diy solar

diy solar

Small off grid cabin questions about batteries and panels

Tat2rtist

New Member
Joined
Sep 6, 2020
Messages
10
I have a small off grid cabin we are building, for the next 5 years it will be used for weekends mostly , in about 5 years it will be used during summer months mostly full time. Right now I want to build a system that will run lights and TV. Microwave and Refridgerator will be run off of generator for now if and when they are needed. I just purchased 4 Lifeline 6 volt 300 ah batteries at a super price that I couldnt refuse. My question is and if I got what I read right, should I go with a 12 volt system that would have 600ah or a 24 volt system with 300ah. I believe from what I read I have the amp hours right. In 5 years, I will have to upgrade I know. I was leaning towards 12v with twice the Amp hours, in case I want to run a keurig coffee pot or a microwave. Just looking for some input and recommendations. And I know people will ask why I bought those batteries, I was purchasing some food grade barrels to make a spring box, and the guy had just replaced 6 batteries in his RV because 2 of them were bad, the other 4 checked out fine but he decided to replace all 6. He offered me all 4 batteries for $200 total so of course I jumped on them. Any help would be greatly appreciated. Also I am figuring to purchase one of the all in one charge controller, inverter systems to make things simple, any thoughts on those would also be appreciated.
 
12v x 600Ah is 7200 watt hours
24V x 300Ah is 7200 watt hours
They have the same power.
If you don't have 12V appliances or loads go with a 24V system.
 
My very first system for my cabin was 12V. I am trying hard to forget that. I've since put together both 24 and 48V systems for different buildings that FAR outperform what I could originally accomplish at 12V. The higher voltage systems just have greater capacity for growth than do 12V systems. And, the SERIOUS, hard-wireable components start at 24V.

You will be able to buy far higher quality components if you purchase individual units rather than an all-in-one unit. Stick to name brands like MorningStar, Midnight, Outback, Schneider, and Victron. You'll remember the quality long after the price is forgotten.

Don't buy solar panels through the mail. Shipping doubles the cost of panels. Try to find local sellers that you can drive to and load onto the back of your truck. Check out your local Craigslist. You'll get better deals buying high-voltage grid-tie panels, but an MPPT controller will transform the voltage down to what your battery string wants. Expect to pay around 25cents per watt.

Study the battery specifications for your Lifelines carefully, and program your controller to match what the manufacturer recommends. Keep in mind that they are already used, and if given good care, you might get a few years out of them. But, I'd start shopping for replacements now so you have a short path to replacement. I'd recommend higher amphour flooded lead acid batteries with snap caps so you can monitor specific gravity.

When designing your system, use the 2X rule. For however many watts you need to supply, buy 2X as many solar panels as that load. So, if your biggest load is a 750W table saw, get 1500W of panels. The 2X rule also applies to everything else from wiring to generators. If you need a wire that has to carry 40amps of current, buy a gauge rated for 80A.
 
My very first system for my cabin was 12V. I am trying hard to forget that. I've since put together both 24 and 48V systems for different buildings that FAR outperform what I could originally accomplish at 12V. The higher voltage systems just have greater capacity for growth than do 12V systems. And, the SERIOUS, hard-wireable components start at 24V.

You will be able to buy far higher quality components if you purchase individual units rather than an all-in-one unit. Stick to name brands like MorningStar, Midnight, Outback, Schneider, and Victron. You'll remember the quality long after the price is forgotten.

Don't buy solar panels through the mail. Shipping doubles the cost of panels. Try to find local sellers that you can drive to and load onto the back of your truck. Check out your local Craigslist. You'll get better deals buying high-voltage grid-tie panels, but an MPPT controller will transform the voltage down to what your battery string wants. Expect to pay around 25cents per watt.

Study the battery specifications for your Lifelines carefully, and program your controller to match what the manufacturer recommends. Keep in mind that they are already used, and if given good care, you might get a few years out of them. But, I'd start shopping for replacements now so you have a short path to replacement. I'd recommend higher amphour flooded lead acid batteries with snap caps so you can monitor specific gravity.

When designing your system, use the 2X rule. For however many watts you need to supply, buy 2X as many solar panels as that load. So, if your biggest load is a 750W table saw, get 1500W of panels. The 2X rule also applies to everything else from wiring to generators. If you need a wire that has to carry 40amps of current, buy a gauge rated for 80A.
Thank you for the info, Im new to solar, have watched a lot of videos, and have been to a few sites, but sometimes looking at all the hardware and equipment can get overwelming. I did price panels from Santan solar, great prices, but shipping was horrible. I have seen some locally that are pretty reasonable. I already considered MPPT controller, the all in one units are nice, but I have a breaker box in my cabin I want to tie into, so buying individual pieces I think would be better because it would allow me to use bigger gauge wire from the inverter to the box.
 
Remember that you get what you pay for. If you buy a very cheap, low cost all in one unit, you get a very cheaply made sub-standard piece of equipment. They're of such mediocre quality that you need special procedures just to turn them on without frying something.

The one area that's surprising though is the solar panels themselves. There's a local reseller in my area that markets "used" solar panels on Craigslist. These are name-brand grid-tie panels that were installed on someones roof, but then they defaulted on their payments, so the panels are retrieved back off the roofs. He then sells those at ~20cents per watt. I've tested them side by side with my retail purchased panels (Renogy), and the best performance I've seen (94%) is with the used panels. Some of them appear almost "like-new" in condition.

It's going to help though using proper terminology when discussing components. I think the breaker box you are referring to is the cabins main "load center". Load centers are always mounted with AC breakers. Compare that with a DC combiner box that also contains DC breakers.

In the real world, the flow of your power would be from your individual solar strings, into a combiner box, that has an individual DC breaker for each solar string with maybe 10 gauge wire. Then the DC flow is directed from the combiner to the charge controller with maybe 6-4 gauge wire. Between the combiner and the controller, you might have what's called a power center, or disconnect box, which contains all the DC related breakers for controlling the main system. https://ressupply.com/power-panels/midnite-solar-mndc250plus-mini-dc-disconnect-box.

Please be aware that you should never use breakers rated for AC in any application carrying DC current. The AC breaker contacts don't open as wide as DC designed breakers, and can arc and weld together.

The MPPT controller transforms the raw solar voltage down to what the battery wants (4 gauge wire). Then the battery fuels the inverter. Here you want VERY substantial wire to convey the high amps from the battery to the inverter. Use between 2/0 and 4/0 gauge (NOT 2-4 gauge).

Once the inverter converts the high amperage DC to lower amperage AC, the ACout line from the inverter to the load center can be lower, 10-8 gauge.

The best inverters are designed to be hard-wired into the load center, and make split-phase 120/240V AC that a standard load center is designed for. Look at Magnum, Outback, and Schneider for the best inverters.
 
Back
Top