Does this design sound correct?

NewLife · August 2019

Hello. We’ve bought 5 acres in central Colorado and need an off grid system as the $38,000-$40,000 to get grid power to the property is not an option at this point. Here is the solution proposed by a solar company. They say the System Size is 48 V, 3840 watts:

Estimated Production: Average Winter Monthly KWH: 456, Average Summer Monthly KWH: 489

System Layout: South Facing Pole Mount. 5 hours sun daily, no shading

PV Modules: (12) Canadian Solar 320 MS

Racking: (2) DPW Top of Pole Mount (6) Modules Each; Concrete, Sono Tube, SCH 40 Pipe

Inverter: Schneider XW+ 5848

XW System Control Panel

Charge Controller: Schneider MPPT 80-600

PDP Mini with By-Pass Breakers

Square D 600 volt Disconnect

FLA Batteries: 16 – Rolls L16, 6 volt S-550 @ 420 Amp/H

4/0 Battery Cable package

One Insulated Battery Box with Power Vent

Design and installation of all solar components. Permit procurement and rebate applications

Excavation and AC Interconnection will cost extra.

Total cost: $27,989.00 USD

Our estimated daily usage is 12kWh, using propane for cooking, heating, laundry. Hard to get a definitive daily usage when we will still need to build the house and purchase appliances, lighting, etc. Plan to be smart with all that selecting as energy efficient everything as possible. We also need to run the well pump which is estimated to be around 200’ deep.

We want to have a scalable system for future enlargement and grid tie when that becomes an option.

We know there are tons of variables to consider as well as intelligent usage during daytime peak hours, so we aren’t expecting grid “city living” capabilities. Heavy consumption will be during the peak hours. Will that cause the PV array to be pulled from during those times and not have enough production to charge the battery bank?

The system sizing with these 12 panels – will they be able to provide enough to get a proper state of charge on the battery bank?

Do we need more panels?

Been reading many articles/discussions on battery and array sizing and FLA batteries not getting enough charge and really don’t want to kill the battery bank and have to purchase new ones because the system wasn’t sized correctly.

We do have a propane generator that we can use to charge the battery bank when needed, but if the system is properly designed and we live the off grid conservative lifestyle that shouldn’t be needed too often, correct?

Thanks for your input and we appreciate the knowledge everyone has here. Very concerned about design, especially with this price tag.

Estragon · August 2019

Generally speaking, pv will produce ~75% of (STC) rating, so ~2.9kw for your array. If a cool, breezy, high altitude location, you may do a bit better.

At 48v charging, that's ~60a. Two strings at 420ah each is 840ah. For full time off-grid, we usually like to see charging capacity of C/10, so you're a bit light, but not terribly so. Loads could move capacity from marginal but okay to inadequate though. If there's 20a@48v (~1kw) loads also running, for example, that takes you down to 40a or 5% of C, which is really getting too low.

Summer monthly average looks reasonable, but winter looks optimistic. Generator use will likely be needed in winter.

840ah is ~40kwh, of which you don't want to use more than 1/2 regularly. At 12kwh/day, this means you'll likely be sparking up the genny after 1 gloomy day. 12kwh/day is on the high side though. Hopefully you'll end up under that.

NewLife · August 2019

I agree with your reply and am trying to educate my partner on the need to increase the PV array. Hoping to do that with some input from real life off-grid experienced folks here.

Is there a rule-of-thumb of battery bank size to array size being array size should be ~ 1.5 larger then battery bank, and is there any good battery bank and array sizing information to educate someone without getting too deep in the weeds with technicalities?

Thanks for looking at this.

mike95490 · August 2019

NewLife said:

......We do have a propane generator that we can use to charge the battery bank when needed, but if the system is properly designed and we live the off grid conservative lifestyle that shouldn’t be needed too often, correct?

That depends all upon the weather. Winter here, I get 6-15 days no sun, so it's all generator power. we get by on about 7kwh daily. Wife requires power! (Northern Calif)

Image: https://us.v-cdn.net/6024911/uploads/editor/17/olhcxroptlru.gif

Estragon · August 2019

The rules really start with design loads. From those numbers (kwh/day and peak kw), the bank is sized to give required days of autonomy (often 2-3) before starting the generator. With the bank size, you then determine the pv required to properly charge it given local climate and willingness/ability to use generator to supplement. As Mike notes, at higher latitudes generator charging in winter is usually needed to keep pv size reasonable.

A rule of thumb would be pv current s/b 10% of bank capacity in ah, so an 800ah bank wants 80a pv current. To get 80a, you need roughly 80a ÷ .75 x 50v = 5300w array.

Like you, I had to estimate loads before the cabin reno was finished and could base loads on actual experience. I listed all the likely loads in a spreadsheet, added a fudge factor, and came up with ~3.5kwh/day. It's ended up being more like 4-5kwh/day, mainly because of a septic system added after which required a lift pump that takes a remarkable amount of power for what it does. Even so, with a bit of load shifting (eg avoid pumping when the sun doesn't shine) we rarely run the genny in spring/summer.

One thing that helped was living off a couple of golf cart batteries and a generator for the early part of the build. Charged them and ran tools off the genny during the day, and used a small 12v inverter otherwise. It helped a lot in figuring out what we could and couldn't live without, which is a highly personal set of choices.

12kwh/day is doable, but when you consider the long term cost of battery replacement (~5yrs), gear (~10yrs), and pv (~20yrs), $40,000 to bring grid in may work out better.

Photowhit · August 2019

NewLife said: in central Colorado

Wow there may be no other state that this could be so much difference in a few miles...
If you water the yard which way is down hill...lol.

Seriously, if there is a town near by, it would be good to check the available sun using PVWatts. The near even availability is surprising for summer and winter (to me). I assume trackers help, They are pole mounted, but you didn't say if they will be tracking the sun?

Using 12 kWhs a day, the system appear a bit under paneled to me, but trackers will help, apparently more than I thought.

NewLife · August 2019

5 hours sun a day where we are. There are no trees and up on a hill. Luckily winter isn't overcast all the time like in the northeast. I need my sun, just like the PV system does. No automatic tracking at this point, just manual. We may have to add that later.

I feel that adding 4 more panels will help. Unfortunately $40K isn't in the budget right now. The solar system cost has climbed up a lot higher than anticipated too, but we must get professional install as this is way above the DIY skills in this household and I want to ensure it's set up correctly.

Shine on everyone! As we get the system up and start the off-grid living experience I'm sure we'll have some more questions.
Thank you all for your input.

Dave Angelini · August 2019

You can always run a genset and add solar later. You can run about 6kw of nameplate solar on the mppt-80-600. Sounds reasonable for the price.

BB. · August 2019

I am going to play Devil's advocate here (don't take any of this personally or as a "slam" against your choices). This is just the engineer in me trying to make sure you end up with educated choices.

Is there anyway you can get your solar power needs down towards 3.3 kWH (3,300 WH) per day... That is a fairly reasonable cost/capability system (Full size AC fridge, LED lighting, Laptop computer, cell phone charger, washing machine on sunny days, "solar friendly" well pump, etc.). A "near normal" electric lifestyle... And use a genset (or several different gensets--Like a smaller eu2000i for smaller loads during bad weather, and a 3.5-5kW for running construction/shop loads when needed, as well as charging the battery bank when needed).

If you are spending ~$28,000 for a solar power system--With real power limitations and maintenance costs (many people "murder" there first battery bank or two, electronics should last 10+ years, but 5+ year failures for repairs/replacements are not unusual).

Vs ~$40,000 for utility power--Basically unlimited power (up to a reasonable limit of 100-400 amp @ 120/240 VAC service--Depending on what your utilty will supply). And the utility (for the most part) will be responsible for maintenance (you will probably need a genset or two for winter outage protection--But gensets usually are relatively cheap and only consume fuel when running/needed).

And, utility power will probably add value to your land... Off grid solar + genests, these are usually sunk costs and you will not be able to "up sell" the property (solar power systems a generally a very personal set of decisions which will probably be different than yours).

Utilities tend to be increasing the costs of new transmission lines/installations--So $40,000 today may be >>>$40k years from now (no rural electrification subsidies these days).

You are paying "up front" for your $$$/kWH with your solar power system... Something like 10 years of predicted energy bills today for the install. And there is ongoing maintenance (new batteries every 5-7 years, can be 15+ years plus for forklift batteries as an example).

As you and your partner get older--Are the monthly/yearly/decadal maintenance issues something that you will/can perform?

And you do need to look closely at your (potential) utility rate plan... In the "olden days", you could have pretty high $$/kWH charges (mine run $0.20 to $0.40 per kHW in Northern California time of use/tiered plan). And a relatively low monthly connection fee ($10 to $50 per month). Today, I am hearing about "low" $$$/kWH rates ($0.10-$0.20 per kWH?) and possibly upwards of $100-$200 per month connection charges....

I suggest to not "fall in love" with any specific solution. Look at this as long term costs and Risk/Reward issues (cost of system and system failures vs possible lower costs and lifestyle choices).

Energy usage is a highly personal set of choices. Your goal of 12 kWH per day usage (360 kWH per month) is not at all unreasonable (North American energy usage is around 500-1,000 kWH per month for the "average home"--Whatever that is).

Solar power is highly variable (long term yearly harvests can vary by +/- 10%, local weather patterns can give you many days or even weeks without significant sun)... For example "5 hours of sun" is not (usually) just that---Here is a random spot in Colorado:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Greeley
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.94	4.60	5.39	5.57	5.70	5.82
Jul	Aug	Sep	Oct	Nov	Dec
5.87	5.70	5.66	5.28	4.30	3.93

And if you have heavy snow falls (what is snow--I am near San Francisco California), you might want to panels near vertical (winter) and/or need to keep them free of snow.

-Bill

MichaelK · August 2019

You can look at the specs of my system down below. I have an approximately 20% larger system than your company's proposal for about one half the price. That's because I did it myself. I had no professional electrical experience before installing it. You can do it too!

BB. · August 2019

There are other options too... There are "E-Panels" today that are basically one or two Electrical panels (boxes/enlosures) that are prewired for the specific equipment that you want to use (Charge controller, AC inverter, etc.). More or less, lets you do the rest locally (main breaker panel, local installer/roofer to put in solar panels, misc. wiring.

https://www.solar-electric.com/search/?q=e-panels

E-panels are very nice for first time DIY folks, and not bad for those that have a 100 miles of dirt road to their place--And no local stores.

And some places will even build out full custom systems in the warehouse, test, break them down, and ship to you (buy batteries locally may be more cost effective--Heavy, truck only shipping, etc.).

Our host is Northern Arizona Wind & Sun out of Flagstaff (I do not work for them, just an unpaid volunteer here) does this... And I am sure there are others (you do not have to buy from NAWS to participate on the board):

https://www.solar-electric.com/search/?q=custom+built+off+grid

NAWS has some sample pricing--Again, I do not work for them. If you have questions, feel free to call their engineers for help.

-Bill

wellbuilt · August 2019

I have a good size system here I’m using 15 295 watt panels . 430 ah golf cart battery’s .
In the summer months I have lots of power.
I’m charged up by 1100 every day .
If it is raining and very dreary I only get in 3/4 watts an hr .
If it’s just not sunny I’ll see 12/1500watts .
As the sun drops in the sky come October my numbers go south .
I don’t get much power between before 1000 and after 400 pm .
And then there is the snow it’s a 4 letter word .
Last winter my solar was off for 6 week .
The panels where covered in ice ☹️
If they do get cleared off I may only get 3/400watts for 4 hour in nov dec , sooooo I’m adding 12 more panels on the ground for winter .
My system cost 13k so far , but I need a second fm 80 charge controller and racking
I’ll problem spend 2 k more .
27 k Dosent seam to bad really .
It sounds like it could be 2/3 k high .
I am a builder in ny but I did not have much PV expereance at all .
The guys here really helped me out .
You could Install yourself I use out back equipment ,
I used a pre wired panel it is not very hard to set up .

spacebass · August 2019

If it was $27,000 for solar or $38,000 for city I would try very hard to raise the extra $11,000 !!!!

Photowhit · August 2019

NewLife said:
I feel that adding 4 more panels will help. Unfortunately $40K isn't in the budget right now. The solar system cost has climbed up a lot higher than anticipated too, but we must get professional install as this is way above the DIY skills in this household and I want to ensure it's set up correctly.

I see heating on your list and no backup generator. Hope you have some other alternate heat source. Understand that the batteries don't last for ever, Rolls are good and should last 7-12 years if not taxed to badly, but understand first timers often kill off their first set of batteries, so ask questions early!!!

Good luck in your "New Life"!

Does this design sound correct?

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Greeley
Average Solar Insolation figures

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Does this design sound correct?

Comments

GreeleyAverage Solar Insolation figures

Categories

Greeley
Average Solar Insolation figures