First Solar System

wkdfarm

I am designing a new retirement home on my WV farm. I will be the GC. I am a retired Electrical Engineer and US Submarine sailor - so I have enough experience to be dangerous re solar, inverters and batteries. Have also built a couple of houses and done my own electrical and plumbing.

So I am planning Grid Tie with battery backup b/c the power is unreliable. In the last ten years we have seen two or three week long outages and loose power for several hours a month. I have my vital loads mapped out and measured or estimated. About 5000W peak and 6kW-hr per day. Utility use in current home is about 10kW-hr per year.

Proposed System: 12kW DC, 35 x 375W panels, 2 SB6.0 US inverters, ~1500 A-h flooded 48V battery and 6kw diesel genset. Generous net metering.

Backup will be by dedicated vital bus with feeds to barn, garage/shop and house. Just less than 48A calculated peak load. Way less in survival mode. Planning on one or two Conext XW Pro. 

I would like to AC couple the SBs and the XW Pro on the vital bus and keep my efficiency high and cost low.

The kicker is I need to be off-grid for about one year b/c my utility requires a house foundation to be in place to do a line extension and I need to build the barn first. 

SOOO...

Is AC coupling reliable enough for a year of lite off-grid use? Seems all the rage in Europe, etc. Could DC couple at higher cost and significant loss of efficiency. But the AC coupled system would obviously be the best bridge to eventual grid tie. Or I could just use the battery and inverter to reduce gen run time. But I don't like the idea of auto gen start when I'm not there.

Any feedback appreciated.

mike95490

i too, don't like unattended auto-start.
Once you have sunk the cost for the off-grid system, how much will the grid hook-up cost you ?
If panels are on outbuilding (not residence) will your local code allow simple string inverter without Rapid Shutdown ?

Rapid Shutdown requirements have removed a lot of choice for solar users.

wkdfarm

WV is under 2014 NEC. So my system is intended to be ground mount and should/does not require RSS. Thank goodness.

I require a building permit but there is little code enforcement except for electrical to get utility hookup. Neighbors say AHJ process is favorable to DIY. No building inspector outside city limits. I want to have a solid plan before I pull permit and meet with AHJ. 

The array will be ground mount, due South, about 5x7 portrait with essentially no shading. Seems good for 2 Sunny Boys with 3 MPPT each. 

Grid connect cost is mostly free for 1000 ft. I am at that limit so I expect $2-3,000 misc fees, land clearing and underground up-charges. Pretty reasonable!

After I get the AC vs DC coupling issue squared away I will meet with the utility and inspector and work out service amperage, disconnects, etc.

Appreciate all input. Any experience with Conext XW + or Pro AC coupled to SMA SB?

mike95490

XW user, and am ordering a XW+ shortly.   I'm totally off grid, so I don't know about the utility interactions. The XW line works well with my backup generator.

aksala

I don't really understand the question. If you're going to sell back when on grid, you're going to build it as an AC coupled system right? I don't see an issue with the XW's as long as the SB's are compatible with how the XW+ does it's Active Frequency Shift Power Curtailment thing. 

From my 6048 manual:

AC COUPLED PV INVERTER COMPATIBILITY

AC power generated by AC coupling PV inverters with Conext XW+ must be consumed by AC loads or used to charge batteries. As an alternative, the excess power produced from a PV inverter can be routed to dump loads. Do not AC couple PV inverters with the Conext XW+ that are unable to reduce, derate or cease the excess PV inverter power in response to the changes in AC line frequency controlled by the Conext XW+. Consult the manufacturer's specifications of your PV inverter and confirm compatibility. 

Failure to follow these instructions can result in equipment damage