Costing out a small, rural solar panel installation

[earplug]

Going AC allows a occasional use of a microwave or coffee pot. They suck watts but make match directors happy.

[Justin]

I will also throw out there, that if you don't have anyone in the club that can help with the install, I can/will (with enough lead time for scheduling).

Thanks for the offer!

At this point, everything is very preliminary. I'll need to throw a spreadsheet together with some numbers to see if the board is interested at all.

[Justin]

Why not go DC with such a small system? ( like RV gear) maybe small inverters for the laptops if you can't get 12v chargers for them...

No inverter costs or losses for the lights, could even look at distributing the batteries and panels to keep losses due to wire resistance down to a minimum...

https://www.superbrightleds.com/cat/...-led-lighting/

Gut reaction is that the inverter would be the way to go. The building is already wired for regular light fixtures, plus the difficulty with sourcing DC-specific laptop chargers would probably make it a no-go.

[Irving]

First off, come up with a demand.

10 indoor 2 bulb fixtures, with 100w equivalent LED bulbs (14W each bulb) is 280 watts.
Laptop, Tablet, Smartphone charging - Figure on 90 watts for a laptop power supply
Outdoor light - Guess on 2 14w bulbs - 28W

Now, add time into the mix.

Indoor lights, figure 5 hours per day - 1400W (280*5)
Laptop, figure the same 5 hours - 450W (90*5)
Outdoor lights, use a photosensitive fixture to conserve engery - 15 hours (worst case in the winter) - 420W (28*15)

Total watt hours you need: 2270W/H per day (assuming this happens every day, unlikely, but plan for the worst).

Now, divide that by the number of hours of sun you get (again worst case). A safe bet for CO is 4 Hours.

2270/4=567.5 watts of solar. Now, I would upsize that by ~50% so that you are able to recover from a cloudy day.

That gives you ~900 watts of panels. In the solar thread I linked a site where 900w of panels is $750, before shipping. There are better prices out there, but they're an easy site to quote prices from.

Now, you have to decide what voltage you want to run the battery bank at, and I would recommend 24VDC or 48VDC. It means more batteries (more money), but it's better from a current draw standpoint.

2270w at 24VDC, gives you 94.5A/H required. That's what the listed devices above will use in a day.

For battery health, you never want to exceed 50% discharge, and staying at 40% discharge or less is better.

So miniumum, you need 200A/H of batteries to support your usage.

I would also suggest that your battery bank be able to run the required items for 2-3 days. Now you're at 400-600A/H.

A good go to for batteries, that I have used extensively, is a 6V 215AH golf cart battery. https://www.batteriesplus.com/productdetails/sligc110
They handle charge/discharge very well.

So, at 6V you need 4 in series to get 24VDC.
That means 8-12 batteries, so $800-1200 in batteries.

A charge controller and a inverter will also be required. For what you are building, I wouldn't bother with higher end solar gear. It's not necessary.

Low Cost MPPT: https://www.ebay.com/itm/4215BN-Trac...53.m1438.l2649
Low Cost Pure Sine Inverter: https://www.ebay.com/itm/2500W-DC-to...sAAOSw~bFWQaBA

MPPT: $260
Inverter: $240

So, Grand total, minus wiring, boxes, etc, is $2450 for 900 watts of panels, 3 days worth of batteries, and a relatively well liked solar controller and inverter.

It's an overbuilt off grid system, but it will work well for what you want.

I have some questions about your example. If he needs 600 AH, and you are recommending some 215AH 6V batteries, he'd need three batteries to make 600AH,but will need four batteries to get to 24V, he'll just end up with 860 AH of batteries? Why will he need 8-12 batteries instead of only 4?

I quickly looked up a 100w panel and it is showing just over 6 amps. Is it correct to scale that up to about 20A for a 300w panel? If so, with three of those we're now looking at 60ish amps. Is that 40amp controller enough?

[68Charger]

I have some questions about your example. If he needs 600 AH, and you are recommending some 215AH 6V batteries, he'd need three batteries to make 600AH,but will need four batteries to get to 24V, he'll just end up with 860 AH of batteries? Why will he need 8-12 batteries instead of only 4?

I quickly looked up a 100w panel and it is showing just over 6 amps. Is it correct to scale that up to about 20A for a 300w panel? If so, with three of those we're now looking at 60ish amps. Is that 40amp controller enough?

Amp Hours don't add up on series, you're just increasing volts, and thus watt hours... so 4 batteries in series is 24V with 215Amp hours... or 5.160 KWh... to increase to 2 or 3 days worth, you'll need 2 or 3 strings of 4 (8 batteries for 430Amp hours, 12 batteries for 645Amp hours) at 24V.

the current on the panels depends on the voltage... lower volts is higher current... 900W / 24V is 37.5A


Oh, and that charge controller is cheaper at their website when they're not paying ebay fees:
https://www.hurricanewindpower.com/e...ge-controller/

I didn't check the other items, but pays to shop around... I always look if a vendor on ebay sells for less elsewhere (like their website).

[Irving]

Thanks for clearing that up.

[Grant H.]

68C is right on the money.

Series vs Parallel will come into play a lot in solar stuff.

Always shop around for better pricing. I do as well, but when I'm posting something here, I generally just grab something easy that has the specs.

One thing I will add to what he shared:

The use of an MPPT controller means you don't have to shop for 24V panels to match your 24VDC battery bank.

An MPPT controller, the one suggested in this thread specifically, will take up to 150VDC in from the panel array. This allows you to daisy chain your panels and run smaller/cheaper wire to your panels. You have to make sure your panel array doesn't exceed 150VDC at open circuit voltage.

As 68C pointed out, 900W at 24VDC is 37.5A, which requires the use of 4AWG wire (6AWG is only rated to 37A).

The panels that I suggested are 39.7VDC at open circuit (3x @ 300W) which gives you ~120VDC at open circuit. Their Max Power Point (where the controller will likely run them) is 32.5V.

So 3x panels at 32.5V is 97.5VDC.

900W at 97.5VDC is only 9.23A. Now we can use 12AWG (on the edge - rated for 9.23A) or 10AWG (rated for 12A).

[68Charger]

Grant really knows his stuff, especially solar specific... I come from a general electrical/electronics engineering background and am not intimately familiar with solar panel and charge controller specs like he is...

at first I was like 9.23A for 12AWG, that's not right... but I realized you're using power transmission spec which is 700 circular mils per amp rule (VERY conservative, to minimize loss)... you're perfectly safe with up to 20A on 90degree C insulation on 12AWG according to NEC (National Electrical Code).. but you will have more loss at higher current with that spec.
That is to say, the choice for 12AWG vs 10AWG in this instance is an economic exercise, not a safety factor.

Of course, whenever you're doing longer runs (than say 50'), you should run a voltage drop calculation to determine if larger conductors are needed to keep voltage drop under 2% in design, in practice (after installation) it should be less than 5%... but this is based on whether or not everything will work, not best efficiency.... you'll find with power transmission spec, you have to get REALLY long runs before this is an issue.

[Irving]

Without derailing the thread too much, what do you think would be the minimum amount of system required to just run a laptop? The system on my van won't do it, but I'm not sure how far off I am.

[68Charger]

Without derailing the thread too much, what do you think would be the minimum amount of system required to just run a laptop? The system on my van won't do it, but I'm not sure how far off I am.

Depends on the laptop... the power bricks they use can have a significant amount of parasitic power loss (sucking power while not actually charging anything, even with the laptop off)... read the power spec on the brick for info.

Some laptops can run on inexpensive DC power cable adaptors to run off 12v power port (simple voltage regulator with cables & connectors)
Example: https://www.amazon.com/CHARGER-Dell-.../dp/B002UI2Y8Q
Others require 15-19v and/or communicate with the brick, so they require more complicated solutions... an inverter and then a brick isn't efficient, so you would have to oversize the system to compensate, but it'll work.