Chest Freezer System Advice

[Rejfoxtrot]

Super newbie here, i'd appreciate in advise you can give me. So i've been wanting to put a chest freezer in an outside shed for awhile now and while it would probably be easier just to have an electric company trench a line and hook it up to the grid i've always been interested in solar. So with a $100 gift card going towards a purchase I bought 4 x 100w polycrystalline panels from Rich Solar on Amazon.

They arrived yesterday and to my surprise I was staring at 4 boxes with two panels each. Checked my order and I didn't over order, they just sent me more twice the amount of panels. I was planning on purchasing a 40amp MPPT controller in the near future, but now i'm unsure if it is too small for 800w. To add to that, I'm not sure that I can safely series link 8 of these particular panels together, which was my original plan with 4 panels. The back of the panels say max system voltage 1000VDC. So max would be 10 panels???

So for batteries I have one free 12v 105 amp hour AGM battery that's about a year old with minimum use. Planning on purchasing another 12v 105 amp hour and make a 24v battery bank.

Stupid question time.....I'll get the same kwh using a 24v that I would using a 12v system right? I've heard that if you parallel connect you double your "capacity" @12v. But if I run the same load off a 24v bank it'll be the same capacity as a parallel connected bank because i'll run 24v at lower amps?

I'm currently load testing my current 21cu ft chest freezer with a kilowatt meter it's a 25 year old freezer. Most likely it will pull too many amps for my bank without adding on. I'm debating on buying 3 more batteries or just a smaller more efficient chest freezer to go outside once I know the results.

Also one more question, Do anyone have recommendation for a 24v inverter?

Thanks in advance!

 

[Dzl]

So first off, welcome to the forum!

Second order of business, I would recommend that you take some time to brush up on the fundamentals. I would start with the first 3 videos in this series.

Its important to understand the relationship between Volts, Amps, and Watts (Volts x Amps = Watts).

Now on to the specifics:

Stupid question time.....I'll get the same kwh using a 24v that I would using a 12v system right? I've heard that if you parallel connect you double your "capacity" @12v. But if I run the same load off a 24v bank it'll be the same capacity as a parallel connected bank because i'll run 24v at lower amps?

Yes! You understand this part pretty well. Parallel connecting your batteries will double your amp-hours, series connecting will double your voltage, but the stored energy (Watt-hours) will be the same either way (2 x 105 ah x 12v = 2.5 kWh). So in terms of battery capacity voltage is irrelevant. But one advantage of a 24v system, is that your solar charge controller will be able to output twice the power compared to a 12v system, because the current (amps) will be half as much compared with a 12v system.

To add to that, I'm not sure that I can safely series link 8 of these particular panels together, which was my original plan with 4 panels. The back of the panels say max system voltage 1000VDC. So max would be 10 panels???

No the figure you saw is Volts not Watts. You definitely don't have to worry about exceeding 1000 volts with your 8 100 watt panels. Your panels are likely something like 18-22 volts each so you could series connect roughly 50 panels before you exceeded that limit.

BUT

That isn't your limiting factor, your limiting factors all relate to your charge controller. There are 2 hard limits and 1 soft limit you need to be aware of.

1. Open Circuit Voltage (Voc) this is the maximum input voltage
2. Short Circuit Current (Isc) this is the maximum input amperage
3. Output Current, this is the maximum output current and can usually be exceeded but the extra energy is wasted. Its worth noting that this limit is the same regardless of whether you have a 12 or 24 volt system (so your controller can output twice as much power (power = voltage x current) if your system is 24v).

You can find these limits on your Solar Charge Controllers spec sheet. Best I can tell from the Amazon link, the max input voltage for your controller is 138V and the max input and output currents are both 40a.

So this gives you your maximum values the controller can handle, now you need to configure your panels in a configuration that keeps them below these limits. To figure out how to do this look on the back of your panels you will see a sticker with the specs, the ones you need to pay attention to are again Isc and Voc (which for your panel are 5.86a and 22.6v).

Keeping in mind that series connecting increases voltage and parallel connecting increases current, we need to figure out a configuration that keeps the total for your 8 panels below your charger controllers max input voltage of 138v and max input current of 40a. If you series connect all 8 you will exceed the max input voltage, if you parallel connect all 8 you will exceed the max input current. But if you wire 2 strings of 4 panels in series and then parallel connect the 2 strings your array's Voc would only be 90.5v and Isc would be 11.7a which is well within the limits. Likewise, you could also wire 4 strings of 2 panels in series and parallel connect the 4 strings and your array's Voc would be 45.2v and Isc would be 23.5a which is also within specs. In either case the output current would be capped at 40a. With 800w it would make the most sense to go with a 24v battery configuration so that you could take advantage of your arrays total output (40a x 24v = ~1000 watts).

Hopefully this helps, I know I'm throwing a lot of numbers and new terms at you, and I know that keeping all the units straight, and their relationship can be complicated. If anything isn't clear or if you need further clarification or have follow up questions, I'm happy to help if I can.

This article gives a good concise but in depth overview of how to size your solar array, definitely read it, its probably a lot clearer than my explanation.

 

[nosys70]

 

[MisterSandals]

I was planning on purchasing a 40amp MPPT controller in the near future

Technical Data
40A MPPT Charge Controller Nominal System Voltage: 12V, 24V
Rated Charge Current: 40A
Rated Load Current: 20A
Max PV Open Circuit Voltage: 150V / 138V
Max PV Input Power: 520W (12v battery) / 1040W (24v battery)

I'm not sure that I can safely series link 8 of these particular panels together, which was my original plan with 4 panels.

8x 22.6(Voc) = 180v (too high)

If you upgrade from 4 panels to 8 panels, you could run 2 strings of 4 panels in series:
4x 22.6(Voc) = 90v
your amps double to 2x 5.86A = 11.72A

The controller you listed will handle this nicely but you would need a 24v battery bank to handle the wattage.

 

[Rejfoxtrot]

Thank you so much for the information! I'll check out the videos and read that article.