Solar on several buildings

[Letmo]

I am planning an offgrid solar setup for my inlaws, but there are a few weird constraints. The whole system has historically used about 1300 kWh of energy in January (which is the only month where the energy budget gets tight). The most power the system has ever used at one time is about 12 kW when there were several pumps running at the same time and somebody was using a table saw. Generating enough energy would take about 20 kW of solar panels on a south facing roof, but there is only room for about 12 kW on the main building. The next best place to add panels is a shop 250 feet away that also consumes about 300 kWh in January. This secondary roof faces east and west so it is not ideal, but it has room for as many extra panels as I need to power the system. The next best option for panel placement would be a ground mount, but there is no good place to put that close to the house, and there are a couple of kids in the neighborhood that would probably come by and break a ground aray. I know that I could just get a couple of sol-arks, but I am looking for something a little easier on the wallet. Is there a relatively inexpensive all in one unit that is capable of ac coupling? Another option I have been considering is long distance parallel operation, but I'm not sure if that is even possible. The last option I have considered is using something like 450 or 550 volt dc lines (depending on the inverter/solar charge controller) from each out building to the main inverter setup.

One other interesting thought is having two independent systems that can support each other during long periods without much sun. The idea would be to connect the ac output of one inverter to the ac input of the other. This would be relatively easy to make it possible to go either direction with a couple of carefully planned disconnects. My main question on this setup is weather or not you could leave the output of each inverter connected to the input of the other all the time or if it would be better to only do it when needed.

I am also planning on adding about 3 days worth of battery backup, but that should be easy with the right inverters.

What would you do in a situation like this?

 

[Shimmy]

Presumably on-grid? What is your goal with 3 days of battery backup? Propane + battery would seem to be more logical, but if that isn't an option your inverter cost should be the least of your problems.

At $900, the Growatt 5kW stackable inverter seems like a good deal; you could add as many as makes sense for your system architecture. At your service entrance you might need a supplemental transfer switch for fully off-grid operation depending on your max amps you want to support.

As for the secondary building, the east+west exposures aren't likely to do much for you in January. You would likely need 8kW for each exposure to match 8kW on a southern exposure in January. Run the numbers and see if it makes sense for your location. (On the plus side, it would reduce your battery requirement slightly with the extra panels.)

 

[Letmo]

It is currently on grid. The house is in an interesting place where the cost of electricity is very low, but the cost to be connected to the grid is a bit higher. This makes it so going completely off grid eventually breaks even, but adding a small system and staying connected to the grid never pays itself off. Their main goal is energy independence.

 

[Shimmy]

I would avoid doing "fake" AC coupling on the inputs; too likely to have problems. Where do you want to put the batteries? Figure out where the panels should go and how much for each location/exposure first, then the rest starts to fall into place.

Is the feed between the house and shop direct, and what size is it?

 

[schmism]

Id go
1) everything on a ground mount inverters 1 location.
1a - ground mount far off (300-500') with high voltage PV 400v kinda thing

2) main roof , additional ground mount - inverters 1 location. (likely a dual/multi paralleled inverter system that would have independent SCC for the 2 different arrays

managing an AC coupled PV array like solar edge can be done with a Solark but its a non-trivial task.

 

[Letmo]

I forgot to mention that the shop has its own grid connection right now so I would need to run an ac line over there anyway. The shop is about 35'X36'.

 

[Shimmy]

If you have to trench anyway, get a manual 2-axis tracking mount for 10kW and build a shed there for inverter and batteries somewhere between the two buildings. Make that the primary master and let the shop and house island with small local batteries sized for diurnal loads only. (Tracker is the grid input for the two other buildings). This location should also have a portable generator input and a big propane tank for the 3-5 days per year that you are likely going to need it.

You can get really complicated, but you seem to want a solution that will give you off-grid reliability for economic reasons. The best solution is likely to be a centralizred production/storage/backup system plus decentralized systems that accommodate daily needs.