Solar recommendations

[HouseWolf]

I found some new 280 watt 24v panels on the local craigslist for $140 each. So I decided to bite the bullet and build a system capable of actually running the house in an emergency.

I have several small systems here at the farm, mostly running gates, or the lights and a backup electric fence controller in the hay barn. But these are all 40 watts with a 12v deep cycle battery and a small charge controller to keep everything kosher.

This will be different. I'm going to set the panels up as a "roof" on the south side of the dairy goat barn. It'll make a nice covered area for the does, providing some shelter from rain (goats hate rain) and shade in the summer.

I've already committed to 12 panels, which will be about 3360 watts.
I see a nice charge controller on Ebay capable of 440 amps, but I'm looking for suggestions on the inverter.

I'll probably build a 2 string battery bank using either 6v or 8v Golf cart batteries, with enough guts to run the house in winter for several days without the sun.
I'm not really considering grid tie, since I want essentially a grid down system, and the grid tie inverters I have seen don't lend themselves to that.

Have I missed something?
Anyone done this already?
What inverter did you choose and why?

There is a xantrex converter - the 3024 thats top right now, but its quite pricey.
It puts out 3000 watts continuous with a 6000 watt peak.
Its oriented toward boats and RV use, and has an AC battery maintenance capability, so I think it would do nicely for the application I'm considering.

I don't want to do this twice so looking for input.

Thanks

[Forest Beekeeper]

Twelve 280 watt panels [3360 watts] is NOT enough to be capable of actually running the house.

I have 4400 watts of panels. It is about 60% of our monthly power.

... I'll probably build a 2 string battery bank using either 6v or 8v Golf cart batteries, with enough guts to run the house in winter for several days without the sun

You need to review your AH math.

... I'm not really considering grid tie, since I want essentially a grid down system, and the grid tie inverters I have seen don't lend themselves to that.

Grid-tied or 'net-metering' systems are way more expensive, they have a lot more regulations and inspectors in the process.

[HouseWolf]

Twelve 280 watt panels [3360 watts] is NOT enough to be capable of actually running the house.

I have 4400 watts of panels. It is about 60% of our monthly power.

You need to review your AH math.

Grid-tied or 'net-metering' systems are way more expensive, they have a lot more regulations and inspectors in the process.

Our house is very modest, and well insulated.
I want to be able to run a fridge, a freezer and the furnace. The furnace is LP, so essentially that means run the furnace fan. All lights are led.

Please elaborate regarding the Ah.

I was thinking the 840Ah/ 20,000 watt hours I had designed would be more than sufficient considering the intermittent operation of the appliances.

[Forest Beekeeper]

How long your battery-bank can go on a full charge is all about the AH of your battery-bank.

I have a 48vdc bank with 600 AH. Our 'Low Battery Cutt-off' is set to about 73% State-Of-Charge.

I have had to shift back to grid at mid-night many times, because our batteries ran low.

We do not always get a good 'full-charge', and some times we may go a few days without much charging at all.

I could lower the preset of our Low-Battery-Cut-Off, but the lower you dip each time, you are shortening the lifespan of the batteries.

[HouseWolf]

The inverter I'm looking at has the ability to charge the strings via AC if the panels aren't able to. Again, the system would be an emergency type, but hopefully able to keep up with a modest load in the event of a grid down.

Detailed input is welcome.

[Forest Beekeeper]

We still have grid power coming into our house. Our inverter can run off grid power, or off solar/battery.

In our township grid power goes down every few weeks, at least once a month. Some months grid power will go down multiple times. There is no way to predict, it may go down down for only half-a-day, or it might be for a week. The power company has it's priorities, and our town is near the bottom of the priority list.

During our first eleven years living here, we did not experience any calendar month without grid outages.

If there is a big storm predicted, we will try to keep our battery bank at near full charge, assuming the storm will blow down trees and knock out the grid.

[PlumBob]

HouseWolf, you didn't refer to it specifically, but remember that you should not drain your batteries down to less than (about) 50% of their capacity. Lower than that rapidly decreases both the total capacity of the bank, and their life span. The way to deal with is - more batteries - more total capacity.

For example, batteries that are rated at 230-amp should be discharged no more than about 115-amps. Our boat has 460-amps total of capacity. We will use no more than 230-amps before recharging. (Speaking here about flooded wet cells - golf-cart batteries)

The total capacity of the battery bank you settle on can also depend on the voltage you choose to use, as you obviously understand. At higher voltages, you can get the same power out to your house with fewer amps out of the batteries.

You really should do at least a "quick and dirty" analysis of what your daily electricity needs would be. Take a look at the plates on the fridge and freezer, and note how many Watts each uses. Same for the fan motor of the furnace. Those are at 120-volt AC. Using that you can see how many Amps each item draws. Estimate how long and frequently each will actually run. Multiply together for total amp-hours needed from the batteries. This is where I gross the whole thing up by 10% or so, to be overly conservative. This tells you how big an inverter you will need - I'd suspect that 3,000 watts will be fine. But no curling irons or Kuerigs. Remember that your inverter will be no more than about 85% efficient, so you will now multiply all of the above by 1.15, to get the total amps you need out of your batteries, (preferably on a 24-hour basis.) From here, you can plan your battery bank and the PV panels you will use to recharge them.

Pant-pant-pant..... LOL And also keep in mind that you will want a "Pure Sine-wave output" inverter. Motors like in the fridge, freezer, and furnace just run more efficiently on sine-wave AC electricity, than on pulsed or stepped DC - (modified sine wave.)

[HouseWolf]

Thanks for the input folks.

I did actually use a calculator I found at "wholesale solar". It indicates the 840 Ah solution should meet the needs I provided.

Granted it would not be "life as normal" but should be able to keep us heated and the food safe.

But I'm still willing to listen to ideas from anyone who has real life experience.

Sales and life are often different... And like I said, I'd like to avoid having to revise midstream.

I am curious about LIon cells. Though the upfront is outrageous, the performance is far superior to the flooded Lead acid.