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diy solar

Sizing a A/C for 120 SF to run 16 Hours off 690 Wh Solar-charged battery

MySuperDikeyDayJob

MySuperDikeyDayJob

New Member
Joined
Jun 20, 2023
Messages
13
Location
New Orleans
I'm building an off-grid hut in the back yard, it's 8x10x12 high. I purchased a 4Patriots "1800" which has a max capacity of 768 DC Watt-hours or 690 AC Watt-hours from full battery to dry. But... I can't find a small enough window A/C or mini-split to run off battery overnight. Recharging the batteries isn't the mathematical problem; presuming 2x100W solar panels (I can add up to 2 more on this solar generator) will power everything while the sun shines. Limiting factor is when the panels stop producing energy, I have max 690 WH to keep me cool overnight (and, possibly, hopefully, keep my food cool but that's a different math problem.) The smallest EnergyStar A/Cs I've found rated 4000 BTU run at around 400W so in theory i get less than 2 hours cooling then sweat the next 12 hours. Since i live in the deep south and hurricanes (i.e. power out) come with the hottest of the weather, I really do need A/C even if not cold.

Some assumptions I am challenging:
1. That an A/C rated at 400-watt is assumed to run 100% of the time. Yes? then with generous (warm) thermostat I might only draw say 25% of the power? If that is the case then I would be theoretically running an average of 100 watts/hr then i might get 6.9 hours of cooling
2. Are there smaller A/Cs out there, with lower Watt draw, preferably mini-split? if so please advise because I'm still looking. Are there DC Air/Cons?
3. Do i really have to spend another $2k on a second generator just to stretch from 2-hours to 4-hours of overnight cooling?
I find it hard to believe i'm the only one with this problem, or maybe i just need to buck up and double my system?

All constructive ideas are welcome, as well as mathematical explanations if I'm calculating things wrong. Product recommendations most welcome.

Thanks! KC
 
Even expensive Bluetti AC200MAX is 2048Wh capacity 2000W continuous power for $1960. I will let others decide which solar generator is best for you since I never used them. Cheapest 5000 BTU window AC uses between 400 - 450 Watts. I would recommend for you to get the inverter drive variable speed window AC like the $400 Midea U-shaped 8000 BTU unit. It consumes ~200 watts on low speed at cooler night time outside temps which would make it last through the night on 2048Wh battery.
 
Last edited:
There isn't a nice way to put it, you overpaid big time for a system that's way to small.

Option 1 - return it, buy an aio and some pre-built batteries and put your own system together, save even more and build your own batteries.
Option 2 - cool with solar and buy a fan.

You could buy an LV6548 and a pre-built battery for about what you paid and have 5kwh of storage and 6.5kw of inverting power...
 
Other options if you want a system...

sungoldpower.com

6.5KW Solar Inverter All in one ESS 48V

Highlight: ★ All in one design: energy storage machine is equipped with 6.5KW inverter, solar charger and 5.12kwh lithium battery to offer uninterruptible power support, its flexibility to plug-and-play installation. ★ 6500W solar inverter: It is pure sine wave outputs, converts 48V DC into 120V...
sungoldpower.com sungoldpower.com

sungoldpower.com

Solar Kit 3000W 24V Inverter 120V output Lithium Battery 800 Watt Solar Panel SGKT-3PRO

sungoldpower.com sungoldpower.com
 
MySuperDikeyDayJob said:
I'm building an off-grid hut in the back yard, it's 8x10x12 high. I purchased a 4Patriots "1800" which has a max capacity of 768 DC Watt-hours or 690 AC Watt-hours from full battery to dry. But... I can't find a small enough window A/C or mini-split to run off battery overnight. Recharging the batteries isn't the mathematical problem; presuming 2x100W solar panels (I can add up to 2 more on this solar generator) will power everything while the sun shines. Limiting factor is when the panels stop producing energy, I have max 690 WH to keep me cool overnight (and, possibly, hopefully, keep my food cool but that's a different math problem.) The smallest EnergyStar A/Cs I've found rated 4000 BTU run at around 400W so in theory i get less than 2 hours cooling then sweat the next 12 hours. Since i live in the deep south and hurricanes (i.e. power out) come with the hottest of the weather, I really do need A/C even if not cold.

Some assumptions I am challenging:
1. That an A/C rated at 400-watt is assumed to run 100% of the time. Yes? then with generous (warm) thermostat I might only draw say 25% of the power? If that is the case then I would be theoretically running an average of 100 watts/hr then i might get 6.9 hours of cooling
2. Are there smaller A/Cs out there, with lower Watt draw, preferably mini-split? if so please advise because I'm still looking. Are there DC Air/Cons?
3. Do i really have to spend another $2k on a second generator just to stretch from 2-hours to 4-hours of overnight cooling?
I find it hard to believe i'm the only one with this problem, or maybe i just need to buck up and double my system?

All constructive ideas are welcome, as well as mathematical explanations if I'm calculating things wrong. Product recommendations most welcome.

Thanks! KC
Click to expand...

You are MILES from a solution.
 
AntronX said:
Even expensive Bluetti AC200MAX is 2048Wh capacity 2000W continuous power for $1960. I will let others decide which solar generator is best for you since I never used them. Cheapest 5000 BTU window AC uses between 400 - 450 Watts. I would recommend for you to get inverter drive variable speed window AC like the $400 Midea U-shaped 8000 BTU unit. It consumes as low as ~200 watts on low speed setting at cooler night time outside temp which would allow it to last through the night on 2048Wh battery.
Click to expand...
Yeah i was wondering if a variable speed drive motor would help. Thanks for the confirmation and references.
 
42OhmsPA said:
There isn't a nice way to put it, you overpaid big time for a system that's way to small.

Option 1 - return it, buy an aio and some pre-built batteries and put your own system together, save even more and build your own batteries.
Option 2 - cool with solar and buy a fan.

You could buy an LV6548 and a pre-built battery for about what you paid and have 5kwh of storage and 6.5kw of inverting power...
Click to expand...
This is helpful. thank you.
 
42OhmsPA said:
There isn't a nice way to put it, you overpaid big time for a system that's way to small.

Option 1 - return it, buy an aio and some pre-built batteries and put your own system together, save even more and build your own batteries.
Option 2 - cool with solar and buy a fan.

You could buy an LV6548 and a pre-built battery for about what you paid and have 5kwh of storage and 6.5kw of inverting power...
Click to expand...
This is helpful. thank you.
 
42OhmsPA said:
Other options if you want a system...

sungoldpower.com

6.5KW Solar Inverter All in one ESS 48V

Highlight: ★ All in one design: energy storage machine is equipped with 6.5KW inverter, solar charger and 5.12kwh lithium battery to offer uninterruptible power support, its flexibility to plug-and-play installation. ★ 6500W solar inverter: It is pure sine wave outputs, converts 48V DC into 120V...
sungoldpower.com sungoldpower.com

sungoldpower.com

Solar Kit 3000W 24V Inverter 120V output Lithium Battery 800 Watt Solar Panel SGKT-3PRO

sungoldpower.com sungoldpower.com
Click to expand...
This is helpful. thank you.
 
MySuperDikeyDayJob said:
This is helpful. thank you.
Click to expand...
You're welcome. See my 2nd post with some options from SGP, I've been running a TP6048 inverter from them since October with no issues (besides minor flickering of lights when my washer agitates). It's way overkill for what you're trying to accomplish so I won't recommend it to you.
I'm not sure what your skill set is but if you can change your oil and/or rotate your tires you could put a system together, especially with help from the forum.

Ps nice username and welcome to the forum.
 
I will suggest you consider what needs to run in your new hut,
Make a list of the loads and what power they will need and total this up. An Energy Audit form is in the resources section of this forum.
For cycle loads like your A/C or a fridge, plug the appliance into a standard wall outlet on utility with a Kil-O-Watt meter and let it run a day or two - the meter will tell you the total kWh used by the cycling appliance and help you to determine the power requirments of your new space.
The equipment you started with is not going to run what you need. Start with the audit and we can help you to pick equipment that will run what you want, more efficiently, for less money.
If you can sell or return what you got, that may be a good option, or use it for camping trips perhaps.
A target budget will help us to guide you on equpment purchases for your hut. Entry level stuff is less money but less efficient, high end tier-one equipment is more efficient and comes with more options, but will cost more.
We can help walk you through this and get to a good solution.
 
A/C's do not have to run at max wattage all the time if the space they are cooling reaches the thermostat set temperature they can do a few things that reduce operating wattage. The simplest of them cycle off the compressor and just keep running the fan. Fans do not draw much power. Some units will have an Energy saver setting that shuts down the A/C entirely once they reach temperature.

Big issue is the level of insulation your space has. If it is poor than your A/C will run full out to try and keep it cool. About the smallest A/C youc an get is a window mount 5000BTU model. It can run more efficiently if you wall mount it instead of putting it in a window.
 
MySuperDikeyDayJob said:
Not super helpful.
Click to expand...

Not in details, but more helpful than you realize. Your perspective is so far off, you need to be thinking bigger by a full order of magnitude - yes nearly 10X. A/C is an extremely demanding application.

What you also may not realize is the power of the A/C unit is only a part of the equation. Your solar heat load and insulation play roles that are just as important.

200W solar? Nope. 2000W is a lot closer.

690Wh? Nope. 6900Wh is a lot closer.

Consider that you're looking at a solution 10X larger than you imagined. Is it even worth pursuing?
 
Somebody check my math.

Assuming an average ussage of 9kWh per day.
200w continuous Misc load 24/7
400 watts Air Cond x 16 hours.

I'll go with 5 hours of avg solar per day, you would need a 2000 watt solar array and 10 kWh of battery storage.

Edit. I took to long typing... @sunshine_eggo beat me to it.
 
42OhmsPA said:
You're welcome. See my 2nd post with some options from SGP, I've been running a TP6048 inverter from them since October with no issues (besides minor flickering of lights when my washer agitates). It's way overkill for what you're trying to accomplish so I won't recommend it to you.
I'm not sure what your skill set is but if you can change your oil and/or rotate your tires you could put a system together, especially with help from the forum.

Ps nice username and welcome to the forum.
Click to expand...
Thanks. I renovated a 120 year old house from foreclosure, did about half the work myself, so I can figure some stuff out. My (female) helper used to call it her Super-Dikey-Day-Job. I'm an engineer, albeit not electrical, but I'm open to learning and I can get past mistakes and failures. Glad this forum is so responsive and helpful. Also good to get confirmation that my frustration in finding in impossible solution was that it was, truly, not possible. I'm having the hut doubly wired- one for solar "emergency" power and separate line hard wired to run off house (grid) power so that I can use the building the 98% of the time that is not emergency. Also buys me a few months to figure out the right system.
 
OffGridForGood said:
I will suggest you consider what needs to run in your new hut,
Make a list of the loads and what power they will need and total this up. An Energy Audit form is in the resources section of this forum.
For cycle loads like your A/C or a fridge, plug the appliance into a standard wall outlet on utility with a Kil-O-Watt meter and let it run a day or two - the meter will tell you the total kWh used by the cycling appliance and help you to determine the power requirments of your new space.
The equipment you started with is not going to run what you need. Start with the audit and we can help you to pick equipment that will run what you want, more efficiently, for less money.
If you can sell or return what you got, that may be a good option, or use it for camping trips perhaps.
A target budget will help us to guide you on equpment purchases for your hut. Entry level stuff is less money but less efficient, high end tier-one equipment is more efficient and comes with more options, but will cost more.
We can help walk you through this and get to a good solution.
Click to expand...
Thank you. I've done the back-of-the-envelope needs, unfortunately that was after i bought the system (lol, duh!) but i will look at your reference. It's a small "guest house" and barely enough for 1 to crash for a couple of weeks. No big HVAC, kitchen, laundry, etc. Tomorrow I have scheduled a call with the customer service where I bought the current system (still brand new). I assume at the end of the call we will collectively come to the conclusion that it's not a fit for my needs, and hopefully take it back. Several other folks on this thread have given recommendations for both battery and A/C systems so i will be following up on those.
 
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