VOLTS How to determine

James Staples V

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The first things that knocks me down is Voltage . 12, 24 48 100 ?? How do i determine which will be best for the job?
 

Bob B

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Please tell us something about your system requirements.

How much power per day needed ..... max and typical current draw / power requirements.
 

smoothJoey

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Have you done a energy audit?
If not, please see my signature for a helpful link.
The results of an energy audit will go some ways to helping determine your system voltage.
Also is this a mobile setup or stationary?
Will you require single phase 120VAC or 120/240VAC?
 

James Staples V

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The first things that knocks me down is Voltage . 12, 24 48 100 ?? How do i determine which will be best for the job?
I used 1920 mvh last month . As high as 3k in the winter [ last winter was an extreme - I hope] why would I want one voltage over another ??
 

MichaelK

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The voltage you select is proportional to the amount of current you need to pull from the batteries to supply the watts you need. A general rule of thumb is you really don't want to pull more than 80-100A through wires. Hence, the following recommendations.
1-1000W loads, select 12V
1000-2000W loads, select 24V
>2000W loads, select 48V

For a whole-house situation, I'd say 48V.

Now, in terms of what you are consuming, assuming BM is correct and you're consuming 1920kWh per month, that works out to be 64kWh per day, which is a LOT. I'm expecting you have electric heat? Electric heating on an off-grid solar system is not very realistic. Your solar resource will be the lowest, when electrical demand is highest. The best option for your situation might be a mixed bag of energy sources, being gas, wood/coal, and solar; in whatever combination works for you.

Do not think that going solar is going to make your electricity cheaper. It will NOT. Off-grid solar is the single most expensive form of electricity there is! Your best option to save money on electricity is going to be installing a grid-tie, where what you produce offsets what you are consuming.
 

Short_Shot

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Hard disagree with 100A rule of thumb.

I'd take it to 200.

A 4/0 cable, though expensive, is well over 50% safety factor at 200A for the shorter runs you would typically use from pack to inverter.

Obviously you need to de-rate for longer runs, which is why you should put the inverter nearby.

Plus most people would agree that 24v is fine for 3kw, rather than 2kw.

Using the 200a "rule" you can comfortably put a 5kw inverter on a 24v pack with plenty of overhead on cables without doing goofy and unsafe things with wiring or needing huge mcm sized cables that these inverters can't even support anyways most of the time.

This also puts your 48vdc pack in the realm of 10kw, which is perfect for 5kw split phase in the US, which gives you 2x 240v @ 20a or 4x 120v @ 20a.


You just need to use high quality components and cables.
 

James Staples V

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I mean. I have used up to 1600 myself.

Typically I'm around 1300.
Yeah it does sound like alot . But, i am wanting to be sure i have excess power for back up in case the weather makes me use more watts than expected, and the sun is kinda not so bright . As soon as my Afib attack mellows out. [been flat down for the most part this week wondering if my son will have to begin my final wishes bullshit if I don't make it the rest of the way through my 30 day heart event monitor] I will dig up my room floor plans and room needs.
I know it is rediculously redundant, but I am thinking of getting a separate units for my fridge, hvac, dialysis machine, washer and dryer. so far i have whittled my usage to 1092 kwh per month. Hanging our clothes helps.
 

JoeHam

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Yeah it does sound like alot . But, i am wanting to be sure i have excess power for back up in case the weather makes me use more watts than expected, and the sun is kinda not so bright . As soon as my Afib attack mellows out. [been flat down for the most part this week wondering if my son will have to begin my final wishes bullshit if I don't make it the rest of the way through my 30 day heart event monitor] I will dig up my room floor plans and room needs.
I know it is rediculously redundant, but I am thinking of getting a separate units for my fridge, hvac, dialysis machine, washer and dryer. so far i have whittled my usage to 1092 kwh per month. Hanging our clothes helps.

Energy conservation is the first step.

Then you may consider splitting off your “critical loads” onto an electrical sub panel and build a system to support those loads, with grid backup, not grid tie. I’ve been doing that for over a year now and it’s been a great learning experience.

IMHO it has already paid for itself simply because during a few power outages so far my wife is impressed that the refrigerators and garage door opener work fine while other things don’t 😎.

Personally, my favorite part is that the sump pump works during a storm induced power outage.

And I will also share that my 92 year old Dad has been in A-Fib for most of the past two decades. So far two battery replacements for his pacemaker and this one should last into his 💯’s. Can’t count how many times we’ve had the final wishes talk and I look forward to many more times! 😎
 

MisterSandals

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WWOOOOOow ! I thought prices sucked here !!! Can you tell me why i want a 12 vs 24 vs 48 or higher voltage ?? For the wattage and whatever else ??
In general, For a given load, amps are (inversely) proportional to voltage.
So for a 12V load, the amps would be 4x higher than a 48V system.

Imagine a 4000W inverter and the wire and fuse sizes needed:
4000W / 12.8V = 312.5A. (Unmanageable)
4000W / 25.6V = 156A. (High, Barely manageable)
4000W / 51.2V = 78A (easy with modest wires and fuses)

So look at your max power draws and work from there what wires and fuses are required. Once you start pricing 400 and 500A fuses, wires, (BMSs!!!),… you’ll see.
 

Supervstech

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If you want multiple setups for isolating loads, multiple 24V is the easiest to work with…

If you want a single system to provide maximum output for the least money outlay and safest from overheating wires, go 48V…

Remember, it is amps that heat wires… higher voltage, lower amps.
 

acdoctor

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The term watts is a singular number to express a quantity of electric power. Volts and amps is what gets the job done. If you use water as a analogy amps is the flow where volts is the pressure. Watts equals volts times amps. So if you need a small amount of watts then 12 volts will be fine because the amps won’t be very large. If you need a large amount of watts and use 12 volts you will need a large amount of amps to get lost of amps you need big expensive wires. But if you use high volts the amps goes way down and so does the wire size. If you were to try to run a high wattage system with 12 volts and ran the wires that were required you would have problems connecting the large wires on the smaller connectors provided. I hope this helps. If it does you may want to study some basic electricity before jumping into solar. Then run your plans by the forum before buying anything it can save you a lot of money.
 

Just John

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Unless you are running 12 volt appliances, go with a 48 volt system. The price is nearly identical, but wires are 1/4 the size. If using LiFePO4 cells, then you need 16 for a battery rather than 4, but for large amounts of power, you will be glad you did.
 

SolarJourney

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Just joined guys and this is my first (hopefully helpful) post. In the simplest of terms, without a bunch of science and confusing details:

The higher the voltage, the thinner the conductor (wire) required to transmit enough power for a given application. It's worth noting that nothing comes for free, and insulation requirements go up as voltage goes up, but since plastic is cheaper than copper, you still come out ahead.

Example:
Take your standard electric 50gal home hot-water heater.

They usually run off of 240v. For the purposes of this thought experiment say for whatever reason you wanted the same amount of electrical heating but you wanted to run it at 12v. Instead of the thin wiring that your house uses, you would need MASSIVE copper conduit to accomplish the same amount of work (power).

The powerlines outside your house take advantage of this, they transmit power over long distances at extremely high voltages, and then when you need power at your house, they "step-down" the high voltage to lower voltages (120/240v here in the US) at the pole and feed it into your house. That's how a tiny thin wire is able to power all the houses on your street and entire neighborhood.

I'm sure you're familiar with the even taller huge power-line transmission runs, these are another order of magnitude higher voltage than the wires outside your house and they are pushing enormous amount of power through relatively tiny conduit. The reason the towers are so massive is because of the "insulation" requirement as I mentioned above. They are so tall and big so that they can separate the conduit far enough (they are using the air essentially as the insulation). You can think of it as a tiny copper wire inside of a GIANT sleeve of insulation.

It's pretty amazing stuff!!! Just to geek out a little more, every few poles you'll see one of those little "Can" looking things. That's where the power is actually stepped down to the 240v you have feeding your home.

On a practical level, the higher the voltage the better in almost every case. Sounds like from your requirements 48v is where you should be heading for your solar setup.

Best,
~e
 
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