Minimum Battery Bank for XW6048

[Altnrgy]

Hello All,
I’m not sure if this is the best place to post this question so please accept my apology if it’s not.

I’m designing a grid tied dual XW6048 system. The manual says a minimum of a 100 AH battery bank for one inverter which I know is ridiculously low. Since the system is grid tied I want to use the smallest feasible battery bank for several reasons. The obvious one is initial cost. The second reason is efficiency as in daily delivered AC KWH’s. As the battery gets larger the higher the charge/discharge and self-discharge losses get, especially as the bank ages. We only need a max of one day of autonomy since we are grid tied and have a generator.

I’ve never had good luck with sealed batteries. We can locally source Trojan L-16H-AC’s with a 435AH capacity @ C20 rating. We are planning to use just one string unless you folks can help me understand that is going to be problem. I don’t like multiple strings of batteries and a single string of the something like Surrette or HUP is totally overkill for a grid tied system plus cost 4x’s more.

The main reason we need the dual XW6048 is because of the 2HP well pump that is grossly oversized but suggestions to replace it with a soft start models are falling on deaf ears. I’ll look over the load calcs again but it might make more sense to go with a dual XW4548 instead. I see that CEC rates it with slightly higher efficiency as well.

I’ve zeroed in on an array of 7.5 to 8.5 kW. That’s all the roof we have so it’s unlikely the array will ever get larger. I like what I see on these MPPT600-80’s charge controllers but have some questions about them which I’ll ask in a separate post.

Any suggestions or requests for clarity are more than welcome.
Go easy on me. It’s my first post.
Thanks,T

[Cariboocoot]

Welcome to the forum.

You're right: 100 Amp hours on a 6kW inverter is absurd. What they meant to say is 100 Amp hours per 1kW. I believe Solar Guppy came up with that as being necessary to get the grid tie to function properly. You can't be him for accurate info, and he' sure to correct me if I've gotten confused about it. Again.

So you need at least 600 Amp hours. If you know you have a need for 'X' Watt hours for one day autonomous, the battery bank has to be able to meet that requirement as well.

There's one thing you won't like about the XW 600-80 MPPT: it's really expensive.

[BB.]

Note that is 100 AH @ 48 volt battery bank per 1kW of sustained load (or solar panel, which ever is larger).

You might look into Variable Frequency Drive (VFD) for your 2 hp pump. Basically a variable frequency inverter that can soft start and adjust motor RPM's by varying the frequency (assuming pump is seriously over sized, you can crank way back on the power requirements). Will work with 3 phase pump motors and single phase 3 wire motors (those with an external capacitor connection--as I understand).

There are a lot of VFD manufacturers out there (google), but I don't any experience with them--Perhaps some people here can recommend brands/vendors that they have had good luck with.

-Bill

[niel]

i'm not a xantrex expert, but regarding the use of the l16s, if the 1 day of autonomy would be no more than 1/2 of the battery capacity it may be ok from a battery capacity use point of view (48v x 435a/2=5220wh), but may be too low for this inverter and you propose running 2 inverters so i doubt it will work so i'll let those that know better in utilizing these inverters comment on this.

if the pumps are the main source of your problem then try to address this before you purchase anything for a solar system. alas, i am not an expert here on pumps either.

that would be a very large array with a potential in the many 10s of kwhs per day and i understand this is to offset your loads, but for 1 day of autonomy based at this rate of power being fed daily will need a much larger battery bank to sustain that kind of indicated draw. to keep a backup of, for example, 1000w you need a minimum of 2000w in batteries to keep some lifespan to the batteries.

is there a reason you a going to this very high voltage for the array that would necessitate this specialized high priced mppt controller? be advised with an array as large as you propose you would need multiple controllers due to the high current that array would translate into at 48v output (remember the inverter is 48v and so must the batteries be as well as the cc output to charge them). controllers do have current limits. 8500w/48v=178a and this is far above any single mppt controller. it may be more cost effective to go more strings of pvs at lower voltage to keep controller costs down.

[Cariboocoot]

Or minimize the battery-based GT to just enough for emergency back-up purposes then feed the remainder of the PV array to a standard GT inverter to get the benefit from the maximum number of panels possible.

[Altnrgy]

Note that is 100 AH @ 48 volt battery bank per 1kW of sustained load (or solar panel, which ever is larger).

You might look into Variable Frequency Drive (VFD) for your 2 hp pump. Basically a variable frequency inverter that can soft start and adjust motor RPM's by varying the frequency (assuming pump is seriously over sized, you can crank way back on the power requirements). Will work with 3 phase pump motors and single phase 3 wire motors (those with an external capacitor connection--as I understand).

There are a lot of VFD manufacturers out there (google), but I don't any experience with them--Perhaps some people here can recommend brands/vendors that they have had good luck with.

-Bill

Thanks for quickness and variety of replies. We're stuck with the pump. It's new and their well/pump guy is adamantly opposed to anything other than what he's already put in there. I'm coming a long a little late in the process. I've politely tried to point out there is a variety of pumping options but I'm the only one who seems to care how much it's going to add the cost of the system.

Thanks, T

[Altnrgy]

is there a reason you a going to this very high voltage for the array that would necessitate this specialized high priced mppt controller? be advised with an array as large as you propose you would need multiple controllers due to the high current that array would translate into at 48v output (remember the inverter is 48v and so must the batteries be as well as the cc output to charge them). controllers do have current limits. 8500w/48v=178a and this is far above any single mppt controller. it may be more cost effective to go more strings of pvs at lower voltage to keep controller costs down.

Good question. Wire size and cost is the quick answer I came up with. The array is about 150' away from the battery bank. Of course, I was planing on more than one charge controller. Using the Xantrex online Sizing Tool I determined I need 3 of the of MPPT-60's. But to stay under a 2% voltage drop I need a pair of 1/0 copper wires for each charge controller. PV prices have really dropped these last few years so maybe 5% is more tolerable but even if I could tolerate that I'd still need #4 wire.

Unfortunately they do not have a sizing tool for their MPPT-80 model but if I did the calculations right I can easily get that many watts on two 80's. Paying attention to the maximum and minimum voltage as in allowing for heat to lower the MPPT voltage and the cold to raise the open circuit voltage I can do 9 modules in series That's put's my MPPT voltage somewhere over 225 on the hottest days. #8 wire results in less than 1% voltage drop. So the answer is even though I'm paying more for the controllers I'm making it up in wire cost.

Thanks, T

[Cariboocoot]

Another controller you might consider is the MidNite Classic 250: http://www.solar-electric.com/misocl250mpc.html
With a 250 Volt input limit, it's something between the typical 150 V in MPPT and the very expensive 600 Volt Xantrex unit.

[mike95490]

I'd like to know why you need a 2hp pump ? I'm irrigating about 1 acre, and running a house, with 1/2 hp, and 160' lift.

How much water do you need, and what is the lift you require ?

The PV array to run a 2hp pump (4,000w consumed) would have to be really big !

Does it have to run at night, do you have elevated storage, or pressure tanks ?

Making several starts a day (every time you turn on a tap) is really hard on the pump and the starting surge uses a lot of power.

[Cariboocoot]

I'd like to know why you need a 2hp pump ? I'm irrigating about 1 acre, and running a house, with 1/2 hp, and 160' lift.

How much water do you need, and what is the lift you require ?

The PV array to run a 2hp pump (4,000w consumed) would have to be really big !

Does it have to run at night, do you have elevated storage, or pressure tanks ?

Making several starts a day (every time you turn on a tap) is really hard on the pump and the starting surge uses a lot of power.

I don't think he has a say in that; somebody else put in the pump and all he gets to do is try and find a way to power it. You know what plumbers are like: always stomping on goombas and spoiling Bowser's plans.