Off Grid - Shed, light loads.

mr.radon · August 2012

Maybe a few people can check over these numbers.

I'm building a small 12X16' shed, want to run a few lights 10W (2.5WX4) LEDs outside), 140W (35WX4) inside. A couple of outlets just to allow me to charge a drill and run a radio. Maybe 50-100 KWhr per year usage; more in the summer, less in the winter.

I have (5) slightly used 50W 14VDC panels laying around...plus (4) Yellow top Optima D31T's (RCap 155, c/20 75) and a 3000W 120VAC inverter w/ a shore power connection.
{Vendor samples from a work project}

So, I figure all I need is a charge controller. Hook all the panels up in parallel with blocking diodes, install them on the roof. Connect the inverter to the battery bank, connect the output of the inverter to the loads via a small service box.

Questions I have are:
1) This sound reasonable?
2) Do you need to pull an electrical permit for an off-grid system? (location Snohomish County, WA)

PVWatts tells me the south facing 250W panels can make 248 kwhr/year, but I figure the losses in an offgrid are higher. Should be good right?

mike95490 · August 2012

Re: Off Grid - Shed, light loads.

> 2) Do you need to pull an electrical permit for an off-grid system? (location Snohomish County, WA)
Some spots in California, want permits for >50w systems, that covers just about everything other than garden path lights !

Your inverter, at 3,000w, likely idles at 100w or so, with no load, which will consume your PV harvest pretty quickly !

I'd say, get a 300w SureSine inverter from Morningstar. Pretty efficient, pure sine wave, has a remote ON-OFF switch, and is intended to be hard wired. Many mod-sine inverters cannot be wired with a neutral ground scheme.

Plan on your daily usage, to be no more than 50% of your PV nameplate harvest.

BB. · August 2012

Re: Off Grid - Shed, light loads.

mr.radon wrote: »

I have (5) slightly used 50W 14VDC panels laying around...

What exactly is the Vmp/Imp/Voc of the panels you have? 14 VDC is a bit of an odd-ball, especially if this is Vmp rating.

Vmp should be around 17.5 to 18.6 volts or so for charging a 12 volt battery bank with a series type charge controller (PWM or MPPT).

A good estimate for off grid systems is around 0.52 for end to end system efficiency (from solar panel rating to usable AC power). PV Watts defaults to 0.77 derating for grid tied systems.

-Bill

Cariboocoot · August 2012

Re: Off Grid - Shed, light loads.

Adding my two cents:
You will not need blocking diodes with a charge controller.
The red top Optima battery is not a good choice; they are starting batteries. The yellow top is the deep cycle version.
You'd have a 250 Watt array. Might get 15 Amps out of it (could be used with the Morningstar 15 MPPT controller if the panels are odd Voltage, although you might have to drop one panel to meet the max input Voltage limit).
Possibly harvest 500 Watt hours AC per day, maybe a tad more with good insolation.

Totally agree with Mike about the inverter: a 3kW 12 Volt inverter is going to be a MSW clunker you won't be happy with.

Dill · August 2012

Re: Off Grid - Shed, light loads.

I'll 2nd (or 3rd) the suggestions above. I have the Morningstar Sure Sine 300 in my 12x16 and I couldn't be happier with it. Great inverter. I run it off a pair of 220ah golf cart batteries in series.

mr.radon · August 2012

Re: Off Grid - Shed, light loads.

The inverter is one I helped design for our trucks. Its a true sine and has 0.010A (corrected the earlier mA error good catch) current draw while it idles, this is an early prototype (I have several more). I also have a remote control station that I'm going to wire next to the light switch so I can shut it off if it does drain the battery or to secure it for winter. Like I said these are all vendor samples from a truck's 120VAC inverter system. I don't want to spend lots of $$$ if I already have these parts.

The panels are Unisoloar US-64 panels. I said 14VDC since that is the operating voltage of typical truck electrical system. So I have 320W. I used to use these to charge batteries on trucks while out doing winter testing. (wired directly to the trucks battery bank no charge controller)

Using PVWatt at 0.52 = 208kWhr with 320Watts. I guess I'll be fine.

Spec's:
Amax: 3.88A
Vmax: 16.5V
Aclose circuit: 4.8A
Voc: 23.8V

Cariboocoot · August 2012

Re: Off Grid - Shed, light loads.

At 16.5 Volts the Vmp of those panels is actually too low to properly recharge 12 Volt systems. Between the wiring and heating of panels the V-drop can take the Voltage down below the point where it can actually charge the battery. It may not, but the possibility is certainly there.

No charge controller is a bad idea for anything larger than trickle charging.

It is not a good idea to make a 3kW 12 Volt inverter. At maximum power the DC current draw is very high and difficult to handle. I also doubt it draws 0.01 mA at idle. 0.01 Amps maybe.

You do what you want. Certainly no one has to listen to me. It's not my money you're spending. :roll:

niel · August 2012

Re: Off Grid - Shed, light loads.

my 2 cents.

the pvs you have would probably work even though the 16.5v vmp is not optimal. this is problematic with batteries that require a higher charge voltage under hot climatic conditions as the pvs may not suffice to provide enough voltage. having 2 pvs in series would overcome this handicap and you could parallel another string of 2 in series which would leave the 5th pv out of the equation and would also mandate an mppt type controller to down-convert to 12v. golf cart batteries are usually in the higher charge voltage range, but at least the ambient temps in your area may help to keep the pv temps down some keeping in mind that sunny days could make the pv temps soar on the pvs. the 2 x 2 pv arrangement would allow you to not worry about the higher battery voltages/low vmp pv outputs. if you could locate another us64 then you could have 3 strings of 2, but you would be in the higher current area than a low end mppt cc may handle and this would require fusing/circuit breakers for the 3 inputs.

will these pvs suffice is hard to say for you are in an area that doesn't see lots of sun. any battery with about 50ah and above may suffice based on the higher 100kwh/year figure and 50% dod max. more capacity would be warranted if you account for days without usable sun, but if you go out too far in autonomy then the battery capacity would warrant more capacity in pv to properly charge them. 4 of these pvs would charge a 12v battery with about 100ah just fine, but 220ah golf cart batteries i'd say is pushing it.

if your inverter works for you then use it. just don't try to put loads on it at more than about a kw or so as the wiring and battery capacity would become much higher and difficult. even loads at 1kw have very large gauge wires and large capacity battery banks, but if you stay low in your load goals as you've indicated then you will probably be fine. i'm not too sure how efficient your inverter would be with the low capacity loads though, but if the 10ma idle current draw is an indicator then you may be ok there.

now simply speaking you could just opt for a pwm charge controller and hope for the best with all 5 pvs paralleled just to try it. it would be cheaper to do and would allow all 5 pvs to be utilized. fuse each of those pvs though.

mr.radon · August 2012

Re: Off Grid - Shed, light loads.

I like the idea of wiring the (5) panels in series and getting a good charge controller. Gave me a few more ideas...

I already managed to strip 30' of 4/0 battery cables and 250A MAXI fuse out of the old trucks, I'm set for wiring to the input of the inverter from the batteries and the battery to battery connections. I really like the Yellow Tops, well at least on our trucks they work quiet well as Hotel batteries. (vibration tolerance)

I fixed the off current draw error in the earlier post. Yes this inverter does real well with simple stuff like cell phone chargers and the like. Its designed for a truck, we care a lot about "key off" loads and efficiency.

Can someone outline the permitting of "Off-Grid" electrical systems like this. I called the L&I permit line and the lady was very confused what "Off-Grid" was. I got nowhere. Maybe Google can help.

niel · August 2012

Re: Off Grid - Shed, light loads.

going all 5 pvs in series may be pushing your luck with the open circuit voltage (voc) limits to the mppt controller on cold days unless you have a midnite classic. a series pv arrangement i do not recommend you do if on a pwm controller as this would be very lossy on power. i'm not sure if the higher voltage versions are available in the classic lite, but the hypervoc would prevent damage to it on the 150 model up to 150v +12v or 162v, which would be plenty of headroom. just to let you know now, i did not go through the calculations on the high voltage limits from the pvs on cold temps, but to go from around 150v to 12v is inefficient for any controller to do even if they can tolerate the high voltages. higher voltages usually come into play when long wire runs must be used or you are trying to minimize the number of strings due to other factors as the major reasons.

ultimately, how you wind up configuring this is up to you.

mr.radon · August 2012

Re: Off Grid - Shed, light loads.

Called the Electrical Permit main office and the Inspector told me yes I need a permit, but what type will be yet determined. Interesting....will have to ask what NEC codes are applicable to off grid wiring? Hope they have no issue with the inverter....LOL

niel · August 2012

Re: Off Grid - Shed, light loads.

nec is difficult for some to read, interpret, and then put into practice. maybe woo the inspector to give you advice as you would be interested in doing this for yourself and respect his opinions on how to do it. brownie points can't hurt. inspectors will take a simple thing and make it complex and expensive so get to an understanding with him and stay in good grace.

he will tell you that all items in a system need to be certified. the inverter would need it too, but i don't think they have applied this certification stuff to batteries as of yet. when j. wiles has nothing better to do he might pull that stunt on us. us64 pvs have a certification if you look on the back of the pv. it could be ul, ce, or a number of other certification stamps and all are pretty much similar.

mr.radon · August 2012

Re: Off Grid - Shed, light loads.

I had a brain storm, I think I'm going to return all my 120VAC parts and go with a 12VDC system. I have enough truck sleeper LED dome lamps to light up the entire inside of the shed, and LED reverse LED lamps to provide accent lighting outside the shed.
I'll have to talk to the inspector about that. Am also wondering if they have a clause in the NEC for experimental setups so every time I want to tweek this setup I don't have to go back and get a freaking permit.

I'll just used the Inverter as a portable power system, no permanent installation.

CDN_VT · August 2012

Re: Off Grid - Shed, light loads.

TA-DA !!
My thought Also !!!.

And you can build great 12V LED lighting , charge the drill using a charging station for many items, I charge all my stuff with the 12 solar .All my grid tie outside lighting has been converted to LED , big pain because most fixture's are made for e26 , Pin type. I had to change internally the bulb mounting to use the correct LED. I dropped 600W to 50 watts for all ranch outside lighting for low key walking. I next need to look at the lighting if the ranch needs daylight at night,(Cougar/bear visit) thats the next upgrade.
I have a 12 volt area , use a MPPT morning star. Im amazed at what you can do with just 12V system. Lived in the QCI's for years that way 60-70's.

Fuse's , and protection so no fires is #1 .
Get a good DVM and learn the amps / draw. this MPPT has huge expansion controller, plus.

Later you can do the AC voltage , but to invert dc 2 ac , you going to use power for just inverting. Your panels are 200w 4 in two strings , series /parallel to the controller.Plus a spare.
Very similar to one of mine.
VT

mr.radon · August 2012

Re: Off Grid - Shed, light loads.

After talking to the head inspector I found out that even the 12VDC components need UL listing, there goes the idea of using truck parts, no UL listing. Oh well. I guess the best option is to just use a small inverter that has a UL listing. Time to research...So what would forum members recommend?

Mind you, I'm not driving very large loads.

vtmaps · August 2012

Re: Off Grid - Shed, light loads.

mr.radon wrote: »

I guess the best option is to just use a small inverter that has a UL listing. Time to research...So what would forum members recommend?

Morningstar 300 watt has a very good reputation:
http://www.solar-electric.com/mosu300wasiw.html
--vtMaps

Off Grid - Shed, light loads.

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