Full Off Grid Solar - Design Help Needed

blaise

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First post here. Sorry for not being a contributor before asking for help. If there is an Amazon referral link for this forum or a donation page, I'll be more than happy to give back.

I'm designing/building my primary residence in E. Washington state. 16ac of very secluded land. No power available anywhere. I have some background back in the day as a 2 year residential journeyman electrician but that was 20+yrs ago and my math is bad. ;)

I've done quite a bit of reading but want any advice from the vets if I'm on the right/wrong track.

Here is where we are:
1. We have done our power audit and we are a solid 8500/11,000(w/loss) daily watts on the worst day (Mini split, laundry, vacuuming). This includes everything except my shop which will have its own system but not for 12-18 months after the house is finished.
2. Just my wife and I and our 2 dogs. No kids and probably not many visitors.
3. Budget build. Every for the house build is cash unfortunately and will have to make every dollar go as far as we can.
4. House is 1 bedroom/1 bath total of 1232sq. Won't have a garage.
5. Well pump power has been figured out and is on its own system.
6. According to all the maps and charts I feel good about the number of 4hrs of sun light a day.
7. Solar panels will be free of any obstacles and on rotating platforms ran by an Arduino board to get the maximum sun.
8. I have decided on a 24v system.
9. Will have a dual fuel generator for those cloudy times and as a backup.
10. This will be 99% DIY. The extra 1% will be the help and advice everyone gives me!

Here are a couple questions I have.

If I'm at 11kw daily at 24v then I need 460ah of battery. Is this correct? (Wh)/(V) =(Ah)

Then I have to double that so I don't discharge more than 50%? So does that means I want 940ah of batteries?

And then I want 3 days of total battery capacity so I want 2820ah of batteries?

I've decided on 12v 100ah (C20) batteries. This is all I can afford unfortunately and I have 4 of these in our Sprinter van so I'm familiar with them. So does this mean I need 29 batteries? Wait, I'm 24v so now I need 58 batteries because I have to double my voltage but it doesn't double my amp hours. Is this all correct so far before I go any further?
 
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iamrich

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If I'm at 11kw daily at 24v then I need 460ah of battery. Is this correct? (Wh)/(V) =(Ah)
You need 11kw of PV.... Battery size is important, but if you don't make at least what you use, you will run out of battery eventually.

Then I have to double that so I don't discharge more than 50%? So does that means I want 940ah of batteries?

If you are using lead acid. If you are using LiFePo, I would figure 80% usable.

And then I want 3 days of total battery capacity so I want 2820ah of batteries?

That's a poop load of batteries... but yes...

I've decided on 12v 100ah (C20) batteries. This is all I can afford unfortunately and I have 4 of these in our Sprinter van so I'm familiar with them. So does this mean I need 29 batteries? Wait, I'm 24v so now I need 58 batteries because I have to double my voltage but it doesn't double my amp hours. Is this all correct so far before I go any further?
Yes, but I don't think that is feasible. You are going to need a second house for you batteries...
 

blaise

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You need 11kw of PV.... Battery size is important, but if you don't make at least what you use, you will run out of battery eventually.
So 11kw of PV. From what I have read this is what I'm figuring. Tell me if I'm doing this wrong please. 190 watt panel * 4hrs * .8 = 600 watts from each panel. 11,000/600 = 18 panels. Wait, I'm 24v so that means I need 36 panels?

The batteries I'll be using are AGM. I know they aren't the best but they are affordable. I'll have more cash in 3yrs to swap them out if I need to. I think it is safe to say 50-60% for these batteries based on my current use in my van.

Is 3 days of reserve too much? I don't feel that my power usage is that out of whack for a normal 2 personal household. What am I doing wrong here?
 

iamrich

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I am not an expert by any means, but I can fake it :)

I would get used panels if you don't have to deal with permits. You can get 250w panels for $50 each and if you are buying a bunch the shipping is not too bad. I would get as many panels as you can fit. I know you are on a budget but panels are the cheapest part (if bought used) and the shipping is for a pallet.

total wattage needed / Hours of sun (worst month calculation like winter sun low in the sky) / wattage of the panel and then I would fudge that by 20%. So 15kw of power a day (go big), divided by 3 hours of sun = 5kw divided by 250w panels = 20 panels + 20% would be 24 panels.

Batteries are another story. I think LiFePo can be competitive with AGM, but it would be DIY.
 

blaise

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So 24 panels but do I have to double that since I'm wiring my batteries at 24v so I need to feed them 24v or does the MPPT charge controller figure all that out?

The cheapest LiFePO4 12V 100ah batter I could find was around $450 where the AGM is $200. Am I a bad shopper? Lol
 

Tony04

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Hey guys , first post here , Blaise are you strapped to 24volts? , with something that large why not go with 48v system ?
 

blaise

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I'm not sure I see the advantage of going to 48v. Wouldn't I have to buy double the batteries?

FYI the maximum watts we would pull at any given time would be 1300 continuous with 1500 startup if we are turning on the mini split. For instance we wouldn't have the AC/heater on and vacuum at same time or run the washing machine and sit and watch tv or run the oven. We've lived for 2 years strictly on solar so we know how to be smart about our power but this was "van life" that isn't the same as living in a house.
 
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boondox

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Go 48V. No, you will not need more batteries. 48V means smaller wiring and better equipment options.

Also, you may not need as much power as your audit indicates. For example, you can do laundry during the day. If so then you don't need to factor it into the battery size. I haven't done laundry at night for over 20 years. I do all my high power stuff like welding, laundry etc. during the day. That way your battery only needs to be sized for what you have to have at night.
 

blaise

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Go 48V. No, you will not need more batteries. 48V means smaller wiring and better equipment options.

Also, you may not need as much power as your audit indicates. For example, you can do laundry during the day. If so then you don't need to factor it into the battery size. I haven't done laundry at night for over 20 years. I do all my high power stuff like welding, laundry etc. during the day. That way your battery only needs to be sized for what you have to have at night.

So with 56qty 100ah batteries I'll have 5600ah if I wire them at 24v or 48v then? I'm fine with 48v then no problem.

What about the solar panels then? Do I wire those at 48v as well then to match the battery bank voltage or will the charge controller manage that?

Also is this generator a decent piece for my needs?
https://www.generac.com/all-product...-7172-16-circuit-transfer-switch-wifi-enabled
 

boondox

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On thing that will help is to think of it in watt hours, not amp hours. It makes it much easier to compare apples to apples. As to the solar panels, I would wire them in a series/parallel arrangement that added up to the highest voltage that your charge controller can handle*. The charge controller will step it down to the required charging voltage.

* You want to make sure that under no load cold conditions that the panel voltage does not exceed the maximum your charge controller can handle. Every panel has a voltage temperature coefficient. You can use that along with the coldest temperature your are will see to make sure you stay within spec.
 

boondox

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Oh, as to the genny, that depends. For one thing, what is the charge capability of your system? This is usually done by your inverter. For fuel efficiency loading a genny to 2/3 to 3/4 will give best fuel economy. However, some chargers (like mine in the Magnum inverters) are in "pass through" mode when genny charging. This means that the inverter is busy charging the batteries and can not invert. SO, any loads that you put on the system will be "passed through' to the genny. If for example the water pump starts up or someone makes a ot of coffe while charging (both common at my place) the genny needs to be able to handle the extra load. If it can't the voltage and frequency will drop and the system will kick the genny offline. It will try again periodically until charge is complete.

In my case I can charge at about 4kW and I have a 7.5kW genny. Occasionally if a bunch of stuff is on it will get kicked offline. When the coffee or shower is done it starts charging again.
 

blaise

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Thank you so much boondox for explaining that. I think that generator should be good for my needs initially but I'll make the final decision once the rest of the pieces and parts are selected.

So now going to the charge controller, I was looking at the Victron 250/70. I'm thinking of Victron because I put the 100/50 SmartSolar in our Sprinter van and I really like the ease of use and the app on my phone. Really simple to use.

So with the panel advice from iamrich above, the 250w panels have a VOC of 37.6 V and max current of 8.27 A. So if I wire these 24qty 250watt panels up series/parallel (6x4) this would bring come to 225.6v and 33 amps. Does that math sound correct?

https://www.victronenergy.com/solar-charge-controllers/smartsolar-250-85-250-100
 

boondox

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Math looks fine. But make sure that on your coldest day your voltage won't be too high. Here is a link to a calculator.

 

blaise

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Oh that is something I didn't consider. Thank you!

So would it be better to wire the panels at 4x6 instead? That would yield 150.4v and 49.62a instead right?

I might be smart to go with the Victron 250/100 for an extra $200 to have a little more upgradability.
 

boondox

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I would do the math. If it works at 6x4 that is the way I would go. Less wires into the combiner box and less breakers. If that is too high voltage on a cold day then 4x6.
 

iamrich

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I might be smart to go with the Victron 250/100 for an extra $200 to have a little more upgradability.
Yes. If I am reading the specs correctly, you need to run 48v batteries as well. 250wx24 puts you at 6000w which is over paneled, but not by much and I think very doable. I might actually go 5P5S which would put you nominally at 6250W and ~150v and ~42a

1619056580730.png

As Boondox mentioned, you have to change your way of thinking. Run everything you can when the sun is up. Your peak "get stuff done" hours will be the same as your peak sun hours. Dishwasher, laundry, vacuum, AC, etc. You should have plenty of PV to run all that and charge the batteries, and then at night you switch to only light stuff (AC if needed, TV/Laptop, led lights, etc.).

For batteries you said 11kw max, but I think I would just start at 1 day (48V 230ah) and go from there. You can always add more. That would be 20 12v 100ah batteries arranged 4S5P (48V 500ah / 50%). I still think you have better options for batteries, but it's up to you. If you are willing to build DIY LiFePo batteries, you can get 16 3.2v 280ah cells for about $100-150 each ($2400) and that will give you 11kw using conservative 80% (charge no higher than 90%, drain no lower than 10%). But there is nothing wrong with sticking with what you know. Chances are you can go for days (weeks?) without laundry, and if the sun isn't out, that AC is probably not on, so your use might be a lot less.
 

iamrich

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7. Solar panels will be free of any obstacles and on rotating platforms ran by an Arduino board to get the maximum sun.
Also I always forget to mention, you can static mount your panels facing different directions to lessen the overall wattage, but spready it out longer during the day. Say 1250w facing southeast, 3750w facing south, 1250w facing southwest. I don't know if a 20-25 panel array is going to work with #7 (I don't even know what that is :) ).
 

blaise

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I've been reading quite a lot this evening. You and Boondox seem to give lots of advice out so thank you for all the contribution to the community. I co-run a very large forum in a completely different subject and it's people like you both that make the difference.

For my inverter I'm thinking the Magnum MS4048. Is there something different I should be looking for besides what this has to offer? It might be a little overkill but the last thing I want are tripping breakers from an under powered inverter (something I've dealt with in my Sprinter van for a while).
https://www.magnum-dimensions.com/product-inverter/4000w-48vdc-pure-sine-inverter-charger-ms-series
 

Blakes

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I too am still learning, but have lived 4.5 years 100% offgrid in Colorado. My system has evolved (and still is) but currently 1.8kw of panels going into 48v renogy inverter/charger and 8 renogy 200ah batteries.

My system requires, I dunno, a good 5 or 6 hours of good sun in order to not run the generator that night.

This is in a log cabin that we are currently building and living in the basement. We WFH so here all day.

I have a couple of little yamaha 2000w generators. Pour a gallon of gas and let it run out.

Works for us
 

blaise

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Also I always forget to mention, you can static mount your panels facing different directions to lessen the overall wattage, but spready it out longer during the day. Say 1250w facing southeast, 3750w facing south, 1250w facing southwest. I don't know if a 20-25 panel array is going to work with #7 (I don't even know what that is :) ).

It is a solar tracker based on data from some database. So the panels move with the sun's angle and such to get maximum solar.

 
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