Battery Input to GTI PV channel to offset PEAK DEMAND

I have a Huawei 10K inverter that has 3 PV input lines (for 3 PV arrays).  I'm only using 2 at the moment.  I'm on the APS "On Demand" plan, which, tracks the highest demand between 4pm and 7pm and uses that value to determine the electric bill.  IN short, if you can limit your total DEMAND rate during this time, electricity is very, very cheap.

In the real world, there's Dinner to be made (ovens and such), clothes to be washed, dishes to be cleaned, TVs to be powered on, Computers to be powered on, all at times right at peak, 4pm to 7pm.

If, somehow, you had a "buffer" of stored energy that can be tapped into during this window, you could limit the GRID demand load.  

All 3 inputs of my GTI have a minimum of 360v with a max of 600v with a 15amp limit.  The GTI doesn't know if the connection is a Solar Array or not, it's just reading VOLTAGE.  So if you had a source coming into the GTI that was over the minimum and below the maximum, the GTI shouldn't care what is generating that voltage.

If I can control the 3rd input so voltage is only available on that 3rd input between 4pm and 7pm, that means, I can connect my battery bank to the 3rd input and it only draws for that time, the batteries can be used to supplement the solar to handle the boost in Demand for that time frame, thus reducing my DEMAND charge lowering my entire bill.

Outside those hours, the battery bank is disconnected from the GTI and are connected to Chargers plugged into my Distribution Panel.  I use GRID power to charge the batteries back up.

The issue isn't HOW much electricity you use, it's how much you pay for each KW used.  The ON DEMAND rate is SUBSTANTIALLY cheaper than any other rate per KW because they know they are going to make up for it using the DEMAND modifier.  If you can make your peak DEMAND remain close to 1 or 2, your bill is next to nothing.

Yeah, you can shut off appliance, don't do this, turn this off, etc, etc.  However, if you didn't want to force changes in your daily routine, this seems to be the ticket.

If you take a bank of 10 48V battery, say 20 or 30 aH, wire them in series to reach the 480v to the GTI, the GTI should pull what it can up to the 15amp limit of the circuit.  But if the other PVs are supplying, then the GTI should balance the pull.  The 20aH should last the 3 hour window as the "DEMAND" buffer of your system.

Granted, 480V DC from a bank of batteries is some serious stuff, but, why should the GTI care if the Voltage is coming from Batteries, a PV Grid Array, a Windmill, all it cares about is converting DC Voltage to AC, and if you can get DC Voltage to it's input, it will convert it....

This seems reasonable to me?


  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    A few observations...

    First comment, in California (years ago, don't know now) they peak power (in California peak KVA not KW--Can discuss further if interested)...

    Basically an issue when they were putting solar panels in schools, there were (roughly) two major parts of the bill. One was the normal KWH per month billing (time of use, etc.)... And the 1/2 (roughly) of the bill was the peak 15 minute KVA per month used (what you are seeing).

    The "gotcha" that was sending electric costs higher for the schools was that the KVA charges did not "care" which direction the power was flowing. I.e., not only "demand" side of bill, but also the "supply" side from GT Solar too...

    What happened was that the usual GT Solar design is that the peak KVA was 4-8x the "average power usage" over 24 hours... I.e., the solar array had "make" 100% of the power over the 3-6 hours the sun is up in the sky. Especially during summer (no school) this sent the "reservation charges sky high" (again, because the school GT Solar peak was much higher when the sun was up vs the normal average loads).

    So you would need to make sure your 4-7pm reservation KVA from the (battery assisted) GT inverter was "balanced" with your actual demand and did not actually "supply" too much current tot the grid.

    Second comment, there are lots of "hybrid" AC inverters that have a normal (48 VDC or so) battery bank and connect to your grid and to your loads... The inverter could be programmed to have "zero load" 4-7pm by using the stored energy in the battery bank (and the bank is recharged earlier in the day. Finding the right Hybrid inverter with this programmable net zero programmable Hybrid GT/Off Grid capable inverter.

    Third comment... Your GT inverter with 3x MPPT solar array inputs... Is this three MPPT chances, or just one MPPT channel with 3x pairs of connectors. If they are 3x independent MPPT channels, then connecting a third MPPT input to an alternative DC source MAY work.

    Fourth comment... Need to know about the input to the MPPT controller. In some MPPT input devices, yes, can connect to a solar array or even a battery bank to the MPPT input. One designer suggested placing a low Ohm value resistor in series with the battery connection to limit surge/switching spikes (solar array have ZERO surge current capabilities, Battery Banks can have very high surge current capabilities and could damage/fry the MPPT input stage). I don't have any detailed knowledge on the MPPT input stages to tell you what would, or would not work.

    Usually a manufacturer will not support customer questions like this... You may end up having to do your own experimentation (and any costs of hardware failure) yourself.

    Fifth comment... You are correct, high voltage DC current is nothing to sneeze at... Fuses/Circuit Breakers (protect battery bank wiring) and switches/relays need to be designed for use with high voltage DC.

    Just an example of AC vs DC switching:

    You may have issues finding high voltage DC relay(s). And then there is trying to charge a 480 VDC battery bank (not a lot of high voltage solar or AC battery chargers out there).

    My guesses.

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