Off grid cabin build! Solar component assistance!

Madmarc · August 2014

Hi! First time poster long time reader of posts! Our only internet is via phone (typing nightmare) so I'm forced to be brief.

Halfway through the build of a 500 square home in northern VT. Off grid. After much research of forums like this I started purchasing solar components but now think I should get more specific info before proceeding (the stuff is expensive!). I have experience with RV solar but his is a different beast!

I'll start with what we are looking to power:

1. 220 volt 1/2 hp well pump, with as large a pressure tank as we can fit to limit pump cycling.

2. 10 cubic ft fridge, its an energy star rated Vassani.

After those 2 large thing just a hand full of lights, small stereo, maybe a blender hear and there.
I know there are formulas to help one figure power consumption but it seems to get tricky regarding the starting current of electric motors.

What I have already purchased:

1.4 sharp 250 watt poly panels

2. 4 trojan L 16 re b's

Questions: more batterie? 12 or 24 volt? Brand and size of inverter/ charger?
I'm having a hard time getting this all nailed down and any assistance would be greatly appreciated!!

solar_dave · August 2014

Re: Off grid cabin build! Solar component assistance!

How long have you had the batteries? You need to get a charge on them if it is more than a handful of days.

Cariboocoot · August 2014

Re: Off grid cabin build! Solar component assistance!

Welcome to the forum.

Batteries are the first thing you should size, and the last thing you should buy. They start to go bad the moment they are made and all you can do is slow down the rate of decline. Leaving them sit without maintenance charging does not help.

Your well pump is a major power user in terms of immediate load. Unfortunately from "240 VAC 1/2 HP" specs we can't tell how much of one. Is it submersible or shallow-well? Soft start? Any current specifications given? There's quite a wide range of possibilities here.

Needing 240 Volts for one item is an issue best overcome with an autotransformer. That way you can use any inverter that can supply the maximum power needs @ 120 VAC and still have the higher Voltage available.

Definitely go with 24 Volts over 12. The lower Voltage should only be used in applications where there is no choice (i.e. mobile/RV). With four 6 Volt batteries you just put them all in series: same stored power, less loss from higher current. L16 is a case size, so these may be 320 Amp hours or 390 Amp hours or some other size. That is important to know too.

1000 Watts of panel on an MPPT controller (necessary item) should produce about 32 Amps of peak charging current. That would work for the smaller capacity L16's but not so good with the larger ones. You will need a charge controller capable of more than 30 Amps, so that means 45 Amp. 60 Amp, or 80 Amp; before you choose think about and plan for possible future expansion.

Knowing how much power you need to supply both is maximum Watts and Watt hours daily is essential to good design. The formulas are a vague guideline at best; using a Kill-A-Watt to measure the loads (or at least similar ones) in real use conditions will be far more accurate.

As it is your battery capacity is probably at least 3.8 kW hours @ 50% DOD. That's a good range for most off-grid cabins. Your array is on the small side, especially for Vermont (not like you have lots of sun in Winter there). That brings up the question of if this is full-time year-around home or vacation spot for Summer only? Also what happens when the sun fails to shine? Got generator? It's a good idea to have one.

mike95490 · August 2014

Re: Off grid cabin build! Solar component assistance!

So your pump/pressure tank, is going to cycle several times daily, and maybe a few times at night. Have you already bought the pump? Any hope to exchange it for 1/3hp model. 2 wire or 3 wire? (3 wire is easier starting load)

Madmarc · August 2014

Re: Off grid cabin build! Solar component assistance!

It's a 1/2 hp. 2 wire Gould. It's in the well currently so I only have an idea of its amperage, 5 to 6 I think. I plan on switching the pump off at night.

Madmarc · August 2014

Re: Off grid cabin build! Solar component assistance!

I have purchased the batteries, will get a charge on them today. They are 370 ah @ 20 hr.

A 24 volt system is a good suggestion. As is a mppt controller.

I can add more batteries or panels if that's what it takes to get it right.

This will be a full time home for my wife and I. We don't need much (meaning we are not expecting the home to have all the conveniences of a grid home). We've spent the last 3 years living and working around he country in our RV.

Thanks to all for the great info. I think I came to the right place!

Surfpath · August 2014

Re: Off grid cabin build! Solar component assistance!

Madmarc wrote: »

I have purchased the batteries, will get a charge on them today. They are 370 ah @ 20 hr.

Thanks to all for the great info. I think I came to the right place!

It looks like we have similar appliances loads (same time use?). If you look at my sig you'll see I have a 48V system and I use 8 L16RE-B's with 2k in panels. I cycle them from 95% SOC to about 65% SOC each day. I actually would love to have another series of panels (660 more watts) for those (thankfully infrequent) cloudy days when I can barely get 250 watts into the bank for bulk.

PV is cheap, I'd advise you to lean towards more panels rather than just hitting the "ideal day" panel requirement.

By the way, for the first few weeks I got in ~ 2 hours of daily absorb into those RE-B's. It was not enough. They are tall case batteries and like longer absorbs (say 4.0-5.5 hrs) to get close to the recommended 1.28 100% SG. I also used a slightly higher absorb voltage than what the manufacturer recommends. They also seem to do better when they are worked hard (laundry days, for eg.). With an RTS (temp gauge) and a battery monitor using end amps you can safely find that appropriate absorb time and voltage that will work for your system.

vtmaps · August 2014

Re: Off grid cabin build! Solar component assistance!

Madmarc wrote: »

Questions: more batteries? 12 or 24 volt? Brand and size of inverter/ charger?

Welcome to the forum,

What about power tools and laundry and vacuum?

Let's say you are doing a laundry and the water pump comes on. While the pump is still on, the fridge cycles on... do you expect things like this to happen? You may need a bigger system than you think. or not. It depends on how you manage your energy usage. The batteries you have may not be able to handle the current draw. Four of those Trojans in a 24 volt string are rated 370 ah. That rating is based on drawing about 440 watts from them. When you draw more than that they may not perform to your expectations. Especially in the winter if they are cold.

Will your batteries and electronics be in the house, or in a power shed? I ask because if the batteries are cold in the winter you need to plan for that. Keeping your batteries and electronics in the house will keep them warm in the winter, but has its own set of issues.

How far are the panels from the battery and electronics? I ask because it may make a difference in whether you build a 24 or 48 volt system.

What about a generator? Manual or automatic start? Where will you keep it... cold starts are tough on generators.

--vtMaps

Madmarc · August 2014

Re: Off grid cabin build! Solar component assistance!

Hey Vtmaps! Thanks for the reply. We expect to manage power usage to a degree. Sure we will use a vacuum from time to time but will pick those times when say the sun has been shining a few days.

All our solar gear will be in a utility closet in the building. So it will stay fairly warm.

It seems like the question mark in our system is the well pump and how to manage that, we can't afford to have it cycle on 5 times a day.

Through this forum I've learned and decided to increase our number of panels from 1000 w to 1500 w. 3 strings. Does that sound about right? Any suggestions out there regarding inverter/controller choice?

Sorry if my posts are all over the place, I'm typing on a 3 inch wide computer!!

Thanks for all the great info! Great forum!

Photowhit · August 2014

Re: Off grid cabin build! Solar component assistance!

Madmarc wrote: »

Through this forum I've learned and decided to increase our number of panels from 1000 w to 1500 w. 3 strings. Does that sound about right? Any suggestions out there regarding inverter/controller choice?

Might consider 3 strings of 2, rather than 2 of 3, though it will require a combiner box, the charge controller works most efficiently at about 2x the battery bank voltage.

I like Midnite Classic's charge controller, since it now ships with the 'Whiz Bang Jr" it will give you a good idea of the state of charge of your batteries, hence save the price of a quality battery monitor. Naws tends to have the best price on them.

zoneblue · August 2014

Re: Off grid cabin build! Solar component assistance!

Our system is 1.8kW 400AH, 24V, and we find here in NZ (40 degrees lat), that it runs a small place with similarly modest loads year round.

One way to get around the pump loads is to have a small above ground tank, and seperate shurflow type pump(s) to presurise the house. That way you can run the well pump once a week, at a time thats convenient.

Product wise, Inverters are really the crux. You want the smallest one that will run your gear. Big inverters use a lot of power just doing nothing, and also create a risk of radically flattening your bank, if something gets left on by mistake.

Off grid cabin build! Solar component assistance!

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