Grid tie size calculator query

Hi all,

This is my first post but very valid question.
I need to calculate a Grid tie system for a client but they are using 3 phase electricity thud they have 230v per phase.
Each phase is using on average 150a per phase during the day as they manufacture plastic and metal products.
They also have 400v machinery running with the offices using 230v.
If they were only using 400v machinery only then we would not have to multiply by 3 to calculate the size of the inverter and panels array amount.
I am sure that I am correct in saying the above and if not please correct me as this has been puzzling me for a while now.
But now that they will be using 230v and 400v on the same Grid tie system will I need to multiply the inverter size and panels array by 3 to make sure that the system can save the client the full 150a per phase?
My logic tells me that if its not done in this way I will only save my client one third of his electricity bill.
Can someone please help me to make sure I calculate these types of systems correctly going forward.

If possible can you include the formula used as I also want to make sure I am using the correct formula to work this out.


  • mike95490mike95490 Solar Expert Posts: 9,324 ✭✭✭✭✭
    Simple, you use 3 inverters, You first have to discover what 3-phase system they have, Delta or Wye,  Do they have an onsite transformer?

    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 ||
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  • mvasmvas Registered Users Posts: 384 ✭✭✭
    edited February 2020 #3
    No, you do not multiply 3 Legs x 175 amps per leg  x 400 volts = 210,000 watts
    Use a 3 Phase Watts calculator like ...

    It may be more cost effective to install a single 3-Phase inverter vs 3 @ single-phase inverters
    Isn't 230 v ( L-N ) / 400 v ( L-L ) typically Wye or Star type 3-phase?
  • BB.BB. Super Moderators, Administrators Posts: 31,911 admin
    First question--You need to check with your utility on if they accept Grid Tied solar power systems or not... And what their bill/payout plans are like.

    Utilities tend not to like GT solar (this is a big "green" thing with government regulators) and tend to do things that make connecting Solar GT systems difficult (various rules/operational regulations, low $$$ per kWH credits/payments, etc.).

    In the US, for smaller 3 phase systems, the three phases cannot be more than 10 kWatts "unbalanced" (at least that was our USA west coast utility requirement years ago).

    From a purely functional point of view, you connect 3x GT inverters to your three phases (picking the correct units for your voltage/frequency/array/utility requirements) to the main power panel (probably need an automatic shutdown of some sort to detect a loss of phase)--And everything will work pretty much "like magic".

    From a very simple point of view, your loads will have no idea that there is a GT inverter connected at all... They will see 230 VAC @ 50 Hz (you are in South Africa?) as normal. And your GT inverters will simply supply line voltage/freqency power into your main panel.

    Your power meter(s) will spin forward (charging for electricity) if the loads are greater than the GT solar output. Or "spin backwards" if the GT solar is greater than the loads...

    For all the world, the Utility looks like a "giant AC Battery Bank"... Your loads "discharge" the battery bank, and the AC Grid Tied system "recharges" the AC battery bank (your utility meter measures the total energy consumed/generated over time). This is not much different than how a car's alternator and DC loads work with the car battery... The only difference is the utility does the voltage regulation, not the GT Inverters (which output 100% of the available solar energy during the day back into the main panel).

    If you have other voltages on the main panel (480 VAC, 277, etc.) those will work fine too... 

    Sizing ghe solar array and 3 GT solar inverters (or more), is usually done in conjuction of how solar power is "charged" with your utilty.

    In California (USA), the idea was for the GT solar to "break even" with energy used (over a 1 year time frame). And, originally, if we generated more energy (kWH) than was used (positive credit), we did not get paid for the excess--It was "free energy" for the utility.

    Say the company used 100 kWH per day (sounds like they use a lot more--Just an example):

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 64° angle (from vertical):
    (For best year-round performance)


    Let's say that you average 6 hours a day (over 12 months of sun--not bothering to do the averaging now).
    • 100,000 WH per day * 1/0.77 solar panel+inverter deratings * 1/6.0 hours of sun per day = 21,645 Watt "break even" array
    • 21,645 Watt array * 0.77 panel+inverter derating * 1/3 phase power = 5,555 Watt inverter (3x inverter, minimum rating)
    • 21,645 Watt array * 1/3 separate arrays = 7,215 Watt arrays (3x, one for each phase)
    Anyway, a very rough way to figure out the average "break even" usage over 1 year period.... Of course, this depends on your utlity's GT Solar policies, actual energy usage of the plant, if you are looking for "break even" or not, etc.

    Also, a gotcha for some commercial power customers in the USA... Many large commercial customers pay (very roughly) 1/2 of their utilty bill based on kWH per month costs, and another ~1/2 based on their maximum 15 minute energy consumption in the last 30-90 days.

    What has bitten some folks (one I read about was government schools that installed GT solar) was these "reservation charges". Turns out, the the utilities charged the reservation fees based on current (kWatts/kVA) moving through the utility feed... Lets say that your customer used 100 kWatts (usually kVA) average loads during an 16 hour work day day. They would have paid a 100 kVA reservation charge....

    But solar power systems (break even design) generate all of their energy during (in this example) 6 hours of sunlight... So, the GT solar peak output may be something like (completely made up numbers to demonstrate a point):
    • 100 kVA * 16 hours of usage per day * 1/6.0 hours of generation during sunlight hours = 267 kVA peak GT solar output
    • 267 kVA peak solar - 100 kVA average weekday load = 167 kVA peak (week day) reservation charge
    • 267 kVA peak solar - ~0 kVA weekend factory load (nothing in operation) = 267 kVA 15 minute peak reservation charge
    The customer may reduce their energy usage (kWH) to zero (one year average solar generation - 1 year plant energy usage), but their 15 minute reservation charge went from 100 kVA weekday peak to 267 kVA weekend solar generation peak, or increased reservation charges by 2.67x because of the GT solar power system... In the USA, the unintended effect was to increase substantially increase the school districts' power bills by 1.67x because of the 15 minute reservation charge increase (sunny summers, schools out of session, etc.).

    Not saying that this applies to your customer--But was a problem for many in our area (not sure how problem was addressed--But most have been by our public utility commission/utilities).

    Also, in some areas of North America, GT Solar is now being discouraged (high connection fees, low payback per kWH) to actively being prohibited (very poor rate plans to actual "illegal to connect" regulations, or refusing to issue final inspection/acceptance permits).

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