Solar for a remote shed

sticky2 · Nov 16, 2015

My garage is fine, but need some help with a remote shed that's about 3 miles from local service.

I'm going solar and have most of it figured out.

I know I want to be able to store 4,536 watt hours and I'm willing to go with a 24V system. How do I convert this to amp hours and how do I size my deep cell batteries?
Thanks

TractorJeff · Nov 17, 2015

Its early in the morning but I would assume you would take the 4536 watt hours / (12VDC or 24VDC) to get the amp hours? Then take that result and guesstimate the number of hours of actual full load use and size your batteries accordingly.
EX: 4800/24=200 amps per hour?
200x8 (hours usage)=1600 amps?
This negates the fact that the batteries will drop in voltage which puts the output power per hour on a Curve.
It will take a pretty good size solar panel or time period to recharge this.
(In My Opinion)

jkwilson · Nov 17, 2015

How do you plan to use the power? Lights, AC tools, or what? What is your biggest power consumer and how long will you use it per day? Where is the shed? Seasonal or year round use?

rockwithjason · Nov 17, 2015

this should help.
http://all-about-lead-acid-batterie...ery-fundamentals/amp-hours-vs-kilowatt-hours/

sticky2 · Nov 17, 2015

It's a remote shed used for support. Main use is lights (it's on a river island, but they are not critical (so it's not like the end of the world if they aren't on, but folks are depending on them being on). I'm looking at 126 watts. 8 hrs/day. My math is:
126 watts @ 8 hrs = 1008 watts x 1.5 for a cushion gets me 1,512 watt hours
1,512 watt hours with storing 3 days = 4,536 total watt hours to store.

I don't know the next calculations to figure out where I need to go. I've never done this kind of stuff and it's been hours just getting to this point.

simpler=better · Nov 17, 2015

TractorJeff said:
Its early in the morning but I would assume you would take the 4536 watt hours / (12VDC or 24VDC) to get the amp hours? Then take that result and guesstimate the number of hours of actual full load use and size your batteries accordingly.
EX: 4800/24=200 amps per hour?
200x8 (hours usage)=1600 amps?
This negates the fact that the batteries will drop in voltage which puts the output power per hour on a Curve.
It will take a pretty good size solar panel or time period to recharge this.
(In My Opinion)

Close but no cigar, that should be
4800Wh/24V=200Ah

200Ah/8hrs=25A constant for 8hrs (which seems high, but he's probably sizing the system to make up for cloudy days, occasional high usage, etc.)

A quick google shows that a walmurr 12v deep cycle has ~100Ah. So you'd probably be fine with 2x sets of 100Ah batteries. If wired in series-parallel you could add capacity as needed.

Are you running an inverter to power stuff? Those are maybe 75% efficient, FYI

simpler=better · Nov 17, 2015

Personally, I'd run a 12V system and use 12V LED pucks intended for automotive use. They're cheap and more efficient than stepping up to 120VAC then back down again.

Plus, any other 12v engine could be used fo rchargin in a pinch (lawn mower, car, etc.)

Zeke · Nov 17, 2015

simpler=better said:
Personally, I'd run a 12V system and use 12V LED pucks intended for automotive use. They're cheap and more efficient than stepping up to 120VAC then back down again.

Plus, any other 12v engine could be used fo rchargin in a pinch (lawn mower, car, etc.)

I assume you mean a small engine running an alternator. And I'd add that a small generator would do the same. And I think even a small one can be rigged to auto start when needed.

However, being out on a river island (love to see this) might make for a noise sensitive situation.

offroadsteve · Nov 17, 2015

simple=better had it most of the way there, but it seems he didn't see your second post.

4536 watt-hrs / 48V = 94.5 amp-hours of storage required.

Deep-cycle batteries like to cycle between about 40% charge and 90% charge, so you need to spec out 2x the battery capacity of what you actually need, therefore 200 amp-hours of storage @ 48v to get 100 amp-hours of useful storage. Batteries are also complicated because the capacity depends on the discharge rate.

Looking here http://www.trojanbattery.com/pdf/TRJN0156_GolfColl.pdf, the smallest set of 6V golf-cart batteries (T-605) would get you there. They are rated at 232 amp-hours at a 100 hour discharge rate, or 1360 watts per hour, which is 10x what you need. But, if you look at the chart, that same battery is only good for 175 amp-hours at a 5-hour discharge rate. You would of course need 8 batteries wired in series to get up to 48V.

As others have said, if you can keep all your loads @ 48V, that will help eliminate conversion losses. But if you have a battery pack with that kind of capacity, I would also install a modest size inverter (2kW) to use for emergencies and such.

theoldwizard1 · Nov 17, 2015

simpler=better said:
Personally, I'd run a 12V system and use 12V LED pucks intended for automotive use. They're cheap and more efficient than stepping up to 120VAC then back down again.

2 - 6v golf cart batteries would work good for this kind of setup.

OP ... Buy a GOOD solar charge controller !

theoldwizard1 · Nov 17, 2015

offroadsteve said:
But if you have a battery pack with that kind of capacity, I would also install a modest size inverter (2kW) to use for emergencies and such.

CONCUR ! You can buy a "modified sine wave" 2kw inverter CHEAP !

The cheap HF $99 generator is not bad for light, occasional use, like a battery charger to top of the batteries. Follow the break in instruction. Replace the crappy original spark plug with a good one (NGK) and buy a couple of spares.

If you ARE going to buy a generator, do NOT store any fuel in it ! Make sure to run it dry every time you use it. Keep a 1 gallon can of pre-mix gas with it. You should make a point to start it and run it with a load every 2-4 months. Again, run it dry after you test run it.

sticky2 · Nov 17, 2015

thanks for the help. For those suggesting LEDs - these #s already assume we're using all LEDs. And the comment about a generator - that's not an option because this has to be able to operate without someone filling up gas etc, which is why we are going solar. We're not going solar to save money but to have less hands on. There are times of year that we cannot get to the location.

I'd already factored in battery inefficiency, storing power for 3 days etc. So the 4,536 total watt hours to store is where I'm left. I just need to figure out the battery setup.

simpler=better · Nov 18, 2015

Makes sense. Stack up 12v or 6v batteries then, so you still have the option of using gas to recharge in a pinch (even if it's not the main plan).

leadunderpressure · Nov 18, 2015

http://www.solar1234.com

This guy has som notes on his solar powered chicken coop that sound close to what you have for requirements.

Bottom of the page has a whole discussion on sizing and what you need.

Sent from my iPad using Tapatalk HD

Browning1 · Nov 24, 2015

I ended up buying a 100w solar panel kit from amazon, 35ah deep cycle battery and a 1000watt power invertor box

Solar for a remote shed

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