XW inverters and panel string sizing

metalguy22

I'm putting together an off grid system (around 5kw) based around the Xantrex XW hybrids. I settled on this as it offers the most future flexibility here in Nevada where the current utility rebate program is a complete mess. There are no State rebates and there is little funding for the utility rebates thus making it not really attractive at this point to grid-tie.

My question here is that while I basically understand the idea behind string sizing for the GT Xantrex units I don't quite get the Xantrex XW sizer. Am I correct in believing that the charge controller in the all XW's is the same MPPT 60-150 unit since they allow no other choices? I see also that changing to different panels gets more modual/string# combinations. Is this due to the differing Voc's of the panels and what should I look for as the best match between panel & XW unit. And what happens when the controller begins to limit the amperage for oversize arrays? Does this mean there is no point in installing anything past the limiting amperage (60a) of the controller?

So many questions, so little time.

BB.

Re: XW inverters and panel string sizing

metalguy22 wrote: »

I'm putting together an off grid system (around 5kw) based around the Xantrex XW hybrids. I settled on this as it offers the most future flexibility here in Nevada where the current utility rebate program is a complete mess. There are no State rebates and there is little funding for the utility rebates thus making it not really attractive at this point to grid-tie.

Check out the net metering plans, if they are available for your area. A good Net Metering plan is still, much less expensive to install and maintain vs a Off-Grid or even Hybrid system.

Unless you also have other needs/costs (emergency power or high monthly service charges).

I would suggest you "put the system together" on paper, and run the costs over twenty years (assuming 1-2 battery bank replacements) and see what you find:

(h/w+install+battery replacement+taxes over 20 years)/(kWhrs per year*20 years)=$$/kWhr

My question here is that while I basically understand the idea behind string sizing for the GT Xantrex units I don't quite get the Xantrex XW sizer. Am I correct in believing that the charge controller in the all XW's is the same MPPT 60-150 unit since they allow no other choices? I see also that changing to different panels gets more modual/string# combinations. Is this due to the differing Voc's of the panels and what should I look for as the best match between panel & XW unit. And what happens when the controller begins to limit the amperage for oversize arrays? Does this mean there is no point in installing anything past the limiting amperage (60a) of the controller?

Xantrex made things very confusing when they call everything the "XW Family"... As I understand it, there are three members of the XW family. 1) the XW Hybrid Inverter--very nice unit. 2) The XW MPPT Charge Controller--also nice, but not required. And 3) the XW wiring panels--very nice but also not required.

The String Sizer only applies to the XW MPPT Solar Charge Controller 60 amp (only released model). The MPPT CC can be used with the XW inverter or just with any other battery based system.

The String Sizer takes the data sheets for the various panels out there and "fits" them to the requirements of the Xantrex MPPT CC.

Basically the operating voltage of the panel needs to be less than 150 VDC and greater than Vbatt. Voc (open circuit) when the panel is cold can over-voltage the controller, and Vmp when the panel is hot can under-voltage the controller.

There are some details that say you should aim for Vmp<~2*Vbatt
as the MPPT CC is more efficient in this area (by a percent or two?). However, wiring issues (such as long runs from panels to charge controller) can be a very good reason to run Vmp nominal towards 100 VDC (you cannot go much higher as Voc cold gets near 150 VDC).

Does that sort of answer your questions?

-Bill

PS: You can use the PV Watts Website to calculate your kWhrs per month/year output.

Use derating of 0.77 for grid tied and GT hybrid systems. Use 0.52 for Off-Grid systems (and Hybrid systems that are not Grid Tied)...

The loss in efficiency of battery based systems is another reason that GT systems tend to cost less to operate vs Off-Grid systems.

Which back-door's into the other recommendations... 1) Conservation--it is cheaper to conserve than to generate your own power; and 2) Carefully evaluate your power needs (peak, by season, etc.), then size your system.

metalguy22

Re: XW inverters and panel string sizing

"Does that sort of answer your questions?"

You bet it does. That pretty much clears up the issues about how to approach stringing panels together using the XW controller. I guess if I want to go above the 3kw limits this controller has I just add another controller to the main panel box.

"Check out the net metering plans, if they are available for your area. A good Net Metering plan is still, much less expensive to install and maintain vs a Off-Grid or even Hybrid system."

This, then again is a more complicated issue. I took a 4 day course from Bill Brooks over in Reno a few months ago and while he is excellent at teaching the basics of design, the utility people that sponsored it were not as good. I guess what I need to look at is whether net-metering benefits offset the extra cost for PV certified installers, Professional Engineer costs, building permits, inspections, net-metering contract requirements, etc., etc. etc. that I won’t have if I do this myself off-grid. I’ve read Nevada’s net metering plan and it is pretty ugly. And the rebates simply aren't there except Federal and I get that in any event. I’m having a hard enough time staying in business now as it is with the rate at which Nevada is piling on the regulations and fees. Just for my peace of mind it might be worth it.

But, as you say, it is likely cheaper as a rule of thumb so I’ll have to look at it more.

Thanks.

BB.

Re: XW inverters and panel string sizing

Yes, you are correct... If you exceed the capacity of one charge controller, you have to add more in parallel and divide the solar panels among them. Some brands have a communications bus where they share data, devices (like remote battery temperature sensors), and operating states (so all controllers transition from absorb to float at the same time).

The bus is not necessary, but can be nice.

Sorry to hear about Nevada (and good luck to you)--that was my first place in my plans to bug out of California on its dive to the bottom. I did not think anyone else could mess it up as bad as they have here.

-Bill