Review my 24v off grid system and solar panel recommendations.

[poly11]

Hi, complete beginner here. We bought a holiday cabin that has an old existing solar system in place, which we would pretty much like to rip out and start anew.

Our required loads are quite low, and we're only there roughly 2 weekends per month, perhaps longer in the summer.
- Water pump ~15 mins per day. (Not sure on the watt load of this)
- LED lights
- USB WIFI router (preferably kept on all the time)
- Occasional phone charging

We're planning to build a system similar to Will Prose's 24v system: https://www.mobile-solarpower.com/2000-watt-24v-solar-system.html

2x 12v 100AH Ampere Time Batteries in series for 24v 200AH
Victron 100/50 MPPT charge controller
- 60Amp circuit breaker
Victron Smartshunt
24v 2000w Giandel Inverter
- 125Amp fuse connected close to the battery's positive
12/24V USB Car charger
2x 150Amp Bus bars

24V to 12V step down and 12V fuse block is not needed yet, might add it later when we start hooking up lights to this.
We'll have a gas generator for emergencies.

We plan to use a RaspberryPI via VE Direct and a USB router for remote connection via WIFI and internet.

So far after plenty of research we've come up with the following wiring and fuse requirements.


Location:
Sweden, plenty of sun in the summer, barely any in winter.
Coldest in winter is a average of -1 to -5 degrees Celsius.
Summer ~ 25 degrees Celsius.


Questions:

• Are the wire sizing accurate? (4awg between batteries and to inverter/ 6awg from bus bar to MPPT)
• Are the fuse size sufficient for the wire and load sizes (2000w inverter at 24v ~123A. Is a 125A fuse cutting it too close, and should I go for a 150A fuse)?
• I tried to search on how to connect USB outlets directly to batteries like this, but couldn't come up with much examples on the forum. Is getting a car/cigarette USB charger like this and just connecting it to the busbars fine? Since the wires they provide has fuses inside, I think I don't need to add my own fuse.
• Does the smartshunt need a fuse?

Solar Panels
There aren't many options in Europe to source our own so we decided to go for this: ELERIX Solar panel Mono Half Cut 415Wp at 200EUR

Maximum Power (Pmax) is 415Wp
Cell technology is PERC Half Cut mono crystalline
Module efficiency is 21,25%
Open Circuit Voltage (Voc) is 41,6V
Number of Solar cells is 108
Color of the frame is Black
Weight is 19.5kg
Dimmensions are 1722 * 1134 * 35mm
Connection cable with MC4 connector

Temperature Characteristic
NMOT (Nominal Module
Operating Temperature) 45°C (±2°C)
Temperature Coefficient of PMAX - 0.36%/°C
Temperature Coefficient of UOC - 0.26%/°C
Temperature Coefficient of ISC 0.04%/°C

We plan to get 2 in series, so it works out to 830W 83.2V

• Does these panels work for our system? The 83.2V seems well within the 100V limit of our Victron MPPT even if we account for cold weather.
• We would like the option to expand our system, to get 1-2 more solar panels. 1 more first to make up for the lack of sun in the winter that could be disconnected in the summer. - But this will then need to be connected in Parallel? Does a 2s1p config even work?
• Can our MPPT handle a 2s2p config? (4x415w = 1660w) (1660w/24v = 69A) 69A seems more than the 100/50 MPPT could handle.
• Side question, these panels (RISEN Tier 1 Solar Panel Mono HalfCut PERC 390Wp) are identical in price, but at 390W, doesn't that just mean it's worse off? Why would one choose this over the other?

Thanks so much if someone can review this! We're just going nuts over trying to make sure all the parts are correct.

 

[DThames]

Right away, the water pump is a big question. Is this a well pump or something like a camper/RV pump that just moves water from a tank to the sink? If a well pump, you need to know a lot more to know what size inverter/battery system will start the pump. If the pump is critical, worry with that first.

 

[poly11]

The well pump currently runs off an inverter, connected to the old set of batteries which I'm highly certain are 12V batteries. So it takes in 220-240V from the inverter. When we run the water pump, the batteries drain roughly a total of 30-40amp each time. Hence I think it will need about 400-600w to fill the tank. (It has an automatic cut off to only fill a quarter of the tank.)
Unfortunately the image is too low quality to see the power requirements and we live far enough that a day trip isn't feasible.

 

[MichaelK]

Several issues. To start, can you find a higher quality inverter than the Giandel? For a cabin, 24V is a good choice, though I'd be looking at inverters designed to be hard-wired into an electrical panel, rather than one with NEMA sockets. I have been very happy with my Conext 4024 inverter.

In series, volts add while amperage stays the same. In parallel amps add while volts stay the same. I question what you are saying here with....
"2x 12v 100AH Ampere Time Batteries in series for 24v 200AH"? Do you mean that you have a total of four of these batteries wired as 2S2P? If it is only two batteries wired in series, then you only have 100Ah at 24V.

I would consider 4 gauge to a battery only marginally acceptable. Look at this ampicity chart. Assuming you can pull an actual 2000W out of the Giandel, that would be 2000W/24V = 83.3A. If you use the 60C rating for wire, that's only rated for 70A. And, that's at 100% conversion efficiency. Real-world amp draw is likely to be somewhat higher. For my own 24V inverter, I'm using 000 wire.


The pic you are showing does not look like a pump? That's a pressure relay? We really need the electrical specifications first before we give you real-world advice. What's your region's standard electrical supply, 120V 60Hz, or 230V 50Hz? This sentence gives me pause "I'm highly certain are 12V batteries. So it takes in 220-240V from the inverter"? They may be 12V batteries, but is it a 12V system? For North American power, I'm not aware of a single 12V inverter supplying 240VAC. Details are critically important here. Here in the US, inverters supplying 240VAC commonly start at 24V.

 

[poly11]

Thanks for your reply, our budget is tight and Giandel is what we can afford. We can consider expanding the system later on, where we can spread the costs. Everything the previous owner has, incl batteries and panels are so old, >10 years, that we figured anything we get now is bound to be an improvement.

Batteries are in series at 24v 100ah indeed.

I mentioned the cabin is in sweden, so that's 230V 50Hz. The inverters here convert from 12VDC to 200-240VAC.
Since we're building a 24v system, I'll get an inverter that's 24V to 230V.

Looking at this chart, a 4awg wire can go up to 100amp, and from our battery to the inverter will be only around 60-100cm at the most. There's too many different wiring charts, which is confusing, but based on reading forum posts here (https://diysolarforum.com/threads/24v-inverter-wire-size.19398/), we thought 4 is fine, could even drop down to 2 if needed.

 

[12VoltInstalls]

our budget is tight and Giandel is what we can afford.

Definitely need more info on the well pump

I’m a Giandel fan as I’ve found them to be reliable for the price. I don’t think that’s an inappropriate budget choice, actually.

Everything the previous owner has, incl batteries and panels are so old, >10 years, that we figured anything we get now is bound to be an improvement.

Good deep cycle batteries not abused can last a very long time and one can think of panel life cycle as 20-25 years.

has an old existing solar system in place, which we would pretty much like to rip out and start anew.

How many watts of panels?

Where I’m going with this is that the existing panels could be added to with the right spec’s, or supplemented with a new system in parallel- just maybe less panels to buy, so money for other areas.

But the critical missing element is the startup and running load of the well pump. Every time I wildhat guess at a workable scenario (or even consider the 12V/24V conundrum) I have no viable suggestion because there’s a huge void in what the well pump needs are.

The other thing about LiFePo batteries is that they have to be a degree or more above the freezing temperature of water if you either want them to survive charging or be anything useful. So those old lead acid batteries might be the cats meow in your situation (or appropriate new flooded lead acid batteries; I don’t like AGM or gel batteries for long-term economics).

So you need to know the startup and running watts on that pump- everything else is essentially easy to work around.

 

[poly11]

I'll look for the well pump information when i go over to the cabin this weekend.

How many watts of panels?

4 panels at 155w each
VMP 17.38
Imp 8.87
Voc 22
Isc 9.75

We plan to also DIY an insulated box for the batteries in the winter.

 

[12VoltInstalls]

Voc 22

So you can buy panels with similar specifications or panels of ~45VOC
Do you can probably still benefit from those 600W of panels 4S and parallel them with the new panels, all running under one new SCC, or let the original panels keep doing what they’re doing.

We plan to also DIY an insulated box for the batteries in the winter

This may not work depending on how cold your area is in winter. In the absence of btu input insulation only can do do much.

 

[metalheaddoc]

Oddball question. If someone is in a cold location but it has a lot of sun, can you build a battery enclosure like a greenhouse to heat the battery a little above ambient temp? Like an enclosure with a glass top to let sunlight in?

 

[12VoltInstalls]

Oddball question. If someone is in a cold location but it has a lot of sun, can you build a battery enclosure like a greenhouse to heat the battery a little above ambient temp? Like an enclosure with a glass top to let sunlight in?

Sure, but glass let’s heat out at night and the you can’t charge until the warm up the next day. Better just to ‘handle’ it electronically imho

 

[poly11]

The pump draws 1.15 kW at full load. We need to run it for roughly 5mins tops each day (typically, we run it 3 times and each time it barely runs for 30seconds, so 5mins is a big overestimation).

Over the weekend, there was too much cloud cover for the existing PV system to have enough charge to run the pump so we supplemented with a gas generator. I expect that with any new system we build, we're bound to run into the same problem unless we have enough battery bank.

The coldest it gets is roughly around -10 degrees at night, and we can add a battery temp sensor to our smartshunt for low temps.
Getting a heat pad and adding it to our insulation box is also possible.