Help With New System Planning

charlievman

New Member
Morning! My father-in-law and I are doing a cabin project together. We are building a 512 sq ft cabin with essentially house construction. The cabin will be a retirement home later, as well as an escape of sorts now. This will be in Northern Arkansas, and Solar Irradiance calculations appear to range from 2-6 ish, depending on the time of year. I really need some help organizing my thoughts around this. I am anticipating a 48v system. Please help?

From a load perspective, we are looking at trying to run panels, batteries, and an inverter to power these items. There is no grid power option. We have a 25amp 3500w generator we could tie in in times of low sun coverage but we REALLY want to avoid if possible. Currently, we anticipate using the cabin in 3-5 day bursts, typically a Fri/Sat/Sun but sometimes we get lucky and can get a 5 day or so week out there.

-Approx 0 to 24 LED bulb lights that are either all off or all on depending on how rowdy we are getting.
-approx 15 cu ft refrigerator that would remain on
-propane stoves electronics always on
-coffee pot for 2 hours a day max
-Mr.Cool DIY 12k 120v unit during hot summer months, mostly running at night but some midday as well
-Possibly a second Mr.Cool DIY 12k 120v Unit that would run the bathroom and bedroom area (running the DIY Multi Zone Would require 230v service). Might not ever run concurrently with other unit, basically cooling Bedroom at night.
-1 ceiling fan
-small LED track lighting throughout
-plugs throughout the house that will charge a max ever of 4 iphones simultaneously, though typically only 2 at a time, 6 hours max in a day.
-plugs throughout the houe that will charge a max ever of 2 computers simultaneously, though typically only 1.
-tankless propane hot water heaters electronics that are always on

I think I have everything there. Or for the most part. I am anticipating picking up with 100 or 200ah lithium batteries, have seen the stuff on the SOK and Ampretime Batteries and am ready to pull trigger on parts, basically from you guys recommendations. My FILaw and I have good working knowledge on how to handle the AC wiring from the home breaker box out in the construction, but this solar calculating and planning is eating my lunch. Any help would be much appreciated.
 

snoobler

Solar Honey Badger
Moderator
Welcome to the forum.

You need to take the generals and make them specific.

Link #5 in my signature for your available solar. If there's not a nearby city, use link #6.
Link #1 in my signature for an energy audit where you take the generals and make them specific by identifying power use and daily hours use.

While mini-splits are as efficient as they get, they use a very large amount of energy. Nighttime running will necessitate a large battery bank.
 

charlievman

New Member
Roger that. Do you know of a better way to run air conditioning than these types of units? And also, should I be looking more at a strict cooling only unit, rather than one that also heats like this? Propane is pretty reasonable and I’m sure I can find a furnace type propane unit like RVs have. Or would that not help on energy draw really?

when you say large battery bank....hit me a a number. What we talking here? 8x 100ah units? 12? 1 million?
 

snoobler

Solar Honey Badger
Moderator
Roger that. Do you know of a better way to run air conditioning than these types of units?

"mini-splits are as efficient as they get"

If you live in a dry climate, evaporative coolers can be very effective, and they typically use less power, but they can consume a lot of water.

And also, should I be looking more at a strict cooling only unit, rather than one that also heats like this?

No. There's no efficiency difference between A/C only and A/C + heat pump. The heat pump gives you the option of heat w/o resorting to propane. If it can't keep up, then you go with propane. I would personally have both.

For the bedroom, a much smaller window unit type might not be as efficient, but it might use less power due to its lower output. A 5-6K BTU unit might serve you better.

Propane is pretty reasonable and I’m sure I can find a furnace type propane unit like RVs have. Or would that not help on energy draw really?

when you say large battery bank....hit me a a number. What we talking here? 8x 100ah units? 12? 1 million?

A 12k BTU minisplit would likely consume 5kW/day assuming it runs full power about 8 hours per day. 4X 200Ah batteries in series for 48V provide 4 * 200Ah * 12.8V = 10.24kWh of which only 8.2kWh should be used routinely for maximum life. Mini-split alone uses over half your usable battery capacity assuming you run nothing else.

The refrigerator may use 1.5-2kWh/day (you can get the annual usage off the yellow energy sticker on them).

The coffee pot may use another 1kWh (you can minimize this by turning off the pot after it's finished brewing. No need to keep running the 900W element to keep it warm).

Once you've done the detailed energy audit, you'll have a better handle on your needs.

PLAN on using the generator for poor weather conditions. It's so much more cost effective to rely on the power of a generator rather than try to size your battery/solar for poor weather conditions. If you get an inverter/charger, the generator can be integrated into the system to charge the batteries through the inverter. If you splurge, you can even make it auto-start/stop, but given the inconsistent utilization of the system, a pull start generator used a handful of times a year is probably the better option as the solar will be able to keep things topped off in your absence.
 

charlievman

New Member
I would personally have both.
100% on that.

For the bedroom, a much smaller window unit type might not be as efficient, but it might use less power due to its lower output. A 5-6K BTU unit might serve you better.
Ok, I hadn't thought about that. Thank you for mentioning!
A 12k BTU minisplit would likely consume 5kW/day assuming it runs full power about 8 hours per day. 4X 200Ah batteries in series for 48V provide 4 * 200Ah * 12.8V = 10.24kWh of which only 8.2kWh should be used routinely for maximum life. Mini-split alone uses over half your usable battery capacity assuming you run nothing else.
Thats the exact battery scenario we were planning on actually. But it was from a place of minimal knowledge on our end.
The refrigerator may use 1.5-2kWh/day (you can get the annual usage off the yellow energy sticker on them).
I wasn't sure if I could trust that sticker, but, the sticker on the model we are looking at is showing 332/year, so am I right to assume 0.9 a day on that?
The coffee pot may use another 1kWh (you can minimize this by turning off the pot after it's finished brewing. No need to keep running the 900W element to keep it warm).
Coffee is serious round here. We will do anything it takes. All joking aside, from the time we hit the on button, all of that coffee is completly gone in 60-75 minutes.
Once you've done the detailed energy audit, you'll have a better handle on your needs.

PLAN on using the generator for poor weather conditions. It's so much more cost effective to rely on the power of a generator rather than try to size your battery/solar for poor weather conditions. If you get an inverter/charger, the generator can be integrated into the system to charge the batteries through the inverter. If you splurge, you can even make it auto-start/stop, but given the inconsistent utilization of the system, a pull start generator used a handful of times a year is probably the better option as the solar will be able to keep things topped off in your absence
With the generator we have (Harbor Freight, Predator 3500w 25amp model, don't clown me too hard, its been faithful for sure) what would be a reasonable expectation of recharge time? I am assuming we could run the generator to cool the house during the day, and top off the batteries, that way we are going into the quiet non-generated night on battery power. Am I right to assume that?
 

MisterSandals

Participation Medalist
I am anticipating picking up with 100 or 200ah lithium batteries
Predator 3500w 25amp model

Edit: Sorry, i was thinking 25amp "charger" but that generator is producing 25amps of 120v AC right?
So the answer on how long it will take to charge your batteries involves how many amps (see "??" below)
your 12v DC charger is. You need to pick a charger (or at least charging amps you will achieve) to know
"how long will it take to charge".

100ah / ??a = number of hours
200ah / ??a = number of hours

Gosh, did i add anything to this discussion. Sorry.
 
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snoobler

Solar Honey Badger
Moderator
100% on that.


Ok, I hadn't thought about that. Thank you for mentioning!

Thats the exact battery scenario we were planning on actually. But it was from a place of minimal knowledge on our end.

I wasn't sure if I could trust that sticker, but, the sticker on the model we are looking at is showing 332/year, so am I right to assume 0.9 a day on that?

It's the best you have to go on. It makes a lot of assumptions. When you're not there not using it, it will use less. When it's warm inside, it will use more. When it's cold inside, it will use less, etc.

Coffee is serious round here. We will do anything it takes. All joking aside, from the time we hit the on button, all of that coffee is completly gone in 60-75 minutes.

No reason to leave the pot on keeping the coffee warm burning juice off and on for that period. IMHO, better to get an inverter microwave and heat to desired temperature if it's not warm enough. 1 minute in the microwave is like 2 minutes running the heating element.

With the generator we have (Harbor Freight, Predator 3500w 25amp model, don't clown me too hard, its been faithful for sure) what would be a reasonable expectation of recharge time?

I have the Predator 3500. Mine is the 240VAC split phase model, so I'm limited to 15A on 120VAC. For $450 with the wheels, I ain't gonna clown you. It's saved my pork belly a couple times.

Your charge time depends on what's charging the battery. You should get an inverter/charger that just plugs into your genny and charges the battery bank. It depends on how much charge current the inverter can handle - 48V models vary, but lets say 70A. 70A * 48V = 3360W - that's 28A on the 120VAC input - too high for your genny. You'll need to select your inverter/charger to be compatible with the current/power output of your generator. I have Victron hardware, so I can tell it to only pull 14.5A from ANY AC source for any purpose. That means when the genny is running, I'm only pulling 1400-1700W from it. Ideally, you want to run your genny about 70% of rated for optimal efficiency and life.

I am assuming we could run the generator to cool the house during the day, and top off the batteries, that way we are going into the quiet non-generated night on battery power. Am I right to assume that?

If you have solar power during the day, I'd choose it first. The best designed systems only use the generator for the times when weather lets you down.
 
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