Is my math/logic sound?

[FlyFishn]

I have a spreadsheet I've been working on for some time analyzing the feasibility of alternative energy.

Does the following make sense and do you agree with my numbers?

What I have figured for a general starting point is 16.366 KWh per day usage. That isn't exact, and may be on the high side, but lets roll with it to illustrate the rest.

I have factored in 3x 0 production days in to my numbers. So by this number - I need to have stored (3*(16.366KWh)) = 49.098KWh.

That is where the numbers probably get a bit wonky. Reason being - the usable capacity of any battery isn't necessarily what the label is.

For lead-variant batteries I am figuring a 20% DoD. That is, only 20% of said label capacity is usable. Beyond this the lifespan of the batteries is impacted.

For lithium-variant batteries I am figuring 60% DoD.

Base system voltage is 48 volts.

So for Lithium-variant batteries at 60% DoD I need ((16.366*3)/.6)= 81.83 KWh. Or, converting to Ah that is (81.83/48)(*1000) = 1705 Ah.

For lead-variant batteries that is ((16.366*3)/.2) = 245.5 KWh, Converting to Ah that is (245.5/48)(1000) = 5114 Ah, or about 3x as much label capacity as the lithium-variant batteries.

***The numbers above have some rounding in them, I pulled them from my spreadsheet without factoring in all the decimals for clarity***

[bigrockcandymountain]

I suggest 50% dod for lead.  The cycle life graph is pretty linear, and they do eventually fail from age even without deep cycling.  Also, your worst case 3 days with zero production only happens a few times a year, so cycling even to 70% dod on lead would be permissible in my opinion.

Also, with solar, there are almost no days with zero production. 

Another factor is your average usage doesn't have to be your cloudy day usage. Run your big loads when the sun is shining makes a world of difference.

I would rather you put your money into oversizing the solar array vs a huge battery. 

If you run with the numbers you have, I expect you'll conclude that off grid is 100 miles the other side of feasible.

[OperaHouse]

At home my utility sends me these charts that say I am an above average user.  At my summer home, I live on a car battery which meets my needs. I still have PV hot water, refrigeration, dishwasher with heated dry, large capacity clothes washer which has its own hot water tank (all cycles are done with hot water).  I chose extra panels vs big battery and to use power directly when it is created. It was a super cheap system because I don't have to do things any time I want. Energy use is prioritized so everything doesn't come on at once. Lifestyle changes make it possible.  Replacing grid power otherwise is not cost effective.

[DamonHD]

Up here in the wilds of London December days can be very low on solar, but I agree that more solar and less battery has worked better for me, and 50% DoD is fine for grown-up deep-cycle batteries.

Rgds

Damon

[kitestrings]

Hi FlyF,

One of the first things I tell folks that are looking at off-grid, is to lower their expectations ;>].  What I mean is start by reducing the electrical loads any/every way possible.  So, no electric heat or hot water, unless it it is thru load diversion (possibly/modestly with heat pumps).  If you can provide those tasks with wood, LP or solar that lops off the (typically) biggest residential loads.  Think LED lights, and not just good, but the very most efficient appliances you can find/buy.  Reduce as much as practical the size of the refrigeration needed.  I sometimes talk with folks who are smug about their new EE frig, but you could park a car in it, and it is only EE relative to comparable's of the same size.  Hang clothes; ditch the AC if possible.  If you focus on those details, you can scale down the battery requirement quite a bit.

I otherwise agree with most of the comments.  We try to 'make hay when the sun shines', so laundry, concerted cleaning, shop work... we try to do more on sunny/windy days.

We've also had real good results with load diversion.  If you can use charge controllers that have that feature it makes it easier.  Midnite Solar is one example.  So, batteries get full... okay lets tuck some of that otherwise lost energy into water preheating dhw, or similar loads.  Storing Btu's can be cheaper than batteries.

All that said, I recommend as robust a bank as you can afford.  We have a family of five, and our bank is 1,200 AHrs @48V, or about 30 kWh at 50% DOD.  Batteries are a major investment, and at best might last 15-ys (lead-acid).  The cycle life vs. DOD looks like this for ours: