Matching battery bank to Radian GS8048 inverter

scott_maui · August 2016

I am building a wood shop that I want to mostly power with solar. I live in Hawaii so we certainly have enough sun for this to be feasible. Maui recently ended net metering so I am not allowed to push power to the grid. The Maui utility only allows approved equipment to be attached to the grid, even when not pushing power. Radian is on the approved list.

I plan on using Radian in its "Grid Zero" mode, in which it will never use the grid to charge the battery but will use the grid to power AC loads not powered by the inverter. So battery charging will only take place from the panels. That is OK as I will only use the shop during the day in which case the panels provide most power. Shop tools have motors with big current draw at start-up, so I am relying on the grid to provide current surges. The Radian can do all of this.

My problem is that Outback says I need 390 AH (@48 VDC) battery for the Radian to work properly. I don't get that since the Radian is never charging the battery and I will rarely draw from it. I get it that the Radian wants to see a constant voltage source for its DC input, and the output of a charge controller needs a load that looks resistive for it to look like a constant voltage source, but it seems like a small battery (say, 100 AH) will work in that role.

Here are the numbers for the proposed design:
5.6 KW (STC) of panels
Typical load = 2 KW
Peak load = 5 KW.
Duty cycle for peak load = 10%
Average energy consumption = 16 KWH
Peak energy consumption = 32 KWH
Average power production for this location for these panels = 22 KWH
Note that all power consumption is for 4-6 hours in mid-day.

I would prefer to not have any batteries. I believe Maui will open up net metering again so I don't want to invest in something I won't need in the future. But as of now I am in a grid-assisted situation, so I seem to need batteries even though I will never be drawing from them for my situation.

Does anybody have any idea how to minimize the battery bank when operating the Radian in Grid Zero mode? Any other ideas? Remember I can only use approved components and the list is not big so suggesting some other inverter probably won't work for me.

Thanks!
Scott

Dave Angelini · August 2016

So you are asking with all the advice that Outback The OEM has given you, if we think it is OK to not follow their guidance?

westbranch · August 2016

maybe this why it is approved, Outback says I need 390 AH (@48 VDC) battery for the Radian to work properly

they want you to be 1/2 off grid???

scott_maui · August 2016

The person I talked to was a sales person not an engineer and he told me the recommended battery size had to with charging behavior of the Radian. Well, the Radian does not charge when in Grid Zero mode. Then he said I could lower the DoD current for a smaller battery. I did not get a lot of confidence in his answer and there is nothing in the operator's manual on how to size a battery when in Grid Zero mode.

So I would like to hear from people who have used this inverter in Grid Zero mode. But more than that I would like to understand how you choose a battery size when the bulk of the load is driven by power coming from the panels, not the battery. That question is really a general design question. Or is it always answered by the inverter manufacturer?

Obviously I am new to all of this.

mike95490 · August 2016

When an inverter runs, it pulls power from batteries, The more power it outputs, the more it draws from the batteries. If the batteries are too small, the AC waveform gets distorted. The general rule of thumb is 100ah battery per 1Kw of output

scott_maui · August 2016

mike95490 said:

When an inverter runs, it pulls power from batteries, The more power it outputs, the more it draws from the batteries. If the batteries are too small, the AC waveform gets distorted. The general rule of thumb is 100ah battery per 1Kw of output

OK. Is that the case even when all current will be sourced from the panels? Maybe what I am missing here is that the battery is needed to do some smoothing of DC panel current even if there is no significant draw from the battery. That would make sense.

BB. · August 2016

In general, from what I have seen/read--AC inverters and MPPT charge controllers do not have very "fast" control of charging current and voltage--Too small of battery bank can be "overwhelmed" by too much charging current (or too much direct load).

In general, using 100 AH @ 48 volts for 1,000 Watts of AC inverter and 1,000 Watts of solar panels (and presumably wind power total) is the usual recommendation for a flooded cell lead acid battery bank--The bank is "large enough" to ride through both loads and charging current without battery bus voltage collapse (or taken over maximum battery bus voltage). This rule of thumb is recommended because it works well as batteries age too (i.e., you don't want the system to work to specifications when the battery bank is new and fully charged--And 3 years down the road having AC inverter shutdown as the batteries age and/or is only at 50% state of charge after a run of bad weather).

We have had our fair share of folks that used "paper specifications" for designing the system (100 AH @ 48 volts per 1,000 Watts was very common "minimum" battery bank size)--Only to have all sorts of issues with battery bus voltage collapse (below ~42 VDC) or out of regulation (>72 VDC).

What you are asking for is not an unusual request--People want to have "zero" energy back into the grid (utility requirement) and want to run their loads from the solar power system... Some people want to do they with battery bank (emergency backup power, and ability to supply stored energy from daytime charging into nighttime use)--And a few others want to have Grid Tied AC Inverters (no battery bank at all)--That can just feed enough power to keep the loads happy and not get charged for power used by the utility (at least during the daytime).

We are sort of getting there with Battery Based "Hybrid" inverter systems (like the Outback and Schneider hybrid inverters)--Albeit sometimes with a minimum of 2 amps @ 240 VAC feed from the grid (to avoid any backfeed?).

However, I do not remember seeing any pure GT system (no battery bank) based systems that can do this (at least at this point in time).

You are correct--Battery banks (and the extra hardware) make a full battery based off grid capable system almost 4x more expensive that a pure GT inverter system.... And may not make economic sense for folks with Grid Tied power.

It is almost a guarantee that if somebody came up with a GT based (no battery) system that did exactly what you want--The utility will change their billing plan to ensure they get their pound of flesh from you...

For example, in California, we have have commercial rate plans that work out (roughly) to 50% is fixed monthly costs based on 15 minute peak power usage (say you use your shop tools a night and/or on stormy day), and the other 50% is based on your actual kWH energy usage (and in fact, we had schools that installed large solar arrays that pushed the "demand charges" higher than their original power bills without solar).

I would suggest doing a paper design and costing based on recommended rules of thumbs--And see what the hardware+installations costs would be--And roll in the ~7-15 year battery life and ~10+ year electronic hardware costs too. See what the real kWH costs would be.

In general, if you have utility power--The whole GT and Off Grid inverter system design and costs are based on your actual rate plan(s) and what make sense... I.e., if you have a time of use plan with high daytime power costs--Reducing your daytime loads (solar, battery, conservation, shifting work hours to off peak) is usually "more cost effective" than just throwing batteries+solar panels+inverters+chargers at the problem.

-Bill "my 2 cents" B.

Raj174 · August 2016

Hi Scott,

I have a Radian GS4048A and use it in grid zero mode. Grid zero mode just allows control over the amount of amperage available to the loads and how deeply the battery bank is discharged. Ultimately, the Radian is an off-grid inverter in that the battery will have to be charged and discharged. So you have a 5600 watt array that will produce 116 amps max and about 89 with hot panels. 89 amps into a 390AH Outback AGM battery bank would be about a 23% charge rate. That sounds about right to me.
Imagine a situation like this, you want to put a 4000 watt load on the system and get that power from the array. To do that set the DOD amps to 17 or higher (AC amps at 240 volts). If the battery is in float the inverter will get the power from the array, however if a cloud moves overhead and the array output drops to 500 watts, where do you think the inverter will get the power to support the load? If the battery can not support the load, the inverter will shutdown. Grid zero gets amps from the batteries first and then the grid for anything above the DOD amps setting. PV power can come and go throughout the day. The battery has to be properly sized to handle the loads unless you set the DOD amps low and allow the grid to make up the difference.

Rick

Dave Angelini · August 2016

Scott, if you can, wait until after SPI in Vegas next month. Should be some interesting new choices.

scott_maui · August 2016

Thanks everyone for the comments. This has been helpful for me.

I think Bill nailed it, in that I am expecting the Radian to work like a pure GT inverter like Enphase or SolarEdge, that needs no batteries to invert. But the other thing I realized after reading Rick's comments is that the Radian switches the AC loads between the inverter and the grid. I was expecting it to add its output power to that of the grid (which is what Enphase does), and use that to power the AC loads - so the more inverter power, the less grid power is needed. But it does not work that way apparently - the power is either all from battery/panel, or all from the grid. That does indeed put a higher demand on the battery than I was expecting.

Hawaii has jilted a lot of people who were planning on pure GT systems for cheap power. All those people (like me) are now trying to figure out how to implement a hybrid that pushes no power to the grid, works without batteries, and can augment the grid's power so that I am not replacing it, just buying less of it. The manufacturer that figures that out will do well in Hawaii since grid power is so expensive here ($0.32/kwh) and sun is so plentiful, and Hawaii utilities have almost entirely stopped net metering and GT systems.

Matching battery bank to Radian GS8048 inverter

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