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Which voltage should I go with?

Amadio1

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Aug 4, 2022
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Hello. I am in the planning stages of an off grid system for a small cabin im currently building. It will be used for typical lighting, refrigerator, water pumps, and in the summer a 800w window Ac for 4ish hours in the afternoons. I plan on a server rack battery and an all-in-one system with 2-3kw of solar panels.
My question is, should I run 12 or 24 or a 48 volt system?
 
48V is a good choice, higher voltage = lower amperage, smaller wire & readily available breakers, bus bars etc. With 48V you can expand the system easily without getting into very large, difficult to manage currents. For example, 200A at 48V (51.2V nominal) is 10kW. At 24V its 400A.
 
Let's add it up first. Making an itemized list of what you mention, with what I think you will consume. You can modify this to fit you exactly.
lighting: 5hr X 100W: 500Wh (assuming CFL or LED)
refrigeration: 1200Wh (factory label)
air-conditioner: 4800Wh (I have the same size, turn it on at ~3:00pm, and solar can power it to 6:00pm. At about 6pm, it has reached the set point is is now cycling on and off. I typically run it till midnight, when it is cool enough outside to shut if off. Cycling on and off is another 400Wh X 6hr = 2400Wh. So, 2400Wh + 2400Wh = 4800Wh.
TV: 250Wh (no TV? Ya gotta have TV?)

So....
500Wh + 1200Wh + 4800Wh + 250Wh = 6750Wh; just call it 7kWh. Maybe that will be ~2.5kWh in winter. Assuming you need 7kWh in the summer, and you get about 5sunhours (sh) per day, that's 7000Wh/5sh = 1400W of panels. Call that six 250W grid-tie panels, which are dirt-cheap right now. Wired 3S2P that might be about 16A at 90Vmp. At 25V charging, that's 60A, while at 50V charging that's just 30A, so with the higher voltage 48V battery, you can get that with a lower amperage controller. Expansion-wise, if you wanted to upgrade the solar in a year or two, you could add more to 2000W or even 2400W with just a 50A controller. Take a look at Epever's Tracer 5420AN. It has a 200V limit, so it can handle four of the above mentioned panels in series.

The one wildcard here is the water pump? What is that, a deep submersible pump? Those are usually 240VAC, and have BIG starting surges. Mine is 9100W. So, what are the specs? A small 120V pump will be far easier to power than an big 240V pump. Get the details before you finalize your design. You don't want nasty surprises after you put it all together.
 
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