Need help setting up first solar system to power chest freezer

Welcome to the forum MauiSun!

MauiSun wrote: »

Hello,
I am new to solar, and I am putting together a small system to power an 8 cubic feet chest freezer.
So far I have:
45a MPPT tristar morningstar charge controller
Two 6v interstate L16s (380ah)
Xantrex or Samlex 2000W pure sine wave inverter (any suggestion/recommendations of one over the other?)
29 260W multicrystalline 60 cell panels

I like to start with the loads and design a "balanced system" that will support your loads... Here, we are bit all over the map, so lets start with the "Heart" of the system--Your battery bank.

More or less, a 380 AH battery bank using typical rules of thumbs for off grid systems that are used daily--It would look something like this:

• 380 AH * 12 volts * 0.85 AC inverter eff * C/8 average recommended continuous current) = 485 Watts typical recommended max continuous inverter
• 380 AH * 12 volts * 0.85 AC inverter eff * C/5 max continuous current) = 775 Watt max continuous inverter (with ~1,550 Watt/VA max surge)
• 380 AH * 12 volts * 0.85 AC inverter eff * 1/2 days of energy storage * 0.50 max battery discharge (for longer life) = 969 WH per day (bad weather, over night energy usage for 2 days of no sun)

Then there is the solar array... Two calculations--First based on size of battery bank (larger battery banks need a larger solar array). Second based on how much sun you get and your loads.

First the size of battery bank--5% to 13% rate o charge is recommended for lead acid batteries... And if you are an off grid cabin with daytime loads, I would suggest 10% or greater is a good idea (such as your refrigerator running 24 hours per day):

• 380 AH * 14.5 volt * 1/0.77 panel+controller derating * 0.05 rate of charge = 358 Watt array minimum
• 380 AH * 14.5 volt * 1/0.77 panel+controller derating * 0.10 rate of charge = 716 Watt array nominal
• 380 AH * 14.5 volt * 1/0.77 panel+controller derating * 0.13 rate of charge = 930 Watt array "cost effective" maximum

Obviously, you have way more solar panels than you need... And if you go above roughly 950 Watt array, we need to talk about current limiting for the battery bank.

The issue with the refrigerator is that most standard AC refrigerator/freezers have compressors that take a lot of surge current (600-1,000 Watts) and about 120 Watts of running current (50% duty cycle or a bit less is typical). And nominally, a 1,200 to 1,500 Watt inverter would run a typical energy start Refrigerator (auto defrost/frost free may take 600 Watts to defrost the evaporator).. Your battery bank is a bit on the small size to "reliably" run a standard refrigerator. However, if you can find an "inverter" type refrigerator (becoming more common now)--The surge current is much less.

More or less--If you could justify, I would suggest a 2x larger battery bank (4 batteries)--And you may need to look at 24 volt system--A 12 volt system is good for ~1,200 Watts maximum (some folks run a 2,000 Watt AC inverters and are happy on 12 volts)... But unless you "need" 12 volts (HAM radios, DC tools/appliances)--A higher voltage system would be better (helps keep DC bus current lower--I like to design for 100 amps nominal or less for charging/AC inverter if possible).

Yes, you read that correctly, 29 panels....
My father works for Trina solar and I got an amazing deal at $0.60/W that we couldn't pass up.
We are planning to design and build a 6kW offgrid system soon, but for now I am just getting my toes wet and trying to get this freezer going.

That is pretty neat--But a 6 kWatt AC inverter system (and roughly 6,000 Watt solar array maximum) is no small system. Put a 600 AH @ 48 volt battery bank (minimum suggested) on that--And you are talking about some real power/money.

We don't have a freezer yet, but probably getting a Danby 8 or 11 cu. Ft. chest freezer, that I'll later be converting to a fridge. But for now, I would like to set up a system powerful enough to run at full capacity as a freezer.

The freezer consumes approximately 220kWh/year, or 610Wh/day.

Refrigerator/freezers are the typical load that pushes folks from "small" to "medium" size systems.

My questions are:
1.how many of these panels will safely work with this charge controller? The panels run at 8a, charge controller 45a. Does that mean I can have 5 panels running off it? It says it rated at 600W @12v and 1200W at 24v max, so I looks like I can only have two panels max on my 12v system. Does that mean I'd be better off with a 24v battery bank that way I could use more panels (up to 5?)......a little confused here. I'd like to go with the most panels possible obviously.

2. Is my battery bank large enough?

3. Will this system work off a 12V battery bank? or will the fridge power surge be too much?

4. what size wires and cables should I be using with this setup?

After a bunch of research I feel like buying two more L16s and bumping the battery bank up to 24v is the best way to go. Am I wrong?

Thanks for any help or input!

Pretty much answered most of your questions above... Just to give you an idea on wiring, if you have a 2,000 Watt AC inverter on a 24 volt battery bank:

• 2,000 Watts * 1/0.85 AC inverter eff * 1/21.0 volts battery cutoff * 1.25 NEC derating for breakers/wiring = 140 Amp minimum branch circuit (fuse/breaker/wire rating).

For the solar panels--60 Cell panels have a Vmp~30 volts at STC (standard test conditions)... But Vmp falls as temperatures rise, so your 30 volt panel will actually be producing near 24 volts on a hot day--Not near high enough voltage to charge a 24 volt battery bank (you need a Vmp~35+ volt panel/array to charge a 24 volt lead acid battery bank).

So, you will need an MPPT charge controller + ~2-5 panels in series (then more panel strings in parallel) to properly recharge the battery bank. MPPT charge controllers are very nice--Just not cheap.

-Bill