Much like CrossCut my mind is still in the 1990s when it comes to battery technology.RonnyRonin wrote: ↑Wed Feb 24, 2021 9:59 pmI had kind of assumed I'd need 1000 Wh of capacity but I am still going to put off doing math until I'm further along in the project.CrossCut wrote: ↑Wed Feb 24, 2021 7:24 amHave to admit when I saw your list of items to power and the size of the battery box it looked like an expensive failure waiting to happen. Looking up the specs on some of the options available now has changed my mind. The advances in offgrid power have come a long way in the last 20 years when I built my first system - T-105 FLA batteries at 120 pounds for the smallest 12v system, only 50% depth of discharge if you wanted them to last very long, and panels costing 4-5 dollars per watt.
Taking your 1000Wh (which I'm calling 85 Ah at 12v nominal) as a benchmark, I am pleasantly surprised to discover that I can buy that in the UK as LiFePO4 for about £400 (US$560), and I could cycle it to 100% DOD every day for 5-10 years.
Or I could buy 260 Ah of AGM lead-acid, which will be bigger and heavier, for £300 (US$420) and cycle them to 33% DOD daily for two years.
Or if I was really cheap I could buy the same 260 Ah of flooded lead-acid for £150 (US$210), and cycle them to 33% daily for 6-12 months.
I guess it comes down to how often I expect to use them. If I was geniuinely of grid, or living in a boat or RV, I would be using the battery every day and space/weight would be at a premium. The LiFePO4 would be a clear winner. The same would be true if I had a time-of-day electricity tariff and I wanted to use the battery to exploit it. But for a standby application in a house or apartment, where I might only use them a handful of times a year decade and the rest of the time I can care for them on float charge, the lead-acid options could be adequate and I'd save a few bucks.
And one benefit of the lead-acid options is that you've actually got 260 Ah of capacity, not 85 Ah. Yes it would destroy your cyclic lifetime but, if you found that you _really_ needed 3000 Wh between charges (maybe the sun isn't shining on your panels, or the grid is down for longer than the power company promised*, or ZOMBIES), you have the option. If you've only got 85 Ah and you've used them all, well there isn't any reserve capacity.
* Interesting aside: here in the UK, at least, there's a regulatory obligation on the power companies to restore power after an interruption. If you're without power for more than 12 hours, the power co is on the hook for compensation. Which is why, the one time in the last ten years that we had a serious outage, after 11 hours the power co parked a humungous diesel generator next to the substation that serves my street and the neighbouring ones, and hooked it up. Every couple of days they'd come around with a bowser full of diesel and refuel it. We had that generator in place for about 2 months, if memory serves, while they found the underground fault made good.
https://www.citizensadvice.org.uk/consu ... power-cut/