my issues are for the house sub panel is... I currently have a 6000w split phase inverter connected  and Grandma lives with the customer and she is very old and get cold that  means a 12a 120v space heater   around (1k-1500watts) the kids are watching TV 300w and the mom want to  cook a quick meal for the kids with the toaster over (another 1200watts) and she forgot she has the curling iron plugged in too. more 1kw
    My inverter per leg is only the inverter see's the over load and shut off. as it should.
    I tried to rearrange the loads/breakers at the sub-panel to try and separate /balance the loads...with out re-wiring the entire house...I could only do so much. the 6kw inverter just won't hold the load.

    All of the heavy loads like A/C and Spa and pool is on the main house panel...thanks god.
    owner wants to get a generator too at some point...another day though.
    so what I am saying is
    the wife is now aware she can NOT run everything in the house on batteries.
    Under minimal use the  (3) refers, the TV and mics lights only run at 1kw to 1.5kw when refers cycle.
    I will get the larger 15kw only because the 12kw inverters are  in the supply chain...somewhere and not in the warehouse.
    I am figuring I  will get 17 hrs at 1.5kw  at 100% which I will  tell them 50% is max so 6.5hrs at 1.5kw  that should get them thru a day or so if used sparingly.
    what do you think am I way off course here or what?

  • BB.
    BB. Super Moderators, Administrators Posts: 33,522 admin
    edited April 2022 #5
    I still don't quite understand your needs... And where you are located (I guessed San Diego or the area)?

    1) Utility connected power? Then GT Solar is the cheapest/easiest method to generate "substantial" amounts of power for your home with a relatively cost effective solution (say around $0.15 per kWH).

    2) Why battery+solar power? For emergency usage? Do you get only a few days a year (on average) of lost power? A genset can be a cost effective solution... The price of fuel is crazy right now--But call it around $1.00-$2.00 per kWH fuel costs. If you have natural gas or propane on site. If not, then gasoline or diesel options. Need fuel conditioner and recycling fuel back to car (such as once a year back to car and refill with fuel+stabilizer--That can be a good week or two a year solution.

    3) Going full time off grid... That is not a cheap solution... Maybe around $1-$2+ per kWH typical for off grid solar. System has to be larger than GT Solar system as you have to supply power 24x7 (solar+battery power) or using backup genset (winter, storms, overcast, etc.)... And while solar panels are historically "cheap", batteries, copper wiring, racking, etc. are pretty much at a high right now... And depending on what battery type you pick, you will have to replace the batteries every 5-10 years or so.

    4) Hybrid Solar system.... Basically an 'off grid' system that also operates with Utility power--Both "Off Grid" and "Grid Tied" capable "hybrid" AC inverter (many OG inverters are really hybrid type right out of the box). The advantage of Utility power, and net metered/GT Solar. But the costs and maintenance issues of OG solar...

    Because off grid solar (and generator) power is expensive.... Generally that means you work to reduce your loads as much as possible... Or in a backup/emergency power situation, you only run a subset of your loads (LED lights, the refrigerators, use propane for cooking, etc.).

    For longer term--You can look at more energy efficient solutions... For heating, a Mini-Split Heat pump is around 3x more efficient than a standard resistance heater (somewhere down to freezing weather). For lighting, LED is much more efficient than filament bulbs.

    For hot water, there is thermal solar, heat pump water heater, or even lots of solar panels heating an electric water heater (some folks prefer over the 'fun' of solar therm panels+tanks+pumps+plumbing).

    If you already have Utility Power--GT Solar (with a backup genset) can be a nice solution--And the GT Solar can save you money (depends on your utility rate plans). But even then, looking at conservation first (energy star appliances, lots of insulation, replacing central heat/air with mini split heat pumps, etc.) can save money--Without the expense of solar (especially Off Grid Solar).

    Getting a Kill-a-Watt type meter to measure you loads (120 VAC single plug)... You will soon see that loads running 24x7 (or at least many hours per day) like TVs, computers, lighting, heating, refrigerators, etc. can really add up quickly. Just to give you some ideas about the math:
    • 120 Watt refrigerator * 24 hours per day * 0.50 duty cycle = 1,440 WH per day = 1.44 kWH per day
    • 300 Watt desktop computer * 10 hours per day = 3,000 WH per day = 3 kWH per day
    • 15 Watt low power laptop computer * 10 hours per day = 150 WH per day = 0.15 kWH per day
    • 1,200 Watt microwave * 1/3rd hour per day (20 minutes per day) = 400 WH = 0.4 kWH per day
    • 1,500 Watt heater * 24 hours per day * 0.33 duty cycle = 11,880 WH per day = 11.88 kWH per day
    A small/very energy efficient home/cabin (near normal electrical existence) can be around 3,300 WH or 3.3 kWH per day (x30 days = 100 kWH per month).

    The typical North American home is around 300-500 kWH per month (using natural gas/propane for heating, cooking, hot water)... A full electric home can be 1,000 kWH per month to 3,000 kWH per month (i.e., central Texas with full A/C and electric home).

    A kill-a-watt type meter:

    And a whole house monitor (can measure large loads like Electric Water heater, central heating, and branch circuit loads):

    The above links are just examples of possible products (starting point for your searches). For many folks, when they measure their electric loads, changing their habits (turning off unused equipment) and picking more efficient devices (energy star fridge, turn off old "beer fridge in garage", replacing desktop with laptop computer, lots of insulation and weather stripping, etc.)--Saving up to 50% on electrical usage is possible.

    Without a firm handle on your needed loads, rough location, other loads like well pump, electric home, etc... It is hard to be very specific at this point.

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