questions about my proposed offgrid setup

nyarelathodep · August 2013

Greetings all. Being pretty new to solar and offgrid, completely lacking any empirical knowhow, but having spent many an hour researching components pursuant to my needs and budget, I have put together a list which could use some experienced critical analysis. I suppose it would be prudent to present what the system will be used for...its basicaly a pretty simple set up. I have built a cabin in the foothills of western maine which will become my fulltime offgrid home. The power demands will be confined strictly to ac and so everything listed is ac equipment.

Lighting: six led bulbs, low wattage
Electronics: 32" led tv with an 8w draw, dvd player, small radio, charging cell phones and laptop
Water pump: sureflo revolution 115vac (0.9a draw at full flow rate)
Converted chest fridge (less than 120w/day)

Thats the list for my essential power concerns. I have put together a list of components for my system, the main concerns of which are dead reliability, effieicency, and quality customer service and reputation. My proposed setup is thus:

Panels: pair of sharp polycrystaline 250w panels
Charge controller: morningstar mppt 45a tristar
Invertor: morningstar suresine 300w
Battery bank: 4 6v golf cart batteries wired to 12v configuration

So my question, if it can be answered with the information given, would be, are these system components suited to each other, and suited to the power needs listed? I dont mind a bit of overkill or room for expantion either. Any input and advice would be much appreciated!

niel · August 2013

Re: questions about my proposed offgrid setup

welcome to the forum.
well you're on the right track, but you're not quite there. you need to figure what the total daily watt hour draw will be for all of your loads and a killawatt meter would do well for this.
http://www.solar-electric.com/kiacpomome.html

there's no doubt you are off on some of your thoughts on wattage draw as there's no way the 32" tv is only drawing 8w. it might be that figure with it turned off and yes it will draw when turned off too. i am suspecting you are underestimating your needs some.

solar_dave · August 2013

Re: questions about my proposed offgrid setup

The morning star is going to have trouble starting the fridge. Start amps on the compressor can easily be 3 times the running draw. If you want some expansion a larger inverter would be my first suggestion.

Not knowing the exact battery specs I. Would assume this is around 450 amp hours, you are going to want to have about 45 amps of charging current, pretty sure those 2 panels are not going to provide it.

I didn't see you spec a generator for those cloudy days.

icarus · August 2013

Re: questions about my proposed offgrid setup

The Suresine will deliver 600 watts for a number of minutes, perhaps as long as a half hour iirc. I would test the fridge strting current before I committed to an inverter. Consider talking to Morningstar.

Tony

BB. · August 2013

Re: questions about my proposed offgrid setup

nyarelathodep wrote: »

Lighting: six led bulbs, low wattage
Electronics: 32" led tv with an 8w draw, dvd player, small radio, charging cell phones and laptop
Water pump: sureflo revolution 115vac (0.9a draw at full flow rate)
Converted chest fridge (less than 120w/day)

Welcome to the forum NY!

You have done a good job of conservation. The chest fridge may draw ~250 WH or a bit more in hot weather.

Of course, the total energy used (average Watts * Hours of use per day) is still needed--But nothing looks bad here. I would suggest a Kill-a-Watt meter to measure the actual energy used per day. A few things may need a switched outlet to turn off when not used (DVRs and some other items can draw a far amount of power when "turned off" or on standby).

Based on your system as defined:

Panels: pair of sharp polycrystaline 250w panels
Charge controller: morningstar mppt 45a tristar
Invertor: morningstar suresine 300w
Battery bank: 4 6v golf cart batteries wired to 12v configuration

Start with the battery bank. Using "rules of thumbs", I would be suggesting 5% to 13% rate of charge:

6 volts * 4 batteries * 225 AH per battery * 1/0.77 panel+controller derating * 0.05 rate of charge = 208 Watt minimum array
6 volts * 4 batteries * 225 AH per battery * 1/0.77 panel+controller derating * 0.10 rate of charge = 416 Watt array nominal
6 volts * 4 batteries * 225 AH per battery * 1/0.77 panel+controller derating * 0.13 rate of charge = 541 Watt array "max cost effective"

And, on average, the amount of stored battery power to plan on using per day (for 1-3 days of storage and 50% maximum discharge, use 2 days + 50% discharge as "well balanced system" design):

6 volts * 4 batteries * 225 AH per battery * 1/2 days of stored power * 0.50 maximum discharge * 0.85 inverter efficiency =

And based on PV Watts for where the cabin is located (my guess), fixed array, tilted to your latitude:... 1,148 Watt*Hours of AC power per day

Well, looks like PV Watts is down at the moment. I will assume that you have "poor" sun in much of winter so assume 3 hours of sun per day as "break even" system design. If you have more sun, you may use 4 hours per day...

2 * 250 Watt panels * 0.52 end to end system efficiency * 3 hours of sun per day minimum for 9+ months of year = 780 WH per day of "useful" AC power

So my question, if it can be answered with the information given, would be, are these system components suited to each other, and suited to the power needs listed? I dont mind a bit of overkill or room for expansion either. Any input and advice would be much appreciated!

I would probably suggest adding another 250 watt solar panel (you can put three panels in series, or three panels in parallel very nicely with the MS 45 amp MPPT controller):

45 amps * 14.5 volt batt charging * 1/0.77 panel+controller derating = ~847 watt "maximum cost effective array"

So, with your planned battery bank and and "extra" 250 watt panel (750 watt array)--Should reliably supply power for ~9 months of the year purely from solar (unless you have a string of bad weather and/or relatives visiting).

Which brings us to a backup genset--Pretty much everyone needs one. A Honda eu2000i (1,600 watt inverter/generator) can drive a ~40 amp 12 volt AC battery charger pretty nicely--And give you backup power if the inverter (or other parts) in your off grid power system fails. You will probably need to use the genset during poor weather in the three months of winter (If I got the hours of sun correct).

The smaller gensets from Honda (and Yamaha) are very quiet and pretty fuel efficient (try to keep them loaded > 25% of name plate rating for better fuel economy).

If you need more power at times (shop, adding on to cabin, backup to your backup genset), you can get a second, larger (3.5-5kW) genset for when you need the power (using as a primary generator on this small of system may use more fuel/be less fuel efficient than a smaller genest). And they will be louder (build a generator shed?).

But, in the end, without knowing the hours of use for each of your energy users (measured with K-a-W type meter), we are really guessing. But 250 WH per day for the fridge and 500 WH per day for a laptop used 10 hours per day--You are already looking at ~750 WH per day. Your daily required loads should probably only use ~66% to 75% of your planned "average system output". Using 100% per day is expecting a bit much (unless you use the genset more, and/or can dump some loads for a few days until the sun comes back).

And remember fuses/breakers/good size copper wiring + solid solar array mounting. You want a safe system that will not "blow away" or get scraped off the roof by snow.

Speaking of snow (the fluffy stuff I see in my freezer sometimes?) -- If you use the system during the winter, you may wish to have an array that can tilt to near vertical during winter/snow season. Self clearing for snow (need panels high enough to not drift at base) and refection from a snow field out front of the array can really help winter production. If you are not there during the winter and disconnect all battery loads, tilting the array vertical is not near as important.

A 1,000 WH per day (1 kWH) is a pretty spartan energy usage profile, and we have some folks here with exactly that size of off grid system. If you think you want more power, ~3.3 kWH per day may give you a much "closer" to modern electrical experience (full size Energy Star rated fridge, washing machine, well pump, lights + computer + TV + Sat dish).

Energy usage is highly personal--It all depends on your needs. A 3.3 kWH per day system would need a 24 volt (minimum) battery bank (recommended). The MS 300 TSW 12 volt inverter is a great unit--But not for a larger power system.

-Bill

mike95490 · August 2013

Re: questions about my proposed offgrid setup

I think you need some real world testing of the fridge/chest freezer, 120Wh seems way too low for daily usage. And as everyone else says, the suresine 300 is great, but may not be able to start the compressor motor. Maybe you have a friend with a PEAK HOLD clamp-on AC meter you can test the starting draw with.

Steve961 · August 2013

Re: questions about my proposed offgrid setup

nyarelathodep wrote: »

Electronics: 32" led tv with an 8w draw

Please excuse the hijack. Would you mind sharing what 32" TV you found that only uses 8 watts of power. I recently bought what I thought was one of the most energy efficient small TVs, a Samsung 22" LED, and it uses about 21 watts at my preferred settings. Thanks.

nyarelathodep · August 2013

Re: questions about my proposed offgrid setup

Wow, thanks for the speedy and detailed replies gentlemen! Concerning the chest fridge, admittedly my understanding is drawn from reading many reviews and testimonials of offgriders. I have read a few who use smallish 6-8cuf models and run them off the same invertor I listed. Of course, first hand experiments are the final arbiter... i have an external thermostat on order so i can test it myself. I figure if the suresine is not up to the task, i can always lurchase a cheaper secondary invertor dedicated to tbat purpose. The 8w stat of the led tv was listed on the back of the unit itself, so it may be the idle draw. My power usage is most concerned with the fridge, lights, and water pumping. The laptop is actualy really a non issue. I use my phone for almost everything internet related. So long as I have lights, water, can recharge a couple cellphones, and maintain a fridge of some sort, im pretty happy. I definitly dont want things like a wash machine, dishwasher, standard fridge, etc. I prefer to do those things by hand, and have the facility dedicated to those ends. Oh, about the generator...should have mentioned that...i have a craftsmen elite 5550 generator, which I used to build my house and charge the current battery bank of 2 golf cart batteries (via a black and decker battery charger...just a temporary thing to get the job done)...and its perforomed excexceptionaly well for two years so far...considering i got it at a pawn shop for $250. I did have my eye on a yamaha 1000w model (like the above mentikned honda...same class of genny/invertor). The addition of another panel is easily doable...but I wasnt sure how many that particular controller can handle...thats one of my weak points of knowledge, the whole panel to controller to bank sizing. I wasnt even sure i wohld need an mppt type unit, but as you can imagine, in my locale you can get several overcast days, so I figuered the mppt would be warranted. The information provided about my bank and watt hours is exactly what i was hoping for, analysis-wise. That gives me a good solid base to work with. Oh, and so far as the panel mounting goes..rooftop is not an option for me, so my plan is to devise a ground pole mounted, adjuatable unit...metal fabrication is more my forte and ability. I cant thank you folks enough for the replies, and i look forward to more inquiries in the near future

nyarelathodep · August 2013

Re: questions about my proposed offgrid setup

Steve, the model was a samsung 32" led that i saw at a wallmart...i just looked at the specs on the sticker on the back of the unit. That could very well just be the idle draw. I dont watch much tv, so it didnt really factor too much into my needs, and as such hasnt been given my full scrutiny. I did think that was awefuly efficient though. Shows what I know, heh.

Desert Rat · August 2013

Re: questions about my proposed offgrid setup

I have a new 32" LED TV, Sceptre brand from Walmart. I checked it with my KILL A WATT meter, and it draws 62W, 68VA during operation. Turned off it shows 0W, 13VA.

zoneblue · August 2013

Re: questions about my proposed offgrid setup

So many people go straight to the design and skip the energy budget. Its not rocket science, enter everythin you want to use into an excel SS and add up the watt hours per day for each one. A little detective work sometimes required to find the real world draw. Times the result by about 1.5 and thats your target production.

From that point you can check out the weather profile of the location and ponder how many days a year you want to try to meet with solar. If you weather isnt too benign, 360 days is readily acheivable, but my partner hails from maine so i know a little of what its like there. A lower number of production target shortfalls will be in your setup for sure.

Anyway your lifestyle sounds simialr to ours, so checkout our setup in the link in my sig. I suspect youll rapdily conclude that you are under panelled.

zoneblue · August 2013

Re: questions about my proposed offgrid setup

Heres our budget i did last august.
Attachment not found.

BB. · August 2013

Re: questions about my proposed offgrid setup

I am on my phone right now. If I forget to post details on MPPT controllers. You must use MPPT controller because the Vmp of the array is not a good match to your battery bank voltage.

In general, cloudy weather does not matter here.

-Bill

BB. · August 2013

Re: questions about my proposed offgrid setup

One of the reasons folks almost "have to use" an MPPT charge controller is to "match" the higher voltage/lower current of the solar array to the lower voltage/higher current required to charge the battery bank.

For example, say you have Sharp ND-250QCS panels (PDF download). With a rating of:

Maximum Power (Pmax)* 250 W
Tolerance of Pmax +5%/-0%
PTC Rating 223.6 W
Type of Cell Polycrystalline silicon
Cell Configuration 60 in series
Open Circuit Voltage (Voc) 38.3 V
Maximum Power Voltage (Vpm) 29.8 V
Short Circuit Current (Isc) 8.90 A
Maximum Power Current (Ipm) 8.40 A
Module Efficiency (%) 15.3%
Maximum System (DC) Voltage 600 V (UL)/1000V (IEC)

Solar panels are, essentially, current sources that will output current in proportion to the amount of sunlight hitting the face the panel. In full sun, these panels will output around 8.4 amps from around zero volts to around 29.8 volts Vmp (voltage maximum power).

Your proposed battery bank is a 12 volt battery which will be between ~11.5 volts and 15.0 volts (discharging to charging/equalizing). If you where to connect your solar panels directly to the battery bank, you will get around 14.5 volts and 8.4 amps into the battery bank. Or:

14.5 volts Vbatt-charging * 8.4 amps noon time sun = 122 Watts into near full battery bank
12.0 volts Vbatt-charging * 8.4 amps noon time sun = 101 Watts into near dead battery bank

That is what you would get if you "hardwired" the panel to the battery, or if you used a PWM (Pulse Width Modulation--Basically an "on/off" switch). Your "250 watt" panel is only producing around 1/2 of its rated power into the battery bank.

An MPPT (Maximum Power Point Tracking) charge controller is a "constant power" conversion device... At maximum charging current (partially drained battery), the power into the MPPT controller is efficiently down converted (usually using a form of Buck Mode down converting Switching Power Supply) to get energy to the battery. Power IN ~ Power OUT

The math for a "typical day" with typical panels+controller (including typical losses), you would get:

250 Watts * 0.77 panel+controller deratings = 193 Watts out to battery bank:
193 watts * 1/14.5 volts charging = 13.3 amps into near full battery bank
193 watts * 1/12.0 volts charging = 16.1 amps into near dead battery bank

Compared with 8.4 amps through a PWM controller--that 13.3 to 16.1 amps is a significant improvement (even with an ~77% conversion "fudge factor").

Another reason MPPT controllers are used... You can send higher votlage/lower current longer distances than low voltage/high current.

So, say you wanted your 250 Watt array to send the power 50 feet to your solar building with the charge controller and battery bank. Using standard rules of thumbs and a generic voltage drop calculator:

A 750 watt solar array. One with Vmp~17.5 volt panels and Imp~42.9 amps.
And voltage drop 17.5 * 0.03 drop = 0.5 volt drop
Voltage Drop Calculator for 50' of wiring=> 1/O copper cable gives 0.51 volt drop

A 750 Watt array, with three if your panels in series: Imp-array=8.4 amps; Vmp-array=3*38.3v=114.9 volts (note, this is probably too high of Vmp for the MorningStar 45 amp controller--we will have to talk later):
114.9 volts * 0.03 drop = 3.4 volt drop
Voltage Drop Calculator for 50' of wiring=> 1/O copper 13 AWG cable gives 2.6 volt drop

So, you can see that the "lowest voltage" array would use a serious amount of copper wiring (something like Car battery wiring or welding cable) with a "PWM" controller--Whereas the "highest" voltage array would use 14 AWG wiring (normal house wiring).

Note that there are several calculations, configurations, and different brand/models of MPPT charge controllers to choose from.

In this case, with a 12 volt battery bank, most likely the MorningStar can only use 1-2 panels in series (3 is too high of voltage). So, your "array" options would be 1, 2, 4, (2 or 4 if you put two panels in series to keep array wire run smaller diameter). If your runs are short, you can get away with 1,2,3,4 etc. with all panels in parallel. You have to figure out the distance from the array to the battery shed/house. Also, if you put 3 or more panels in parallel, you have to use a fused/breaker combiner box for electrical protection (we can discuss more).

And a second not--Vmp-array has to be larger than Vbatt charging... For example your Vmp~29.8 volts will drop to ~24 volts or so in very hot weather. If you had a 24 volt battery bank, it would need around 29-30+ volts to recharge. So, the Vmp-array would need two panels in series (at least) to get high enough charging voltage--etc. etc. etc.

This is a relatively complex set of decisions--And why we try to work on the "Loads" first--Then step through the options.

Rules of thumbs are not perfect--But they are usually close enough for solar work to quickly design a solar power system that will reliably meet your needs.

Questions, comments?

-Bill

nyarelathodep · August 2013

Re: questions about my proposed offgrid setup

My sincere thanks, Bill, for taking the time to relate all this to me. Im still reading through it all and going off on research tangents to understand it all. What I hadnt considered yet was the options and limitations in connecting the panels.

When wiring ones panels...say you have four in your array, does how you wire them govern how much of a charge your bank is going to get, or is it more an issue of what guage wire you will have to use and what contoller you would have to get?

Lets say that my array will be less than fifty feet from the controller, and lets say i have four of those 250w panels, and lets also say I reconfigure the bank to four 6vs wired for 24v output @210ahs, with the intent on eventual expansion to double the array with a better battery bank (dont want to speculate too much on that except to say im only using the golf cart batteries as a starter system to learn on and have lights and water for now)...what class of charge controller in your estimation would best fit the bill?

Right now, i am looking at my system design from a sort of backwards lens than the typical solar system investor...I have a certain amount of money (~4k) and so long as I have lights and water, i will tailor my lifestyle around the limitations of that budget.

At the same time, i want quality equipment with the ability to expand into a somewhat to incorporate the chest fridge, led tv and other relativly low wattage diversions...nothing so much like the typical household uses. If Im not throwing too big a variable in the mix, I did have an eye on the eventual use of a 24v 850ah new forklift battery...as I have the means to move one around and like the life expectancy.

BB. · August 2013

Re: questions about my proposed offgrid setup

NY guy, I added some paragraph breaks. Much it much easier for us to figure out the points/questions you are raising.

nyarelathodep wrote: »

When wiring ones panels...say you have four in your array, does how you wire them govern how much of a charge your bank is going to get, or is it more an issue of what guage wire you will have to use and what contoller you would have to get?

It is a mix... PWM design rules are quite different when compared to MPPT type controller design rules... And MPPT can be complex enough that many companies create software and websites where you can input your requirements and then it will suggest different supported configurations.

We use a lot of rules of thumbs here to try and keep things "simple" and make for quick back of the envelope caluations. But in all cases, you need to read the specifications/use the tools to finalize/confirm your design will operate within the capabilities of the controllers.

Lets say that my array will be less than fifty feet from the controller, and lets say i have four of those 250w panels, and lets also say I reconfigure the bank to four 6vs wired for 24v output @210ahs, with the intent on eventual expansion to double the array with a better battery bank (dont want to speculate too much on that except to say im only using the golf cart batteries as a starter system to learn on and have lights and water for now)...what class of charge controller in your estimation would best fit the bill?

More or less, it is "difficult" to cost effectively do increase the output of a solar power system by 2-3x or more without a major redesign and some major equipment/wiring change outs.

If you really have two different design requirements (now and 4 years from now)--I would suggest that you do paper designs for both.

In general, any of the major MPPT charge controller vendors make units that will support 12-48 battery banks--And those can migrate. However, at $400-$600 or more a copy--A 400 watt or system, it simply may not make economic sense to use an 80 amp MPPT charge controller.

And, when you go with larger systems--That 12 volt AC inverter + battery bank is not going to scale--You should go to 24 or 48 volt battery bank and that will cost you a new AC inverter... And high powered TSW inverters are not cheap--Again making a "small battery+panel system" wired with a 4kW 48 volt inverter (and its 20-40+ watts of "Tare losses") uneconomic.

Many people will make a smaller system and keep it for a use in a cabin later, or even give to a friend or sell on EBay.

Right now, i am looking at my system design from a sort of backwards lens than the typical solar system investor...I have a certain amount of money (~4k) and so long as I have lights and water, i will tailor my lifestyle around the limitations of that budget.

We can design based on $$$ (don't we all in the end). But, we would really like to nail down your minimum load requirements. If a $4,000 system will not run your required loads--Then it may be worth $zero.

At the same time, i want quality equipment with the ability to expand into a somewhat to incorporate the chest fridge, led tv and other relatively low wattage diversions...nothing so much like the typical household uses. If Im not throwing too big a variable in the mix, I did have an eye on the eventual use of a 24v 850ah new forklift battery...as I have the means to move one around and like the life expectancy.

As soon as you ask for something with a refrigeration compressor--You are in a 3x larger off grid power system. You can look at RV refrigerators, but they are not cheap (and many times, not even that efficient).

If your fridge requirements are not constant (say less than 9 months a year), a propane fridge (get a used one from an RV) may be a better choice--Again loads vs requirements vs costs.

-Bill

zoneblue · August 2013

Re: questions about my proposed offgrid setup

I agree with Bill, for 4K the fridge is going to be tight. It maybe doable, but at the cost of not leaving much left over, as well as the upgrade problems.

If you want to go the upgrade route, rather than cut everything down now, id be inclined to try to leave one whole of something out, and the only thing that comes to mind is the inverter. Buy a used RV 24v fridge (danfoss compressor) and some 12v leds and whatnot, and go DC. heck we run 95% of our house on 24vdc. Then add the inverter when the cash comes later.

FWIW I did do a design two years ago for our motorhome and as a rule of thumb i worked out the fridge i had would need the entire output of 360W of PV. Thats in our climate, pv flat on the roof, etc.

questions about my proposed offgrid setup

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