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Old 15-10-2020, 20:56   #1
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House Bank Sizing Question

Hi All, it's time to replace and up-size my house bank. I have my total amp draw per 24 hours, now I need to know what formula to use so that I have say, 60% state of charge remaining at the end of that time. Thanks!
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Old 15-10-2020, 21:53   #2
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Re: House Bank Sizing Question

Amount disharged(Ah)/capacity used.

Example, say you use 100Ah between charging cycles (24 hours or whatever you prefer) and you are looking for 40% used (60% remaining) 》100/0.4=250. You will need a 250 Ah bank to be at 60% SoC (remaining capacity) after using 100 Ah.

Decide on your deepest desired discharge and your worst case charging scenario. Say you disharge 100Ah/day, mostly overnight because.you have solar. On good days solar replaces that all. On bad days it only replaces 50Ah. At the end of day 1 you are at -50Ah, then you go through another night, now you are -150. If you think one day of bad solar is your worst case then 150/0.4 = 375.

Same basic thinking applies regardless of how you charge, evaluate the conditions that would cause you not to get to full charge and how long it will be until you do get a full charge.
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Old 15-10-2020, 22:32   #3
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Re: House Bank Sizing Question

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Originally Posted by Dsanduril View Post
Amount disharged(Ah)/capacity used.

Example, say you use 100Ah between charging cycles (24 hours or whatever you prefer) and you are looking for 40% used (60% remaining) 》100/0.4=250. You will need a 250 Ah bank to be at 60% SoC (remaining capacity) after using 100 Ah.

Decide on your deepest desired discharge and your worst case charging scenario. Say you disharge 100Ah/day, mostly overnight because.you have solar. On good days solar replaces that all. On bad days it only replaces 50Ah. At the end of day 1 you are at -50Ah, then you go through another night, now you are -150. If you think one day of bad solar is your worst case then 150/0.4 = 375.

Same basic thinking applies regardless of how you charge, evaluate the conditions that would cause you not to get to full charge and how long it will be until you do get a full charge.
Sure would be nice to only have to worry about making it 1 day without sun on the panels I estimated 3, and in fact we've seen 5 straight days multiple times with less than 50% production during those days. House bank starts getting mighty thin ...
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Old 16-10-2020, 00:09   #4
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Re: House Bank Sizing Question

Ah used per day x 4 or 6.., Gives you 2-3 days To 50% without charging.
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Old 16-10-2020, 03:55   #5
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Re: House Bank Sizing Question

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Hi All, it's time to replace and up-size my house bank. I have my total amp draw per 24 hours, now I need to know what formula to use so that I have say, 60% state of charge remaining at the end of that time. Thanks!
Simple math. Take your 24 hour energy consumption and divide by 0.4 (40%, the amount of battery you use). For example, if you use 300ah per day, you need a 750ah battery bank (300! 0.4 = 750). Whether you should over-size or plan for multiple days is another question. But the answer to the exam question is simple math.

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Old 16-10-2020, 06:07   #6
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Re: House Bank Sizing Question

Thank you for that post, Dsanduril. I learned from it.

Blaine, it sounds like you need to be doing another calculation on genset charging capacity. Pretty soon you're going to sink your boat with your house bank....
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Old 16-10-2020, 09:00   #7
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Re: House Bank Sizing Question

This was not a how-do-I-charge, nor what-type-battery-bank question, but both are more important than the size of the bank.

FLA (Flooded Lead Acid) are the least expensive per amp hour capacity, but have the downside of extended periods at float to fully charge. Solar does a nice job of topping off other charge modes or, if you have massive overcapacity on solar, the ability to do it successfully most days (which presumes a good controller as well).

OTOH, if you're up for the science project aspect of it, LiFePO4 batteries can be much more fully discharged (ex-FLA boats commonly do well with half the AH capacity) as well as accept full charge amperage right on up to full.

Or a variety of other scenarios which will impact how you plan your charge routine and necessary bank capacity.

So, what battery type and charge sources are you expecting?
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Old 16-10-2020, 09:12   #8
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Re: House Bank Sizing Question

Yes, the question as asked is simple, and yet an entire chapter in a suitable book would probably be devoted to the topic.

The OP says Marina del Rey and Grand Banks 42 M/Y. I'll go out on a limb here and guess the use case:

Batteries are charged at the dock in MdR from shore power. Leave Friday afternoon and head over to Catalina. Batteries still fully charged on arrival (it's a motorboat). Pick up a mooring at (Fourth of July Cove?). Family enjoys Saturday morning swimming and playing in the water. So, battery discharge is about 24 hours (unless you ran the generator in the morning to make coffee or use the microwave). Saturday afternoon family wants burgers, fries, and milkshakes for dinner, so run down to Avalon or Cherry Cove. Batteries get some charge (maybe to 90%?). Wake up Sunday, still not fully charged, but not too deep. Sunday night back to MdR to be ready for the Monday grind?

In that kind of use case LA batteries are perfect. They spend 40+ weeks a year fully charged on shore power. If the use case really is a weekend and/or the generator runs every morning or evening then all is pretty good with the OP's original question. If the use case gets to spending a week or two away from the dock in the Channel Islands, and minimizing generator usage then it becomes a whole other can of worms. And if the use case becomes heading up the coast and spending summer off-the-grid in the PNW then it is entirely different again.
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Old 16-10-2020, 09:30   #9
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Re: House Bank Sizing Question

Following. Lithium batts still scare the *** out of me, and I'm reluctant (yet) to go down that road. We've used LA 6-volts(Trojan T-105s) for years, and its tough to fix something that's not broke! As full-time live-aboards, but with solar +wind+genset that we run daily(we have 230v AC refer and 230v AC watermaker), our 6 T-105s have worked well so far. We seldom see less than 85%SOC.
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Old 16-10-2020, 09:49   #10
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Re: House Bank Sizing Question

Well, since this seems to be expanding a bit, I use two values to calculate bank size; typical (daily) depth of discharge (DoD - i.e. capacity used) and maximum occasional DoD.

For LA batteries and 365 boat living I like to see daily DoD at or less than ~20% because that gives the best cycle life. I also like to use ~50% (or maybe 60%, depends on frequency) as the worst case DoD. Using some round numbers for ease of calculation:

My usage is 50Ah/8 hours (that's 150Ah/day). I have solar that can easily handle all daytime loads and bring the batteries back to full on a good day. So, I start discharging at 4pm and start charging at 8am. That gives 16 hours of discharge = 100Ah. For 20% DoD that gives 100/0.2 = 500Ah bank size. With nice sunny days all the time that's all I need.

Estimating the worst case becomes trickier. If I have no solar production then at 4pm on day 1 I am -150, and at 8am on day 2 I am -250. If I again get no solar production, at 8am on day 3 I am -400. Say that I'll fire up a diesel charging source in that case. 400Ah/0.5 (worst case DoD) = 800Ah bank size. It's going to take some charging time to get that 400Ah back in the batteries.

Impossible from a distance to know what use patterns someone has and what charging profiles are available and likely. But if you put those numbers together you can come up with a reasonable way to estimate bank size. In this example 500Ah would be minimum and 800Ah would be preferred. What fits (both physically and the wallet) then start to come into play.

The best money you can ever spend on batteries is reducing your usage so you don't have to buy the batteries. Better refrigeration, LEDs, low power TV - whatever is your biggest power user, look at lower power alternatives and see if replacing something is less expensive than the batteries you need to buy to power it.
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Old 16-10-2020, 09:59   #11
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Re: House Bank Sizing Question

What he ^^^^ said!
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Old 16-10-2020, 10:01   #12
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Re: House Bank Sizing Question

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The best money you can ever spend on batteries is reducing your usage so you don't have to buy the batteries. Better refrigeration, LEDs, low power TV - whatever is your biggest power user, look at lower power alternatives and see if replacing something is less expensive than the batteries you need to buy to power it.
This is certainly the big one. Every W you can trim compounds throughout the day. And if you're hoping to get through multiple days of no sun, trimming 10W can suddenly become 500Wh+ saved. Multiply that times a dozen lights on board, and you've earned yourself a whole battery bank.
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Old 16-10-2020, 10:13   #13
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Re: House Bank Sizing Question

Yes, multiply typical daily use by three, gives a little over 60% but remember you bank will slowly loose capacity over time so the extra is good. What is critical is not the final SOC (which will vary every day) but ensuring sufficient backup power in case of electrical system or engine problems.
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Old 16-10-2020, 11:02   #14
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Re: House Bank Sizing Question

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Sure would be nice to only have to worry about making it 1 day without sun on the panels I estimated 3, and in fact we've seen 5 straight days multiple times with less than 50% production during those days. House bank starts getting mighty thin ...

OP has a 42' Grand Banks so wouldn't it have a diesel genset?
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Old 16-10-2020, 11:04   #15
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Re: House Bank Sizing Question

Not a direct answer, but I have priced up every type almost of battery including a tesla battery...

I am either going to go for a tesla 24v battery. Or lifep04. Size of the bank and ability to deep discharge without worry, super low internal resistance means everything is easier.

Its taken me months of research and review reading, and more research. You won't see me post the install till next year because I'm currently on 12v and tesla is 24v, but when I do I will post figures etc to justify it 😁

I hope this helps in some way!
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