One thing. 5kW for the AC plus whatever might happen to be running at the same time. Dishwasher + washer + electric dryer + ???. Got to size the inverter for worst case, including the surge that large motors can pull in the first seconds when they start.

My inverter is 4 kW continuous with a 8 kW for 10 seconds rating. I’ve got a Trace 4024.

The Xantrex 6048 will give you 6 kW continuous and tolerate a 12 kW surge. Looks to be about $3,500.

(Trace changed its name to Xantrex)

http://www.altestore.com/store/Inverters/Off-Grid-Inverters-Capable-of-Grid-Tie/Xantrex-XW4024-4000W-24VDC-InverterCharger-865-1010/p5954/

(I don’t know this company – altestore.)

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“To charge we would need the ability to push in 60/6 = 10KW average charge load.”

My experience is in the < 1 kW charger range. I use a IOTA like those on this page.

http://www.backwoodssolar.com/catalog/batteries.htm#ELECTRONIC BATTERY CHARGERS

That actually seems very high to me, but that what the numbers say – for a $200 bill per month you’re using 57 KWhs per day!

And as far as maximum current draw – an AC unit takes 1KWh/Rated Ton. Average house has either 1 or 2 – 2 ton units, so maximum draw of 4 KW plus some change, so figure 5KW for a safe margin? Just to discharge the whole pack we would have to draw 60/20 = 3KW average load. To charge we would need the ability to push in 60/6 = 10KW average charge load.

I believe that’s all the necessary numbers – but idk that using the price of new batteries will necessarily work, hence the use of old, “worn out” ones that should be substantially cheaper, but we can still run the numbers and find out.

OK, let’s try this with lead-acid batteries. Trojan has a new lead-acid designed for off-grid use. Golf cart size but thicker plates.

T-105 RE. 6 volts. 225 amp hours. 1,350 watt hours. 1.35 kWh.

Don’t cycle them more than 20% and they should last 4,000 cycles. 10.9 years. (4,000 is Trojan’s number, so I would expect more than 4k.)

If you cycle only 20% then you’d get only 0.27 kWh per battery. You’d need 67 to store 18 kWh. At $150/battery that would be a bit over $10k. Just for batteries.

I’m not sure how to price out inverters for AU. What we have here are ones with 120 vac output. Some can be stacked to give 240 volts but that doubles the price. And we don’t know the maximum load that would need to be serviced. Let me just guess $3k.

And another $1k (?) for battery chargers. I’m not sure what the best way to charge that many batteries might be.

When I said $4k above I was thinking a very efficient house. Something that uses about 2x as much as I do. 18 kWh is a honking lot.

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And then Otis seems to be saying that Texans use 44 kWh per day?

Or am I too tired to be attempted this right now?

And we’d probably want to know the maximum draw (ACs can suck a lot).

Granted that’s assuming that the $4K pack can last from 4AM until 10PM even during the summer, and charge completely in that 6 hour window, but if that’s anywhere near true the ROI on that is incredible!

Best idea, I suspect, is to install solar panels and provide your own peak power then buy cheap off-peak from the grid.

Apparently inverter prices are going to fall in the future as rumors are floating that bunch of Chinese companies are going to get into the business. That will turn them into commodities and make them a lot cheaper.

BTW – how much would one of those systems go for? Because here in Texas we have a lot of companies offering “free nights” where the cost of electricity actually drops below 1c/kwh… Could have a very fast payback to store all day’s power…

The rule of thumb for off the grid is about three days reserve. So take your average daily use and multiply by three.

I’m off the grid and have pared my usage down more than most. Plus I live where AC is not needed. (I use a fan a half dozen days a year.)

I sized my system to give me 1.2 kWh per day or 3.6 kWh total. Our cloudy days are almost all in the winter and when it’s cool my refer pulls well under 1 kWh per day.

The Prius PHEV has a 4.4 kWh battery. Changed out when it drops to 80% capacity one would be pretty much what I need. But it might only be about one day storage for a more typical house.

Now if I could get an 80% LEAF 24 kWh pack – I’d be good for over two weeks.

And how many KWh of battery are in the average hybrid? I can look this up tomorrow, but it might make a nice pairing to take them all out and use them as home storage for a decade or two after their life in a car – help make the grid more flexible.