Need help designing off-grid system

[Bigbus]

First some background.... I am trying to design an off-grid system for my tiny home in Seattle. An energy audit showed that I will use (By my best estimate) 1670wh dailly. I already have 6 245 watt panels that I would really like to be able to use. Everything that I am using will be 120v. So, lets get down to it.

Charge controller - I'm confused by this. I have looked at countless string calculators and have yet to find a CC that can handle my 1470w array at 12v. Does that exist? Am I stuck with making a 24v or 48v system? If I end up needing a 24v or 48v system because of my panels, how will I use my 120v loads? At what point does that get sorted out?

Batteries - tell me if I'm doing this right. 1670wh * 2 (for 50% discharge) * 3 (for cloudy rainy Seattle days) = 10,020wh of storage needed. Thats 835ah if its a 12v system and 417.5ah if its 24v. I am thinking about using 6v batteries because they are inexpensive. So that would be 2 or 4 in series to get 12v or 24v and then the corresponding amount in parallel.

Inverter - According to my calculations, my peak load will be 1709w. That means I need a 2000w inverter right?

What else do I need to think about? Someone drop some knowledge on me. (I'm looking at you @Sunking)

[Sunking]

You have a hurtle you cannot overcome. You live in Seattle WS, the absolute worse place in the lower 48 for solar. Months from about mid November to end of March you have 2 Sun Hours or less. Christmas week and you are down to 1.2 Sun Hours. To give you an idea of what that means if you built a system to generate 1 Kwh of usable power would require 1200 watts of panels with two very expensive 50-amp MPPT charge controllers . Same requirement in say Dallas TX would only need 300 watts and a small 20 amp MPPT Controller.

Bad news does not end there. With so little Sun, you have to generate a lot more power in a short period of time to overcome not having enough Sun Hours. The charge currents would exceed all Flooded Lead Acid specifications, forcing you to use AGM batteries. Why is that bad news. Because AGM cost twice as much, and last half as long as FLA making them 400% higher cost.

[Mike90250]

If you can change your location eastward, and get out of the gloom, you would be in better shape. But as it is planned, you will be on generator power for recharging 3-4 months a year.

As to battery bank voltage. 12V is out. 24 or 48V are your only choices. And then it's only AGM or Li-IonLFP batteries, so you can charge them quickly with minimal generator run time.

As to your inverter size, WTH - "peak load will be 1709w" That's crazy. Unless you are making toast while blowdrying your hair. That's going to make me ask to see your calculations and what you have done.

[Bigbus]

Thank you for the reply! Ok, I'm sorry I misspoke. I am going to school in Seattle but I will live closer to Yakima. Clearly that makes a big difference because I will get 5.7 kwh/m^2/day yearly average and that drops down to 2.8 in December. I fully expect to have to use a generator to keep my batteries healthy during the worst weeks of it. But of course I want to minimize that as much as possible.

As far as batteries go, given what I just said, am I still stuck with no 12v and no lead acid?

Yep, you're right. Sorry again. I got mixed up because I have a clothes dryer that I want to be able to use in the winter months. Without the dryer, daily usage is 1670wh at a peak load of just under 300 watts. With the dryer, I get bumped to 2840wh at a peak load of 1709w. Now because I only have to use the dryer during winter months when I will likely need to use a generator anyway, I don't necessarily need my panels to be able to support it, however, I do want the rest of my system to be able to run it. So I'm not sure where that leaves me with the inverter?

Thank you guys so much for your help!

[Mike90250]

You NEED a propane fired clothes dryer. Using a generator gives you an expensive gasoline dryer. Remember all the generator run hours require maintenance, usually an oil change every 50 hours

[Sunking]

Why on God's Green Earth do you want to pay 5 to 10 time more for electricity the rest of your life (vs buying it from the POCO) and become a heavy polluter?

Please explain that in detail. As for the clothes dryer is never going to happen like air conditioning, electric hot water, and electric cooking is not going to happen off-grid. Do you have any clue what you are asking for? The very fact you uttered 12 volts tells all of us you are stuck inside a 12 volt box. Batteries are not 12 volts.

[J.P.M.]

Thank you for the reply! Ok, I'm sorry I misspoke. I am going to school in Seattle but I will live closer to Yakima. Clearly that makes a big difference because I will get 5.7 kwh/m^2/day yearly average and that drops down to 2.8 in December. I fully expect to have to use a generator to keep my batteries healthy during the worst weeks of it. But of course I want to minimize that as much as possible.

As far as batteries go, given what I just said, am I still stuck with no 12v and no lead acid?

Yep, you're right. Sorry again. I got mixed up because I have a clothes dryer that I want to be able to use in the winter months. Without the dryer, daily usage is 1670wh at a peak load of just under 300 watts. With the dryer, I get bumped to 2840wh at a peak load of 1709w. Now because I only have to use the dryer during winter months when I will likely need to use a generator anyway, I don't necessarily need my panels to be able to support it, however, I do want the rest of my system to be able to run it. So I'm not sure where that leaves me with the inverter?

Thank you guys so much for your help!

Seriously: Get a solar clothes dryer. Cost is close to zero and maintenance is effectively zero. Known by it's more common name: Clothes line.

[jflorey2]

fully expect to have to use a generator to keep my batteries healthy during the worst weeks of it.

OK good.

As far as batteries go, given what I just said, am I still stuck with no 12v and no lead acid?

12V is appropriate if you are going to use very little power (i.e. under a few kwhr a day.) Beyond that, go with 48 volts.

You can get 120VAC inverters for almost any DC voltage input, and 48V inverters are almost as common as 12V inverters.

I got mixed up because I have a clothes dryer that I want to be able to use in the winter months.

As others have said, absolutely not on an AC heat clothes dryer. Get a propane one. (Can also use the propane for generator, heat and cooking.)

Solar is cheap nowadays so I'd plan to overpanel and limit with your charge controller to the max your battery bank can handle. Plan for 3-5 days autonomy with a max discharge to 50%. So that's a minimum of a 10kwhr battery bank. That would be a minimum of 8 T-105 batteries wired for 48 volts. Your max charge rate for those will be ~30 amps. I'd go with a ~2000 watt array and current limit to 30 amps. (You'll almost never see that high a power anyway.)

[Mike90250]

Bigbus. You need to post using a computer, not a phone or tablet, the anti spam software senses the oddball character set and truncates your posts

[Bigbus]

About the clothes dryer - it's a washer/dryer combo that I will use as a washer without the dryer 95% of the time. The only time I ever want to use the 1.08kwh it uses to dry is during the winter when it is freezing outside and I can't use a clothes line. If I already need a generator to make it through the winter, why can't I use it for an extra hour every couple of weeks to dry my clothes?

To explain in detail why I need to be off grid - I am a back country ranger. While some back country ranger stations have electricity, many do not. It's as simple as that. I can't buy electricity because its not out there and I cant pay to have it put out there because its government land.

Batteries - Ok, I get that I need to have a 24v or 48v system. That answers my question.

Lets move on.

I will have 6 245 watt panels wired 3 in series and 2 in parallel in order to give me higher voltage and therefore smaller wires.
These will go to a 150/70 charge controller.
The charge controller will charge a 24v battery bank capable of storing 14.7kwh equivalent to 612.5 ah. I got this number by using the reverse of the calculations Sunking does in the stickied post "solar off-grid battery design". I want to maximize the 1470w PV I already have so I plugged it into the equation, panel watts = (daily wh x 1.5)/sun hours, which for me is 1470 = (daily wh x 1.5)/3. That gave me a pretend daily wh of 2,940. I then plugged that into the next equation which is (5 x daily wh)/battery voltage = battery amp hours. That puts me at 612.5 amp hours. To get a bank that size, I will have 12 6v batteries. 4 in series and 3 in parallel.
From there, the power will go to a 24v inverter. I still don't have an answer about the inverter. I am going to the use dryer sometimes, albeit rarely. My thinking is that it will be drawing its power from the batteries, even if I have to charge the batteries with my generator in order to do that. In that case, I would need an inverter that can handle the load of my dryer. Which is why I said a 2000w inverter. Now, other than the few times a year that I use my dryer, I will have a peak draw of less than 300w. So I understand that a 2000w inverter is bigger than my usual needs. So my question is what do I do about that? I know that I want to use this dryer sometimes. Will you please help me figure out a way to do that? I suppose one solution would be to just plan on unplugging the dryer from the outlet and plugging it into the generator. That would be fine. Any other ideas?

I understand that my system is bigger than what I have estimated as my daily needs. But those daily needs are bare minimum. Because I already have the panels, I think it is worth it to spend the extra money to have a system that matches those panels. Rather than telling me I don't need all of it, or trying to talk me out of it, I would love to hear if my calculations are sound and if they are safe.

jflorey2 any reason you say to do 48v instead of 24?

[bcroe]

Your system can be far more economical if you plan to run your dryer, etc directly from the generator,
or use propane. 300W is one thing, a 2 KW inverter is a game changer. Bruce Roe

[Mike90250]

> 12 6v batteries. 4 in series and 3 in parallel.

48V, I'd suggest only 8 batteries, all in series, and use a small inverter. Run the dryer directly from the generator. Then you don't need to plan for a huge battery

When I was refreshing my NiFe bank, I ran my site for a week, off 4, group 27 batteries, Charged daytime by solar, and night, with a honda eu2000

[Xplode]

Mike is right with suggesting 48V. Given the 10KWh storage you're looking at, It seems a good way to go.

If you haven't done the reading on how to recharge batteries yet, you should poke through the stickies up top in this forum. as sunking mentioned, charging rates for 12v battery banks are pretty limiting. The TLDR version is that you need to size your charge controllers to match somewhere around the C/8 - C/10 of your battery bank. so for your T105 batteries that's about 22-29 Amps. That makes a 30A Charge controller right on the upper end of perfect. Any higher and you'll boil your batteries, drastically shortening their lifespan. Since they're the most expensive part of the system, treat them like your baby.

If using a bank at 12V you can charge @ 30Amps max, that is about 360Wh of stored energy per hour (in perfect sunlight) that you can inject into your batteries... (This is overly simplified and ignoring losses/charge voltages/decaying panel outputs/ect you will get less) So really, you'd only be able to utilize about 2 panels properly. Any more is a waste of money and may overpower your charge controller, depending on what you decide to use.

Bump that bank to 24V and you can charge 720W per hour... Sooo 3-4 Panels will be usable...
Bump that bank to 48V and you can charge 1440W per hour... Sooo 5-7 Panels will be usable....
Think about how that works with only 2 Sunlight Hours per day in the winter months.

And guess what? You didn't need to buy bigger wiring or charge controller because you're still at 30Amps charging... which should help keep your equipment costs down too...

Now, the idea of paralleling your batteries might seem good to increase capacity while keeping the voltage lower, but I'm led to understand that it becomes difficult to keep the individual batteries healthy and balance charging currents evenly. As the batteries age, they will become unbalanced and cause strain on the other cells leading to a shorter lifespan.

Now - I'm new to the game so I might not have explained it the best, and I'm just trying to be helpful, but I just built myself a 24V off-grid system for my part-time living cabins using all the readings/advice on this forum and it's working great. so these guys will correct me if I'm wrong :P

[Bigbus]

10kwh doesn't seem like enough. That only gives me about 2 days of usable energy. But I suppose the alternative is 16 batteries. Which would be insane. However, putting them only in series makes an issue for me with internal resistance, right? I would have a battery bank that is 48v and 215 amps. So that would be 215/8 = 26.9. Meaning the max current I can apply to my battery is 26.9 amps. Isn't that a problem?

[Mike90250]

...... Meaning the max current I can apply to my battery is 26.9 amps. Isn't that a problem?

But you forget the other half of the equation. Higher voltage = lower amps. Wattage stays the same.

1500w load @ 12V = 125A
but at 24V it's only 62A and 48V only 32A With the benefit you can use thinner cables