Need Help With System Design

kaycee

I have an idea or a plan in my head about setting up an off-grid system but I would really appreciate some input or advice on the feasibility or practicality of my planned setup.  My system voltage will be 48V.  My planned array size is 4400W and at this time I have (6) LG 300W panels and I want to add (6) Canadian Solar 440W panels.  I also have a 60A Outback charge controller and I want to run another 60A or 80A Outback charge controller with it.  Last but not least, I have a 4400W MS4448PAE Magnum inverter/charger.  For batteries, I am planning to buy (16) 430 AMP-HRS Crown batteries of the L16 category.  My estimated daily energy usage is 4000W which includes using a 240V submersible well pump.  For backup charging I have a 9000W and a 3500W generator.  I also have a Trimetric battery meter. I know that I am omitting some vital needed info but just ask and I will do my best to get the needed facts to you. The electrical specs for the solar panels are

 LG 300W   Vmpp  31.7V,   Impp  9.47A,  Voc  38.9V,   Isc  10.07A   Max. Sys. Voltage  1000V   Max. Series Fuse  20A  

Canadian Solar 440W   Vmpp 40.7V,  Impp 10.82A,  Voc 48.7V,  Isc 11.48A,  Max. System Voltage 1500V,  Max. Series Fuse 20A
 

Once again, any and all help is greatly appreciated.

BB.

Welcome to the forum KayCee...

Just to make things a bit "easier for me" and give you a design that is accurate for your location/needs/etc...

Roughly (nearest major city) where will the system be installed? Figure out Hours of Sun per day, etc.

How much will you be there? Summer Weekends, 9 (no-winter) months of the year, 12 months a year?

You are looking at lead acid batteries... A good choice for cold weather as they can operate below zero if kept charged.

Li Ion batteries typically must cycle at 40F to 50F minimum (need to be kept "warm" as below freezing, you cannot cycle the Li Ion batteries).

I suggest a minimum full time off grid system starting point is around 3,300 Watt*Hours per day for a "near normal" electrical existence (using propane, wood, etc. for cooking/heating/etc.). Larger systems are certainly "doable"--But the costs rise pretty quickly.

Note I am saying "Watt*Hours" for total energy usage... Watts is a "rate" like driving at 50 miles per hour. And the total distance is 50 mph * 10 hours of driving = 500 Miles driven.

If you have a 4,400 Watt inverter... That is the "peak" continuous power output... Typically spit into 120 VAC @ 2,200 Watts on L1/N and 120 VAC on L2/N--I.e., your max load per "phase" is 2,200 Watts.

If you had a large load, then you could use L1/L2 for 4,400 Watt loads (such as well pump).

Note when looking for "appliances' for off grid use...You want very efficient appliances (like a smaller Laptop computer vs a large Desktop system). And a "solar friendly" well pump that may take 1,200 Watts starting/running vs a standard well pump that may take 2,000 Watts running and >4,000 Watts starting.

For example you could have a desktop computer that runs 10 hours per day (work/play/etc.) vs a Laptop with integrated monitor:

• 30 Watt Laptop * 10 hours per day = 300 WH per day
• 400 Watt Desktop * 10 hours per day = 4,000 WH per day

Getting a "Kill-a-Watt" type meter to measure your "plug in" loads:

https://www.amazon.com/s?k=kill+a+watt+power+meter

There are larger AC meters and DC power Meters too... More or less, for "larger systems", sticking with 120 VAC appliances vs trying to pick 48 VDC (or other) appliances.

Your initial numbers (inverter size, array size, battery bank size) are probably a good starting point. But I like to go through the math, and your location/hours of sun per day, loads, etc. to design a "balanced" system.

-Bill

kaycee

Thank you very much for responding to my post Bill.  I will answer your questions to the best of my knowledge.  I live approx. 50 miles SE of Couer D'Alene, Idaho.  I live here year round and in the summer we have an avg. of 6 hrs sunlight per day and in the winter an avg. of 3.5 hrs of sun per day. The entire system will be installed in a partially below ground walking height crawl space.  All heating and cooking is done by wood and propane.  The well pump is a 1 hp 230 volt Red Lion.  As best I can figure, my estimated daily watt/hrs usage will be 3640 and I will round it up to 4000 watt/hrs.  I have nothing to base it on right now because I currently live in a tent.  Our house will be completed late this summer or early fall.  The appliances we now have are a 24v 6cu/ft chest freezer and a 24v 16cu/ft refrigerator. we have one laptop, two iPhones and two iPads to charge.  Our internet is Star Link satellite and we will purchase an energy efficient washer and gas dryer. we also have a 750w micro wave, lighting will use LED bulbs, one flat screen 32-40 inch television, and one ceiling fan.  House is wired for AC and DC receptacles.  Hope that I did not miss anything you mentioned.

Dave Angelini

I doubt the Magnum will run the well pump. You might consider a Radian and then all of your electronics can be on the same network.
DC receptacles in a new permitted house are pretty much not code and will not pass inspection. Your county will tell you this and you could void insurance if you carry it.

There are 2 kinds of people, those who want solar and those who want more. I would go 2X or more on solar wattage. The more solar you have the more days you will not have to run a generator. 

 New to offgrid folks love a genset. As time goes by,  the genset Love just is not there as it use to be.

Good Luck !

kaycee

Hi Dave and thank you for reading my post.  Could I parallel stack another Magnum and run the well pump without any difficulty?  Checked out the price of a Radian and they are quite spendy.  Also, what is the max solar input that I can have with two charge controllers?  I too would like to maximize my system if I can do it as economical as possible.  I wish that I had found this site much sooner. I have been purchasing equipment little by little for three yrs.  Finish line is in view and I hope to not miss anything this time around.

BB.

KayCee,

The following is how I suggest starting a "paper" system design (always do a full paper design down to the exact equipment before you start buying). This is a pretty simple/quick design based on our rules of thumbs for an off grid home. As always, feel free to adjust, ask questions, etc. about any of this.

I like to:

• Measure/estimate energy usage per day (by season if applicable)
• Review loads... Refrigerators, Freezers, and Well Pumps are usually "problem loads" that add to size/cost of system.
• Size battery bank to support daily loads.
• Size battery bank to support "peak continuous loads" (surge loads are assumed to be ~2x peak inverter rating)
• Size solar array to properly charge battery bank (use 5% for weekend/summer useage, 10%-13%-25% for full time off grid solar)
• Size solar array to size daily loads (by season if applicable)
• Review results and add a 50% to 65% of Predicted harvest "fudge factor" (or as David says, size system, then make 2x larger solar array to bring generator usage to near zero hours per year).

At this point, 4,000 Watt*Hours per year. A full size energy star fridge, LED Lighting, a "solar friendly well pump", LED TV, Laptop, Cell phone charging, clothes washer (during good sun or with backup genset).

The well pump--Standard well pumps with induction motors (Franklin, etc.) are not very efficient, and (usually) have high starting surge current.
 
Solar "Friendly" well pumps are typically Permanent Magnet 3 phase motors with a "variable frequency drive" (a.k.a. a variable frequency inverter). The PM motors are much more efficient and the VFDs are "soft start" and can vary pump RPM based on water needs and available energy (some pumps can be connected directly to a solar array without any batteries or generators--The VFD takes available solar energy and modulates the pump RPM base on available solar power).

These solar friendly pumps are not cheap--But make designing and running a solar power system much easier. Grundfos pumps is a high end pump mfg. And their SQF Series are the most flexible in-well pumps. Reliable, lots of options, and no surge current over running Watts. The SQF is solar panel, DC, and AC power compatible. From our host, NAZ (Northern Arizona Wind & Solar):

Product Documents

Grundfos 6 SQF-2 Pump Curve Data

Sales Brochure #1

Sales Brochure #2

Product Guide

SQFlex Setup and Sizing

SQFlex Service Instructions

Pump Motor Specifications

Grundfos SQFlex Warranty

There are other alternatives. A "slow pump" (lower power) to cistern, then use a pressure pump from cistern to house (can even be an RV style pump for cabins and outbuildings). In some areas, off grid homes may have building regulations that require water storage tank(s) and fire pumps (some areas in California require fire water/pumps).

OK... The math & modeling. The battery bank assume that you have 2 days of "no sun" and discharge to 50% of capacity (longer battery life). This is usually the sizing for Lead Acid/AGM batteries. If you use Li Ion, smaller AH battery bank can make sense.

For cold climates, Lead Acid are nice... They work down to 0F pretty nicely (temporary loss of capacity when cold--Can heat/insulated box to keep warm). Li Ion batteries really need to be around 50F or warmer to cycle. Cold Li Ion batteries cannot charge without being damaged.

• 4,000 Watt*Hours per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max planned discharge * 1/48 volt bank = 392 AH @ 48 VDC minimum suggested
  

The minimum suggested Lead Acid battery for an AC inverter is 100 AH @ 48 volts per 1,000 Watts of inverter. Your 4,400 Watt inverter, would suggest 440 AH @ 48 VDC minimum (2x larger bank is not bad if you have heavier loads or ~880 AH @ 48 volts).

Now sizing the solar array... Firs based on battery bank capacity and recommended charging. 10-13% minimum rate of charge works well for Lead Acid when used for full time off grid homes. Li Ion batteries can take much higher rates (charge faster/more efficiently/less time than Lead Acid).

• 440 AH * 58 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,314 Watt array minimum off grid array
• 440 AH * 58 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,308 Watt array nominal off grid array

Of course, if you choose a 2x larger AC inverter with a 2x larger AH battery bank, then need more solar panels. Or spend money on a "solar friendly" well pump and don't go with the larger inverter and/or battery bank.

Next is sizing the array for the amount of sun you get... Fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Coeur d'Alene Idaho
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 42° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.12	3.30	4.37	5.04	5.24	5.53
Jul	Aug	Sep	Oct	Nov	Dec
6.07	5.88	5.11	3.71	2.33	1.88

For the three months of winter--It appears you may not have a lot of sun... If we design for 9 months of sun, and assume less power/more generator runtime in winter... Pick February at 3.30 hours of sun per day as "break even":

• 4,000 WH per day * 1/0.52 off grid system eff * 1/3.30 hours of sun per day (Feb) =  2,331 Watt array "Feb break even" array

If you assume that you only use 50% to 65% of predicted harvested solar power:

• 2,331 Watt array (Feb break even) * 1/0.50 "solar fudge factor" = 4,661 Watt array (to carry better through winter, support additional loads when need, and reduce genset runtime).

So it looks like you can "justify" a 4,308 to 4,661 Watt array if you are living full time in the house.

Note that I am using 4,661 numbers to avoid round off errors and so you can follow the math. In reality, solar "math" is only accurate to about 10% (i.e., 4,661 +/- 466 Watt array). Don't feel like you have to use a 440 AH battery bank if you are using 415 AH batteries...

In general, batteries are expensive and have a (relatively) short life (5-7 years maybe for "good quality" Lead Acid. 15+ years for "forklift" batteries, etc.). Batteries are also the easiest to "murder" (typically through under charging, over discharging, poor maintenance).

Generally, the #1 place to put "extra money" is the solar array. Solar panels are "cheap" and last 20+ years. And a well charged battery bank is a "happy" battery bank.

The next place to put your money is, possibly, Lithium Ion batteries (LiFePO4 typically). More efficient, charge faster (large array in short periods of sunlight). However they tend to be more expensive and you are a bit more on the "bleeding edge" of technology. Li Ion batteries need a BMS (battery monitor system) to ensure your expensive batteries are operating withing voltage/state of charge/temperature/current specifications. Lead Acid batteries tend to be cheap(er) and more forgiving. Li Ion batteries, done right, have made for many happy users.

Nothing above is "written in stone"... Just a starting point to see how well this will work for you and figure out where your money is going.

The idea is to get the requirements documented and everything planned out.

-Bill