Designing my off-grid system without starting with load

bluepigfarm · June 2018

Okay so I understand the standard process to measure usage and work back from there to size your system. I've been currently living off-grid for the past year with a 2000w generator for charging computer, phone, and tool batteries. I'm ready to step up to a dedicated solar system with my primary need a chest freezer (plenty in the 300 kwh/y range). The limiting factor for me is 100% budget - I already own 6 320w panels that were extra's on a friends project and I will buy batteries locally (leaning toward's 8 Trojan 105's in two strings for 24v). For everything else I have a firm budget of $3000. I'm located in central Maine so assume I will need to use the generator some (especially in the winter) to keep the batteries charged. I don't have any codes or permits where I live, but will be hiring an electrician to help with the actual installation and wiring, but primarily will be DIY.

Now to the questions:

What components are either necessary or would make sense based on what I have available? Could I get by with just a Midnight Classic 150, a 3000w pure sin inverter, and a battery monitor? I assume all the wiring and electrical connectors are going to add up quickly so I want to keep some room in the budget there. What components are okay to go cheaper with? For example the Outback interters are way out of my price range, but will a cheaper one (but still pure sin) be a big mistake?

Anything I end up with is going to be a whole lot more useful than what I'm doing now, but I really want to maximize what I can get out of this system. I'm not worried if it's still limited - I'll just adjust my usage to what I'm able to produce.

Estragon · June 2018

A 1500w PSW inverter should start & run a chest freezer. Running likely 2-300w, but you need headroom for starting. If you have a good size (~25-40a) 24v charger you could save a fair bit by going for something like a Samlex 1500w inverter instead of a larger inverter/charger.

A classic 150 would be a good fit (if the array will be fairly close and the string Voc etc works out) and would give a bit of room for later expansion if needed.

Aside from wire, boxes, breakers, and maybe conduit, the other thing to consider is racking/mounting.

Photowhit · June 2018

You have 6 - 320 watt panels, are they 72 cell panels (have a VMP of around 35-37 volts?) It's likely.

With a 24 volt system, you would like to have an inverter charger. Otherwise you would need to buy a stand alone charger to go with your generator to charge the battery bank. About the cheapest would be a Samlex Evo at 2200 watts. I really don't think you will need a larger inverter. I run a house with window air and fridge on an 1800 watt, and do work arounds to run a water heater! $1200 but make an account and see what the pricing is, usually less!

https://www.solar-electric.com/samlex-evo-2224-inverter-charger.html

If the panels have a vmp of around 35 volts, you could use a 80 amp PWM charge controller. I would recommend a Y-Solar brand but see some one got poor reviews on Amazon. I had someone I was helping through a 48 volt setup who got one and has been very happy. About $80.

You will need/want a combiner box, about $120 with breakers, you could make a fuse based box for $50 or so...

You will also want some breakers in the power center, a Midnite DC breaker box for around $250 comes with main breaker, add breakers for input and output of the charge controller.

Add wiring and framing for the panels...

bluepigfarm · June 2018

The Vmpp is 36.4 - I've been thinking that I had to go Mppt no matter what, but honestly don't quite grasp when that's acceptable vs not. I appreciate both okaying using a Samlex, something I was wondering about the reputation of.

Do the breakers in the power center need to be solar specific like that Midnite, is there a disadvantage to use off the shelf parts?

Is there a preferred material for DIY raking? I was planning on using something like this for a ground mount:
https://www.homedepot.com/p/6-ft-14-Gauge-Electro-Galvanized-Half-Slot-Steel-Strut-Channel-ZB14HS6EG/300905355?MERCH=REC-_-PIPHorizontal2_rr-_-300905308-_-300905355-_-N

Thanks for the help

Photowhit · June 2018

The breaks don't need to be by Midnite, but do need to be DC rated. DC cam maintain an arc much easier than AC.

You can use DC rated breakers by CBI or Square D QO breakers are rated for up to 48 volts DC and often locally available.

petertearai · June 2018

consider 48 volts if you can

mike95490 · June 2018

bluepigfarm said:

.... I will buy batteries locally (leaning toward's 8 Trojan 105's in two strings for 24v). ......

Then bite the bullet and go to a 48V system Aims makes a 48V , 2Kw inverter charger with 30A transfer switch

Midnight Classic has 3 cooling fans, they make a fair amount of noise - Morningstar MPPT-60 is silent

Combiner box, you really want to use breakers, because the brand midnight sells are switch rated. If you use fuses, you cannot break an energized circuit, you have to wait till sun goes down. first or 2nd fuse that blows, you are in the price range of breakers

Photowhit · June 2018

It's rare but I pretty much disagree with all the advice Mike has given...

mike95490 said:

bluepigfarm said:

.... I will buy batteries locally (leaning toward's 8 Trojan 105's in two strings for 24v). ......

Then bite the bullet and go to a 48V system Aims makes a 48V , 2Kw inverter charger with 30A transfer switch

I've got no problems with suggesting a 48 volt system if you can find a reliable inverter charger in your price range. I, and many others, don't find Aims reliable. You can search the forum here for past problems with Aims inverters.

mike95490 said:

Midnight Classic has 3 cooling fans, they make a fair amount of noise - Morningstar MPPT-60 is silent

I like to recommend the Morningstar inverters for campers, they are good reliable units but have somewhat limited features. Be aware that the Midnite has fans, as do most others from Outback and Schnieder etc, but they have a lot of features that allow the user to start and use loads when they batteries are approaching full and a battery monitor that allows measuring the current actually flowing into and out of the battery.

mike95490 said:

Combiner box, you really want to use breakers, because the brand midnight sells are switch rated. If you use fuses, you cannot break an energized circuit, you have to wait till sun goes down. first or 2nd fuse that blows, you are in the price range of breakers

Most systems have a combiner box at the array and a breaker at the power center in front of the charge controller. Once you throw the breaker at the power center, you have no power going through the fuses and can safely remove them or check and replace them.

Solar panels aren't likely to produce more current than the panel is capable of and are generously rated. You 320 watt panel that generates at STC, little less than 9 amps normally will have a 15 or 20 amp fuse rating. Even in a warm/hot environment they aren't likely to blow unless you have and even that they are protecting you from. I haven't had one blow in the 12 years of using them.

Image: https://us.v-cdn.net/6024911/uploads/editor/nq/gb9lv66igkog.jpg

That said, I think I will built my own combiner box next time around with a NEMA box, as I've picked up a couple a section of din rail and use Midnite's or other din rail breakers. THIS WILL NOT MEET CODE! I think NEC code is no touch now, where you can't have exposed electrical anywhere. My previous fuse type combiner box met code in the past, but I believe this has been recently changed so that no exposed connections are allowed in the combiner box.

MrM1 · June 2018

Agreed on 48v. If you are getting 8 batteries anyway, you can run them all in series rather than series / parallel. Better for charging and battery life. And at 48v, the available stored power will be the same with 8 batteries in series as 24v 4S/2P.

Trojan T105 225 AH

24v 4S/2P is 450 AH x 24v = 10,800 watts hours

48v 4S is 225 AH x 48v = 10,800 watts hours

At 50% DOD both setups would have 5,400 available watt hours.

And 48v = smaller battery cable wire AND smaller battery main breaker. In hind site I wish I would have went 48v.

I would possibly suggest the Schneider Conext SW 4048 Inverter / Charger

- Shop around you can find them in your budget

- You will also need the SW System Control Panel

- And a nice add on is the MidNite Solar E-Panel custom designed for the SW line of Inverters.

- Comes with the 175 amp DC breaker needed for the Inverter / battery connection (if you go 24v Schneider recommends a 250 amp breaker for battery / inverter connection of the SW 4024)

- The E-Panel makes a very nice and clean connection sub-panel for all your other breakers and connections

- And the E-Panel for the SW that is made by MidNite Solar is the lowest priced of the E-Panels for various Inverter brands. (Magnum, Schneider SW, Samlex EVO, etc)

DC breakers are hard to find off the shelf IMHO at least for my area - especially when you start looking at DIN rail breakers, and MidNite's are pretty competitively priced. They would have everything you need for breakers both AC and DC. Once you go out of the Inverter AC you can easily go into an off the shelf Main Panel with standard AC breakers for the house branch circuits.

Estragon · June 2018

Panel frames are aluminum, and if in contact electrically with galvanized, galvanic corrosion could be an issue, particularly if you're in coastal Maine. If so, the sacrificial zinc will get eaten away fairly quickly, and structual integrity may be comprimised. Commercially made racking is generally made of aluminum with stainless steel connectors to prevent this.

It may still be possible to use the galvanized strut for the racking with careful design, but just wanted to point out the potential pitfall.

mcgivor · June 2018

@Photowhit said
I like to recommend the Morningstar inverters for campers, they are good reliable units but have somewhat limited features. Be aware that the Midnite has fans, as do most others from Outback and Schnieder etc, but they have a lot of features that allow the user to start and use loads when they batteries are approaching full and a battery monitor that allows measuring the current actually flowing into and out of the battery.

Just for the record, Schneider charge controllers do not have fans.

Photowhit · June 2018

mcgivor said:

Just for the record, Schneider charge controllers do not have fans.

Sorry, My mistake.

bluepigfarm · June 2018

Seems people think I should just go with the 48v if I'm buying 8 batteries anyway. I need to price out the components to see how much more 48v will cost. I don't really understand what the trade off is from 24v to 48v, but above MrM1 says it's better for charging and battery life - are there downsides or just the increase component cost?

Thank you everyone that has helped push in the right direction!

bluepigfarm · June 2018

Also if I went 48v does anyone have a recommendation for an inverter / charger? I can't find any 48v ones on this site unless I'm missing something.

MrM1 · June 2018

If you are buying UL 1741 home grade inverters, I do not think there is much of a cost increase over 24v.

I'm sure there are others, but some of the key differences for 48v would be:

1. Smaller wire / battery cable

2. I believe conversion from 48v to 120v is easier and possibly less losses??, but those smarter than me will correct me

Look for the Schneider Conext SW 4048. This is a very versatile inverter / charger

- Webo Solar has it for $1420 and when I got my SW 4024 they price matched even lower.

MrM1 · June 2018

Parallel battery set ups can suffer from unbalanced current during charging. This will result in overcharging some cells, while under charging others = shorter battery life if not well maintained and EQ'ed. Series battery banks do not suffer these unbalances. It is sometimes suggested that one should get the battery voltage / Amp Hour need and avoid parallel all together. 48v vs 24v allows for lower amp hour batteries. As noted above 48v allows for lower AH batteries yielding the same result as larger 24v.

bluepigfarm · June 2018

Photowhit said:
Most systems have a combiner box at the array and a breaker at the power center in front of the charge controller. Once you throw the breaker at the power center, you have no power going through the fuses and can safely remove them or check and replace them.

As I'm drawing out the wiring - this would be from the panels into a combiner box, then out of the combiner box and into the breaker, then the charge controller? The question I'm double checking is that there isn't anything between the combiner box and the breaker.

Thanks!

MrM1 · June 2018

Maybe this will help

Designing my off-grid system without starting with load

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