Off Grid System Design Considerations and Battery Types

People are regularly surprised that my solar powered office is driven by a moderately sized bank of lead acid batteries instead of a fancy modern lithium bank.  I mean, I do work on lithium batteries regularly, and I could certainly build myself a large lithium bank at fairly minimal cost (compared to purchasing one, if I were willing to do that much spot welding).  But, in a deck box behind my office, I have a boring set of 8 Trojan T-105RE batteries in a 48V/225Ah (10.8kWh) bank.  Why?

This is a companion discussion topic for the original entry at

(Comments from Blogger)

2018-04-23 by carl

i have a question that is sort of off topic. do you think roof mounted solar panels help keep a house cooler by shading the roof from the sun? the gap between the bottom of the panel and the shingles seems like it would vent pretty well. i think it would keep the attic a little cooler and maybe the house a bit.

2018-04-23 by Russell Graves

Yup! That’s a common observation with rooftop solar - cooling loads actually go down, quite a bit, because the panels shade the roof and the hot backside of the panel drives a good bit of cooling air through the gap.

2019-09-17 by peifferc

Thank you for this. Two questions:

You say "Sitting fully charged […] is really hard on lithium cells - and an off grid power system is likely to be doing this for a large part of the year if it’s been designed for year round use."

I assume that in your case w/ the office setup it never discharges in summer because you use it during the day mostly? But in a daily home cycle where there’s pretty significant power use from 7-10 pm and then some overnight as well won’t that be a good cycle of charge and discharge for the lithium? Or am I not accounting for something?

Your other big warning about lithium is the temperature range. I have a big basement dug… that’s already literally a sunk cost. The basement gives you some thermal regulation from the surrounding earth for “free” and of course it’s connected to the top half of the house which is climate regulated. Does available basement space change the calculus for lithium ion? You do say “So, what this means for off grid use is that you really either need to have the batteries inside where they’re thermally regulated…” I’m wondering if there are other gotchas behind that. Still too risky?

2019-09-17 by Russell Graves

If you back off the charge voltage for lithium so it’s not charging past 80% or so in the summer, the sitting fully charged becomes less of an issue. It does require you to manage the charge voltages on an annual cycle.

Even if it’s getting used from 7PM-10PM, sitting fully charged most of the day is hard on the cells. If it’s recovered at 11AM, and sits at 100% SoC until 7PM, that’s going to stress the cells. If you’re doing lithium and care about longevity, I’d suggest keeping them to 4.1V/cell max year round, and maybe backing off to 4.0V/cell during the summer. You’ll want a BMS that can balance at those lower voltages, though, and I’m not aware of such things being easily found.

If you have a basement, you should be fine with lithium on the temperature front.

Do you have to meet NEC? If yes, then the answer will come down to, “Can you convince your local AHJ and inspector that your lithium bank is compliant?” If it’s a pre-built solution, they should allow it, though those tend expensive. I’m not sure you can get a homebuilt solution to pass electrical inspection. But I don’t know your area. I live in an irritatingly strict area that apparently has inspectors who refused to believe that a PowerWall was a battery and not an EV charger (because it said Tesla on it).