24V Off Grid Solar Sizing Questions Hawaii

[Islandsun]

Hello to all on the forum!

I am working on sizing a small 24V system for my cabin in Hawaii and would really appreciate some input from more knowledgeable individuals! I am a total rookie when it comes to electrical/renewable energy but after reading through the forums and websites through the last year I believe I am starting to get a grasp on the different components I will need. Any advice would be much appreciated!

The specs- 400 sq. foot cabin, 4-6 hours of sun, 24V system, 2 small chest freezers (one with fridge conversion), LED lights, 24V Flojet variable speed pump from catchment tank, phone/ comp charging, ipod player, compost toilet fan.

The loads- From my calculations with Kill-a-Watt I'll be squeaking by using 1400-1500 watt hours per day. Both the fridge and freezer pull 70 watts on start up from what I can tell and the pump will be around 144 watts.

The system- After speaking to a few Off Grid suppliers today this is my tentative component list. I am planning on over-sizing a bit on panels and batteries and possibly on the inverter to leave room for growth later.

• 4 Renogy 150W panels wired in series. Vmp- 17.9 Imp- 8.38
• 10 awg MC4 wires 75-100 feet to CC.
• Midnite Solar KID 30A Charge Controller. Max 150V input.
• 4 Costco 6V Golf Cart batteries wired in series. Should be around 200AH from what I can tell but having difficulty here.
• Samlex 24V 600 watt Pure Sine inverter. I am thinking about bumping this up to a Xantrex PROsine 1800 watt for future growth (and energy efficient search mode) but based on posts on here I have read that the inverter should be roughly same size as panel array V.
• Fuses and Breakers- This is where I am mostly in the dark.. fuses on positive wires from panels to CC, CC to Batteries, Batteries to Inverter, Breaker or fuse from Inverter to AC Panel??

Thanks in advance to anyone who can help me understand this a little better! I feel pretty good about it but have no idea really. On a positive note a pro electrician friend will be helping me set it up.

Aloha!

[sensij]

TL;dr - 200 Ah sounds small for your loads, but the system they've proposed has reasonably well-matched components.


OK, let's apply conventional design rules (as described in this sticky thread) for the first pass through.

1400-1500 Wh daily x 5 = 7000-7500 Wh battery. At 24 V, that is around 300 Ah. Trojan L16RE-A might be a good choice, if you can get it. Have you ever worked with FLA batteries before?

With 4 kW/m2/day average insolation, PV should start at (1500 * 1.5) / 4 = 560 W. 4 x 150 W = 600 W, so that is an ok starting point. The Renogy panels tend to be more expensive per watt than larger panels, you might want to look into the availability of ~260 W panels (60 cell) or ~300 W panels (72 cell).

A 30 A charge controller on a 300 Ah battery is ok (C/10). You could consider a 40 A charge controller if you want some room to grow. 600 W of PV won't regularly produce much more than 20 A of charge current... if you end up going with a 40 A CC, 900 W of panels would be a better match. Lower end alternatives to the KID CC you might want to consider include Victron's mppts (100/30 or 150/35) and Tracer series (3210A, 4210A). More equivalent might be the Morningstar TS-MPPT-30 or TS-MPPT-45.

Where are the solar panels being mounted? If you have shade or sub-optimal orientation, you'll need more array power to compensate.

The most important fuses are on any connection to the battery. If the system is grounded, the fuses only need to be on the DC+ side. If you have just two connection to the battery (CC and inverter), Blue Sea Systems MRBF fuse blocks, mounting directly to the battery post, are a nice choice. No fusing is needed between the panels and the charge controller, as long as you have just one or two strings.

[Islandsun]

Awesome thanks for the response!

I thought 200 AH sounded a little small and actually one of the people I talked to told me 400AH. I would love to get some Trojan batteries but the cost of shipping to Hawaii is astronomical- hence the Costco batteries for $85. And no I have never dealt with FLA batteries and not looking forward to it really.

Could I remedy the battery situation by going with 2 separate strings of 4 batteries in series? Still a little unclear on AH and batteries...

Renogy panels are the best bet for me as Amazon will actually ship them Prime- only up to the 150W though. I was originally planning on 3 280W panels.

I like the Midnite KID as it can be paired with another for future growth but if that is not large enough I was looking at the Renogy Commander 40A MPPT.

How does this look-

5 Renogy 150W panels wired in series. Vmp- 17.9 Imp- 8.38
Midnite Solar KID 30A Charge Controller. Max 150V input. (should I ditch this for 40A??)
8 Costco 6V Golf Cart batteries wired in series, two separate strings.
Exeltech XP600 24-volt 600 watt inverter

Thanks

[Mike90250]

I have doubts that a 600w inverter could start a fridge, and very unlikely if are are other loads on at the same time
Also, with energy saving search mode inverters, they don't play well with fridges with electronic controls. The electronic brains don't boot fast enough to start the compressor and by then the inverter is back asleep.

[Islandsun]

Thanks for the input Mike!

I just plugged the freezer (3.5 cubic feet) into my Kill-a-Watt and its start up surge peaked at 185 watts. Add another freezer (fridge actually) and 144 watt water pump, coupled with some lights.

The Exeltech XP600 looks like it surges to 1100 watts. Do you think that will work out? If not I can bump up to the Exeltech XP1100 that surges to 2000W.

Thanks!

[Mike90250]

Kill-a-watt cannot measure starting surges, That takes a $300 meter with Peak Hold, You might be able to get a decent Electrician to measure it for you, for less then the cost of the meter.
It's safe to assume the fridge will take 5-10x the running current, to start, usually for less than 2 seconds,

[Islandsun]

Wanted to get back on here and post a drawing of my final design with a few questions, mostly on Grounding. I have learned a lot since my first posting here and actually feel pretty good about my system now. Please feel free to pick apart any wiring, breaker, equipment issues etc..

As of now I am going to be installing-
-2x 300 watt Solar world panels (might add 1 more if needed in future),
Solar Epic 40 Amp MPPT CC with MT50 Display
8 Golf Cart Batteries 400 AH
Outback GFX1424 120V 1400 Watt Inverter/Charger
Honda 2800 Watt Generator

I have watched a few Mike Holt videos and it seems like he says to not have more than one Grounding Electrode due to lightning, but then I have read many times that you need one for DC and one for AC... Honestly a bit confused.

My main questions on Grounding are-

1. I am putting the panels on my neighbors property to access sun, and I don't think she wants me to pound a ground rod in. Can I run 6AWG bare wire from panel frames in the trench with my conduit to a Ground Rod outside my shed where equipment will be? Distance is about 130 feet, which from my understanding is kind of like a Grounding Electrode in itself.

2. Once at the shed current plan is to have 2 rods 8 feet apart 6 feet from outside wall, bonded together, 1 for DC and 1 for AC. If both rods are bonded together can I just get by with the 1 rod for AC and run the DC components to that? Soil is very rocky but also moist dirt so if I can get away with 1 rod that would be great.

3. I am planning on grounding DC Neutral at the battery terminal and then to all of the components (see drawing). And then AC Neutral at the Panel which is where I will connect the Inverter chassis ground (see drawing).

Thanks for the help!!

[littleharbor]

[QUOTE=Islandsun;n366389]
As of now I am going to be installing-
-2x 300 watt Solar world panels (might add 1 more if needed in future),


What is the max Voc. of that controller? Some of their 40 amp controllers are only 100 Voc. With 2 series wired 72 cell panels you are pushing the limit already. You wouldn't be able to add a 3rd panel.

[Islandsun]

Max is 150V. The panels have a Voc of 40V.

[Matrix]

Looking at your drawing, you might want to check with MidNite Solar about grounding the battery negative. I know they say with the Classic 150 if ground fault protection from the classic is used, DC negative ground should NOT be connected to the system grounding conductor anywhere in the system as this will defeat the GFP function.