New small Off-Grid System Design and Evaluation

lockwhip · February 2013

Hi Everyone,

This is my first post so hope I'm in the right place! After many weeks and hours surfing, reading articles and watching videos "I think" I have come up with the components to assemble an expandable off-grid solar system.

I started with my power needs and sized a battery bank accordingly: (4) times 6v, 200Ah AGM in series for 24V, 200Ah (4800 watts fully charged - but will discharge 20-50% max, so planning on 960-2400 watts usable).

Next I sized the array to charge the battery bank at a 10% rate and came up with (200Ah x 10%=20amps x 24V=480 watts x 1.25 (1 amp out of the battery bank requires 1.25 amps to recharge (I think)) = a 600 watt array.

There are so many good MPPT controllers on the market, I spent alot of time on these and decided on the Midnight Solar Classic 150 so I could expand the PV array and Battery bank later ( and use two 300 watt PV panels to start).

I did not want to depend upon an inverter/charger to recharge the battery bank so selected an inverter only to meet my power needs and will solely depend on the PV panels to recharge the battery bank.

I live in AZ and average 4.7 sun days winter, 7.1 sun days summer (I think).

Here's my last cut on a system component list:

(4) 6V 200Ah AGM batteries in series for 24V, 200Ah
(2) 300 watt PV panels in series
(1) Midnight Solar Classic 150 charge controller
(1) Midnight Solar MNBCMS battery capacity meter
(1) Cotek 1500 watt, 24V Inverter
(1) Midnight Solar Mini-DC Disconnect box with 250 amp breaker

So looking at this to start knowing the inverter will handle all my loads but may want more battery bank and PV panels to "last longer".

I have really enjoyed reading this forum and look forward to any input and advice you all can provide to steer me in the right direction.

Thanks a bunch... lockwhip (my favorite dance happens to be west coast swing!)

westbranch · February 2013

Re: New small Off-Grid System Design and Evaluation

Welcome to the forum, good choices so far ,
now just complete the puzzle and tell us the loads, being as specific as possible (knowing that this is a plan) so we have something to go on as all good systems are based on the load to be supported.

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

I would suggest a bit larger array based on battery charging rates (I am not sure I understand "your" formula):

200 amps * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 753 Watt Array
200 amps * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 377 Watt Array Minimum

600 Watts is fine too--Really depends on how many Watt*Hours per day you want and any seasonal variations (summer fans, winter/poor weather occupation, etc.).

For example a typical system, your AC output would be:

377 Watts * 0.52 system derating * 4.7 hours of sun = 921 WH per day
600 Watts * 0.52 system derating * 4.7 hours of sun = 1,466 WH per day
753 Watts * 0.52 system derating * 4.7 hours of sun = 1,840 WH per day

You are blessed with lots of sun, on average, so your array to battery size can be a bit smaller and still get lots of AC Watt*Hours per day even in winter.

-Bill

vtmaps · February 2013

Re: New small Off-Grid System Design and Evaluation

Why do you want AGM batteries? In some systems they make perfect sense, but in other systems they do not. I especially don't recommend them for a first system where you are learning.

You want an expandable system? That is doable within limits. There are many stories of woe on this forum when folks realize just how non-expandable their system is. You need to plan now for where you are going. The easiest thing to add to any system is more panels, but even that may have issues with the size of your existing cables or the capacity of your controller. Do not plan on adding more batteries to an existing bank, although you can replace your entire bank with new, larger capacity batteries.

Also, do you plan to have a generator? If so, you will want a battery charger, or an inverter that has a charger mode.

--vtMaps

icarus · February 2013

Re: New small Off-Grid System Design and Evaluation

Potentially adding to the battery bank in future, is another strike against expensive AGMs. I would stick with cheap(ER) FLA batteries as a "training wheel" set. When you kill the first set (which most folks do...prematurely, then you can make different battery choice an option.

Tony

NorthGuy · February 2013

Re: New small Off-Grid System Design and Evaluation

I would go with 3 panels instead of 2. You still will have only one string, and it'll be easier to charge batteries even when you have a good part of your solar harvest going directly to your loads.

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Here are the loads. I won't be running the microwave or coffee maker at the same time.

Item Description watts hours/day watts/day ah/day 24v
Laptop computer 20 4 80 3.33
Modem + Router 12 4 48 2
(1) 60w lightbulb 60 4 240 10
Clock Radio 8 24 192 8
Cordless Telephone 5 24 120 5
Fan 60 8 480 20
Microwave oven 850 0.5 425 17.7
Pool pump 84 8 672 28
Coffee Maker 900 0.25 225 9.38

Totals 1999

2482 103

lockwhip

Photowhit · February 2013

Re: New small Off-Grid System Design and Evaluation

lockwhip wrote: »

Here are the loads. I won't be running the microwave or coffee maker at the same time.

Item Description----watts
hours/day---watts/day----ah/day 24v
Laptop computer
20
4
80
3.33
Modem + Router
12
4
48
2

Nice effiecent Laptop, My small netbook uses about this, have you checked this?

lockwhip wrote: »

(1) 60w lightbulb----60
4
240
10

Do you not use compact floresents? light output of a 60watt for 15watts...

lockwhip wrote: »

Clock Radio
8
24
192
8

Likely the only thing I use disposable batteries for is a couple clocks, run audio from laptop or an MP3 player or radio when you need it.

lockwhip wrote: »

Cordless Telephone---5
24
120
5
Fan
60
8
480
20

We have a fan thread somewhere here, I really like the O2cool 10" fans really put out a lot of air for minimal wattage.

lockwhip wrote: »

Microwave oven
850
0.5
425
17.7
Pool pump
84
8
672
28
Coffee Maker
900
0.25
225
9.38

Where are you getting the water for your pool?

Current draws greater than c20 rate (1/20th of your battery capacity, 200/20=10amp x 24 volts= 240watts) will depleat the battery faster than the indicated wattage.)

lockwhip wrote: »

Totals 1999
2482
103

Sure looks like your trying to use every watt you produce, problem is you simply can't do that with an off grid system...

Are you going to have a refridgerator?

Looks like you system will be seriously short, usually expect 50% of panel rating (X) the #hours of sun as the available usable wattage through to output AC, for the system losses. For example your inverter is only 85% effiecnt max so you can multiply your total x1.2 for your usage...

NorthGuy · February 2013

Re: New small Off-Grid System Design and Evaluation

IMHO, the system looks too small for these loads.

Your peak loads are about 50A load on batteries and come in the morning when batteries are discharged the most. This may be more than you want for 200AH bank.

Even with your mostly day-use loads, your daily production may be able to support all your loads only on a good day. If you want to have at least some power on cloudy days you would need to produce this power on a sunny day and store it. With all the inefficiencies, after you put your power through batteries, you will get no more than 60% back (10% lost in charger, 10% in inverter, 20% in batteries). So, if you want to use 2500Wh in a cloudy day, then you must have produced and strored 2500/0.6 = 4200Wh the day before. That is in addition to at least 3000Wh that you had to produce for immediate use. So, your production needs to be at least 7200Wh.

There could be several cloudy days in a row. You need to have a plan how to deal with them. You can conserve energy, or you can grow your system, or you can add a generator. If you have a generator you can use it to charge batteries on cloudy days. This will let you get away with smaller system.

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hello Northguy, Westbranch, BB, Vtmaps, icarus and photowhit:

Thanks for your all your input on my off-grid design. I went back and did some "watt hour soul searching" and got my absolute needs down to 1200 wh/day.
Some of my calcs where off, I deleted a few loads I can do without and am using more efficient devices based on your suggestions.

I am thinking now about the following system for my 1200 wh/day load:

(2) 315 watt PV panels for 630 watts total
(1) Midnite Solar Lite 150 CC
(1) Mini-DC Disconnect 250 A Power Center
(1) Cotek 1500W, 24V Inverter
(4) AGM 6V, 200aH batteries (sorry, gotta have sealed - or the wife leaves me... I understand the wet cell golf cart can teach me a thing or two, but the wife refuses to maintain them - acid............... turkey baster................ well you get the idea!)
(1) Trimetric TM-2025-A battery monitor w/ 500amp,50mv shunt

I still have not decided how to mount the panels, ground fixed or pole x-axis...

What do you think? Thanks a bunch.

lockwhip

p.s. I have a tri-fuel eu2000i

westbranch · February 2013

Re: New small Off-Grid System Design and Evaluation

OK, now which loads are you still operating? what they are does make a difference...

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

First, how do you like your tri-fuel conversion? Run it much yet? Who (or what kit) for the conversion? Much runtime yet?

It seems like your system is doable... With 200 AH @ 24 volts--You probably should not plan on more than ~1,000 Watts of solar array (may have MPPT controller/battery bus voltage stability issues).

Is ~1.2kWH per day "enough"? Will this grow to a fridge, washer, etc. to keep your spouse happy? You might look at 3.3 kWH per day (100 kWH per month) type system. Your inverter should already be large enough to run an Energy Star Fridge?

Full time, weekends, spring/fall, etc. occupation?

-Bill

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hi westbranch, here's the loads:

LOADS rated
Item Description watts hours/day watts/day amps ah/day
Ipad 2 10 4 40 0.1 1.7
Modem + Router 12 4 48 0.1 2.0
Radio 3 8 24 0.0 1.0
(1) 15w fluorescent lightbulb 15 4 60 0.1 2.5
Cordless Telephone 5 8 40 0.0 1.7
Box Fan on low 35 8 280 0.3 11.7
Swimming pool pump 84 6.5 546 0.7 22.8
Coffee Maker 800 0.2 160 6.7 6.7

Total 964

1198 8.0 49.9

lockwhip

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hi BB,

Just installed the tri-fuel conv kit last weekend so only a couple hours but runs fine on propane and reg unleaded. Still have to plumb a convenient nat gas line to check that out.

I purchased the kit at propanecarbs.com - excellent cust service and installation instructions.

After looking at the PV Panel/ Battery cost, the wife said just run the big stuff on the gen and stockpile propane!

Yea, the cotek 1500 watt inverter is sized to run the frig in a pinch.

lockwhip

westbranch · February 2013

Re: New small Off-Grid System Design and Evaluation

Are you going to use a propane stove? if so it would be good to look at that for perking your coffee and using a large thermos for keeping it hot...

that 800W is the killer....???

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hey westbranch,

You are so right - the coffee maker is the killer - and the propane stove will do just fine.
I did nix the microwave demon from the load calcs substituting the propane stove but overlooked the coffee maker - cudos to you!

lockwhip

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Any ideas on PV panel mount?
Read some "stuff" about maximizing PV array output with simple X-Axis manual rotation ( I'm home most days ).
Is this worth any marginal or major consideration?

Thanks, lockwhip

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

You can play with PV Watts for a location near you... It has fixed (you enter the tilt and compass number) and 1 or 2 axis tracking. You can see how it plays out for you.

In general--I like less maintenance/playing around the system. Solar panels are "cheap enough" to do a fixed array just a bit larger. If you do tracking--there is the argument that you will get more hours of sun--which means more hours of charge time on the battery bank (i.e., lower average current for more hours is better than high peak current over shorter period of time).

Good quality trackers are not cheap. I have seen mixed reports on reliability.

-Bill

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Thanks BB for the PV panel tracking info.

I'll check out the x-axis tracking on pv watts to satisfy my curiosity.
I do have to agree with you that, simplier is better...

-lockwhip

Photowhit · February 2013

Re: New small Off-Grid System Design and Evaluation

lockwhip wrote: »

Hi westbranch, here's the loads:

LOADS..........................................................rated
Item Description.......watts....hours/day..........watts/day ................amps........ah/day
Ipad 2.....................10...........4..........................40.....................0.1............1.7
Modem + Router........12..........4..........................48......................0.1...........2.0
Radio........................3...........8..........................24.....................0.0............1.0
(1) 15w CFL..............15..........4...........................60.....................0.1............2.5
Cordless Telephone.....5...........8..........................40.....................0.0............1.7
Box Fan on low..........35..........8.........................280.....................0.3..........11.7
Swimming pool pump..84........6.5........................546.....................0.7..........22.8
Coffee Maker............800........0.2........................160.....................6.7...........6.7

Total.......................964...................................1198.....................8.0..........49.9

lockwhip

HTML doesn't allow for multiple spaces, so if you hit the period or dash it will be easier to read your info.

So your around 1000 watts of AC so with the basic "50% through put" array wattage x hours of solar isolation/2 = usable wattage , we know 4 for isolation and 1000 for watt hours for usable wattage. so multiply both sides by 2 we get array wattage x 4 = 2000 divide both side by 4 and we get array wattage = 500 so your in the ball park. Please note your inverter is only 85% effiecent so I'd say your closer than the numbers, and of course you have to have waste, or you always want to have more wattage than needs. This doesn't account for any cloudy days or automony so cloudy days you need to run your generator. Since you have no extra to store.

I wouldn't invest in a trimetric if you go with the classic lite, if your computer literate Midnite has announced a shunt based modual for the classic should be out this year, I suspect this will allow the classic to better understand where your batteries true state of charge is and allow it to better charge your batteries if they receive charging from other sources. While they are saying used with the classic display I can't imagine it won't work via the network with the classic lite, though I haven't asked yet.

inetdog · February 2013

Re: New small Off-Grid System Design and Evaluation

Photowhit wrote: »

HTML doesn't allow for multiple spaces, so if you hit the period or dash it will be easier to read your info.

Another, perhaps simpler, solution is to paste the formatted table you are copying from into a CODE section. To do this hit the Go Advanced button below the text entry area and select the # icon in the expanded formatting bar.

Here is an example:

Heading    Column1   Column2  
line 1      1:1       1:2
line 2      2:1       2:2

I just constructed the table WYSIWYG using a monospace font in a text editor and then pasted it into a CODE field.

Now if you are trying to copy and paste from a table which was constructed in a document publishing application and uses TABs, then you have a more difficult problem.

vtmaps · February 2013

Re: New small Off-Grid System Design and Evaluation

lockwhip wrote: »

(4) AGM 6V, 200aH batteries (sorry, gotta have sealed - or the wife leaves me... I understand the wet cell golf cart can teach me a thing or two, but the wife refuses to maintain them - acid............... turkey baster................ well you get the idea!)

(1) Trimetric TM-2025-A battery monitor w/ 500amp,50mv shunt

lockwhip wrote:

After looking at the PV Panel/ Battery cost, the wife said just run the big stuff on the gen and stockpile propane!

She may leave you when you murder those expensive AGMs. Can you promise to buy her some AGMs just as soon as you've murdered the GCs?

Photowhit wrote:

I wouldn't invest in a trimetric if you go with the classic lite, if your computer literate Midnite has announced a shunt based model for the classic should be out this year

A few years ago I waited and waited for Midnite's classic... finally I couldn't wait and bought an outback fm60. A beginner needs a battery monitor, especially if you have AGM batteries. If you can't wait for Midnite, buy a trimetric now.

--vtMaps

Photowhit · February 2013

Re: New small Off-Grid System Design and Evaluation

inetdog wrote: »

Now if you are trying to copy and paste from a table which was constructed in a document publishing application and uses TABs, then you have a more difficult problem.

Then you "save as" a gif and add as a photo..

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hello to all you good folks at wind-sun:

You have been so helpful... thanks a bunch...
I really like the term "harvesting" - some how makes me feel like a solar farmer ( true farmers are a cut above me, for sure... ).

So here's my havesting question:
If I have a 24V system with 600 watts of panels and 5 sun hours and derate the entire system by a factor of 0.52,
can I expect to replenish my 200 Ah, 24V battery bank with approx 66 amp/hours per day (no load of course)?

I'm running numbers constantly to try to get a better understanding on system expectations before I purchase stuff.

Again, thanks a bunch...
-lockwhip

niel · February 2013

Re: New small Off-Grid System Design and Evaluation

i get 65ah "net harvested". maybe cut back on the loads or add more pv.

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

lockwhip wrote: »

So here's my havesting question:
If I have a 24V system with 600 watts of panels and 5 sun hours and derate the entire system by a factor of 0.52,
can I expect to replenish my 200 Ah, 24V battery bank with approx 66 amp/hours per day (no load of course)?

This is how I run the basic math (pretty much "worst case" assuming loading at night and charging during the day).

First, how many WH per day from 5 hours of sun:

600 watts * 0.52 system eff * 5 hours of sun = 1,560 Watt*Hours per day (AC power)

The DC power available would be less the 85% inverter efficiency. 0.52 total AC sys / 0.85 inverter eff = 0.61 DC eff (if not using an AC inverter):

600 watts * 0.61 DC system eff * 5 hours * 1/24 volts = 76 Amp Hours @ 24 volt system

So, the available power for DC loads is greater than your 66 AH per day requirement.

We also suggest 5% to 13% rate of charge for a battery bank. A 600 watt array would support:

600 watts * 1/29 volts charging * 0.77 panel+controller derating * 1/0.05 rate of charge = 318 AH @ 24 volt maximum battery bank
600 watts * 1/29 volts charging * 0.77 panel+controller derating * 1/0.10 rate of charge = 159 AH @ 24 volt nominal
600 watts * 1/29 volts charging * 0.77 panel+controller derating * 1/0.13 rate of charge = 122 AH @ 24 volt minimum
600 watts * 1/29 volts charging * 0.77 panel+controller derating = 15.9 Amps "typical" maximum charging current
15.9 amps charging / 200 AH battery bank = 0.0795 ~ 8% rate of charge

So, your 200 AH battery bank does fit into the "comfortable" charging range.

Your 66 AH / 200 AH = 0.33 discharge which is OK--A 2 day / 50% maximum discharge would be 0.25 or 25% discharge per day. It depends on how may days of "no sun" backup power you need. Right now, you would probably need to drop your next nights loads, or run the generator before the next night's load if there is not good sun.

-Bill

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hello All,

Again, thanks a bunch for all your empirical advice. It's given me alot of confidence going forward with my system design.

I have sized the inverter for my system (Cotek 1500W, 24V) on the off chance that I may need to run a very heavy AC load: 648 watt, 5.4 amp air-conditioner.
Because I will have only a 200Ah battery bank, I think I will only be able to run this load for 3.7 hours with a fully charged battery bank before depleting the battery bank by 50% max. This heavy load would only be run in dire circumstances.

So my questions are:
1. Under normal conditions, and only needing 300-600 watts AC total, is the Cotek 1500W, 24V inverter a good choice? Or am I oversizing the inverter and designing inefficiently?

2. Should I use two inverters? If so, of what wattage(s) and how would they be wired to the battery bank?

Thanks again...

-lockwhip

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

What is the "idle current" of your present inverter?

If your loads are light, and you can find a smaller inverter with significantly lower idle current (more efficient with lighter loads)--You can come out ahead (less wasted power in general). And you can still use the larger inverter when needed.

-Bill

Photowhit · February 2013

Re: New small Off-Grid System Design and Evaluation

I think if you are considering another inverter, which can be a viable way to reduce your systems needs, I'd rather you spent the money on more panels and have more to work with.

FWIW- I don't know the rating of the AC your looking at running, I have run a 5300 btu for 4-5 hours a night running my 4 golf cart batteries down near or below 50%, but I recall the draw as being under 500 watts running in energy saver, and of course this is on thermostat so it would cycle on and off during the run. (Please note this was a sole intent of my system, and had very few other loads. I would even disconnect the fridge for longer A/C run times)

Also you have a 200 amp hour battery bank so loads/drawing the battery down at rates above 1/20th of the total capacity will reduce the capacity quicker than normal since battery capacity is figured on a 20 hour discharge rate. So loads above 20 amps x 24 volt x .90(inverter effiecentcy) or about 430watts will reduce the battery capacity at a greater rate.

inetdog · February 2013

Re: New small Off-Grid System Design and Evaluation

Photowhit wrote: »

(Please note this was a sole intent of my system, and had very few other loads. I would even disconnect the fridge for longer A/C run times)

Which would also reduce the heat load in the house!

Photowhit · February 2013

Re: New small Off-Grid System Design and Evaluation

Yep, truely designed for A/C, kitchen, and composting toilet(uses a heater) were in a different building, though the fridge is in the cabin.

New small Off-Grid System Design and Evaluation

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