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

Hedges said:

While I do enjoy mocking or being snarky, I try to help as well, such as the simple math I gave for Wh to W.

When you need to start a motor, it isn't quite that simple. In the link below I show current waveform to start and run a window air conditioner.
While it is rated 4Arms (500W), starting surge was 21.5Apeak (maybe 15Arms, 1800W) for half a second.

Is 600w enough

I plan on putting 600w of solar panels on the roof of a small camper. The Panels will be able to tilt so I can get as much power as I can. My main use is to run a 5000 BTU AC tha draws 450w in the summer and a 500w space heater in the winter & a few little items like phones, tablets and led...

diysolarforum.com

Patriot Power Solar Panel Generator 1800

Our best-selling portable solar-powered generator includes a solar panel & never needs gas. Perfect for survival situations, off-grid adventures, & blackouts.

4patriots.com

"

• Output - Continuous: 1,800 W
• Output - Peak: 3,048 W
• Storage Capacity: 768 Wh"

Looks like wattage should be sufficient to start and run. Usually can't count on data sheet "Peak" unless they say for now long (needs to be a second, not 30 milliseconds), but continuous matches starting power of my A/C.

What you need is more battery.

500W x 16 hours would be 8000 Wh, if running 100% of the time. Which will vary with temperature. You need an idea of total watt hours over that 16 hour period.

This inverter is $750

Midnite Solar DIY Series Inverters

NEW from Midnite Solar - the DIY Series off-grid inverter/chargers. Affordable and simple, with excellent surge capability for demanding loads of off-grid cabins or homes.

www.altestore.com

This battery is twice the capacity of Patriot for $1350

SimpliPhi Lithium Deep Cycle Batteries

SimpliPhi Lithium Ferro Phosphate (LFP) Deep Cycle Batteries are safer than lead acid batteries with a much longer cycle life.

www.altestore.com

This battery 3x for $1000

SOK 24V 100Ah LiFePO4 Solar Battery | Current Connected

Experience reliable and powerful energy storage with the SOK 24V 100Ah Battery. It's built to last and packed with features!

www.currentconnected.com

... just to give you an idea. I don't have first-hand experience with any of those models, but others here can make recommendations.

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Thank you. Helpful. I know an inverter is beneficial but i am fuzzy on why. Need to pull out my old EE books and read up a bit more. Your point about startup amperage is a good reminder, i had forgotten about that phenomenon.

Hedges said:

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



690 Wh / 16 h = 43W

You just need to buy yourself a nice little 43W A/C.

May I suggest this chest freezer, and use it to keep your feet cool? Consumer energy label 249 kWh/year / 365 days / 24 hours = 28W

https://www.lowes.com/pd/Hotpoint-6-9-Cu-Ft-Manual-Defrost-Chest-Freezer/5002179205

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Snarky perhaps but it was a good joke and it did give me a big belly laugh. Plus i love the visual... me melting in the heat with toes frosty inside a freezer. Good one.

Different direction...Ecoflow Wave 2 portable heat pump 5100btu cool 6100 btu heat.
This is mostly for van dwellers, but who knows

MySuperDikeyDayJob said:

Thanks. Brand new walls so we are packing them with insulation. since the space is small, I'm looking at a mini-split wall mount.

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With 2" wide studs 16" on centers, same amount of heat is lost through wood as through fiberglass insulation. Whether 4" or 6" thick walls. Better insulation becomes diminishing returns.

Staggered 2x4 with 6" top and sole plate would break the straight through wood path.

A layer of foam board would be another way to reduce heat loss through wood.


I put in aluminum double-pane windows. The Milguard had plastic injected inside then were cut for thermal break. When I did another building, Home Depo no longer carried Milguard so I got different brand, and those the metal frame sweats with condensation. (ugly) vinyl of course doesn't conduct heat as well.

Air tightness, efficiency vs. breathing fumes from all the synthetic products. I prefer plywood but OSB or particle board are used in everything.

MySuperDikeyDayJob said:

Thank you. Helpful. I know an inverter is beneficial but i am fuzzy on why. Need to pull out my old EE books and read up a bit more. Your point about startup amperage is a good reminder, i had forgotten about that phenomenon.

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You're going to have to explain your user name, avatar, EE books if you're an ME ... I can't figure you out.

Inverter drive motors are 3-phase motors driven by a variable frequency drive. A brushless DC motor, with VFD usually fed AC.
Cheap ones have rectifier/diode front end, horrible power factor, draw higher current than a "real" (resistive) load or PF corrected circuit would but only during part of each phase.
But cheap or good, they ramp up RPM slowly with PWM of varying frequency, never drawing more current from the supply than they draw at full load. Zero surge.

Apparently European mini-split are often PF corrected. Whether PF corrected or not, Variable speed A/C compressor let it run steadily at reduced RPM to maintain desired cooling. That is probably a bit less efficient than running compressor and cooling system at optimum speed. And of course it you can get a water source for cooling, more complex but much better performance than exchanging to hot air. That stuff may be getting into your field; I'm only a hack at servicing refrigeration.

sunshine_eggo said:

You are MILES from a solution.

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So @MySuperDikeyDayJob

You are an Engineer, so it is best you perform a design prior to buying “things & stuff”.

I did not catch what discipline of Engineering, but Electrical Design is performed from the loads back & HVAC Design is performed about the same ,, Loads back or cooling needs & heat gains & weather back to your AC & that gets you your electrical needs.

@sunshine_eggo blunt statement is correct, but I believe made in order to help you understand.

It is just math & science. I believe there are many here that can help you or point you in the right direction.

What size AC & power consumption do your calculations detail you need? That would be a starting point.

RV8R said:

heat gains & weather

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Some of which you may be able to improve.
Can you put the PV panels out in the sun, with building in the shade? (Same issue for RV with rooftop PV, maybe some portable panels.)

Hedges said:

Some of which you may be able to improve.
Can you put the PV panels out in the sun, with building in the shade? (Same issue for RV with rooftop PV, maybe some portable panels.)

How to Plant Trees to Conserve Energy for Summer Shade - Climate Change at arborday.org

Plant trees to reduce your carbon footprint… and for all their benefits. For 35 years, The Arbor Day Foundation has been helping and inspiring people of all ages and walks of life to plant trees.

www.arborday.org

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But build it first, then design, & then renovate ,, right ?

Just to give you a comparison in an actual real-world setup. I have 6000 watts of solar panels and 22 KWH in Lifepo4 batteries. I have a 12000 BTU mini-split and two chest freezers that run exclusively from the off-grid system. The two chest freezers draw on average 300 watts continuous. The mini-split draws between 700 watts to 900 watts on average. It tends to go closer to 900 watts when it's hotter outside and 700 watts at night when it's cooler. Without doing the math, if it has been a clear sunny day, I have no problems running everything 24 hours straight. If there has been some cloud cover, I may shut off the mini-split around midnight and turn it back on when the sun comes out in the morning. I do tend to be more cautious on the battery because I do not want the power to be cut off to the freezers.

Here's some estimates for the math.
Freezers 300W x 24 hours = 7200 WH
Mini Split 800W x 24 hours = 19200 WH
Total usage is 26400 WH.

Solar power generation
4000W x 7 hours = 28000 WH
Total solar power available is 28000 WH.

You might be asking why am I estimating 4000W from 6000W panels? My panels are facing south like they are supposed to. But also, angled 30 degrees. In the summer, the sun is much higher in the sky than in the winter. I could get more solar power if I put the panels flat so that it is nearly perpendicular to the sun. But then I'd have to keep repositioning it in the fall, winter, spring, and again in the summer. 30 degrees is ideal for my location in Las Vegas throughout the year. Why only 7 hours of sunlight? Without any obstructions, usable sunlight starts around 9am and ends around 4pm in the summer. I still get power generation before 9am and after 4pm, but it's only going to be a small fraction of 4000W. Thus leaving out the the small amount of power generation is negligible.

Factor in some conversion overhead and inefficiencies, I can barely make it if I run the mini-split for 24 hours straight. All this is based on a perfect day with no clouds.

When @sunshine_eggo says "You are MILES from a solution" he's pretty accurate in that assertion.

MySuperDikeyDayJob said:

Thanks. Brand new walls so we are packing them with insulation. since the space is small, I'm looking at a mini-split wall mount.

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If your studs are 2x4 you are limited to R13 in fiberglass insulation. It does no good compressing it since you actually lose R value. Better choice is spray in closed cell foam insulation it has ~6.5R per inch (2x4 stud wall would reach R23). Plus which the foam seals better and does not suffer from sagging in the wall. Ceiling and floor should be insulated as much as you can. Do not paint the outside a dark color as that increases the heat it receives during the day.

Avoid having too much window area. Windows are a hole in the wall that leak heat.

My opinion on mini splits is that they are not necessarily the best choice.

I have found "Reflectix" to be a great choice against the outside walls, then standard insulation over that on the inside. It is a foil encapsulated bubble pack like material that just staples up, easy to cut to fit, and works amazingly well. A bit pricy for a roll but well worth it. 3 different types are available, I used the one that is reflective foil both sides with the bubble pack center.

Hogheavenfarm said:

I have found "Reflectix" to be a great choice against the outside walls, then standard insulation over that on the inside. It is a foil encapsulated bubble pack like material that just staples up, easy to cut to fit, and works amazingly well. A bit pricy for a roll but well worth it. 3 different types are available, I used the one that is reflective foil both sides with the bubble pack center.

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Reflectix doesn't suck, but it does NOT deliver anything close to its claimed R-value unless it's installed in a particular way and even then, it's very unimpressive. Put it in a 2x4 built wall with OSB exterior and sheetrock interior wall, it's only R6.1. That's with 1.75" airspace on both sides.

Hedges said:

Some of which you may be able to improve.
Can you put the PV panels out in the sun, with building in the shade? (Same issue for RV with rooftop PV, maybe some portable panels.)

How to Plant Trees to Conserve Energy for Summer Shade - Climate Change at arborday.org

Plant trees to reduce your carbon footprint… and for all their benefits. For 35 years, The Arbor Day Foundation has been helping and inspiring people of all ages and walks of life to plant trees.

www.arborday.org

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Another option is spraying water on a building during peak sun, the evaporation cooling effect does wonders. Assuming you have free water.

sunshine_eggo said:

Reflectix doesn't suck, but it does NOT deliver anything close to its claimed R-value unless it's installed in a particular way and even then, it's very unimpressive. Put it in a 2x4 built wall with OSB exterior and sheetrock interior wall, it's only R6.1. That's with 1.75" airspace on both sides.

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Crushed up newspaper prob provides equivalent R value to that ?

740GLE said:

Crushed up newspaper prob provides equivalent R value to that ?

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Sometimes a pathetic R value is better than nothing. Even R1 or 2 can seem amazing. R0.1 is about 100 times better than an uninsulated shipping container wall...

740GLE said:

Crushed up newspaper prob provides equivalent R value to that ?

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R-value isn't everything. I installed reflectix stapled to the underside of my attic rafters, so it has an air space on BOTH sides. My attic temps went from 139.9°F (thermometer sensor doesn't go any higher) to 120°F. That made a HUGE difference inside the house where we used to be able to feel the ceiling get hot by the end of the day. I added a solar attic vent fan and got the temps down another 10°F. We definitely need to add more attic insulation, but the reflectix made a huge difference in our comfort inside the house.

As for the OP's problem, I can attest the Midea U-shaped 12000 btu A/C can run as low as 150-200 watts when maintaining temps. Mine is running off of my solar as I type this, but I would need a lot more solar and batteries to run it full time. I have 10kWh battery but only 1000 watts of solar installed at the moment. I have another 1200 watts of solar waiting to go up as soon as I get off my butt and build my patio cover. The 8000 btu Midea should work for you, but you still need a lot more battery, or a generator.

Also, I'm in Louisiana. Hurricane prep is what got me here. I started with a generator. That was fine for me until a week-long power outage made gasoline hard to come by.

sunshine_eggo said:

Sometimes a pathetic R value is better than nothing. Even R1 or 2 can seem amazing. R0.1 is about 100 times better than an uninsulated shipping container wall...

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Investing in white paint is always an option too!

Q-Dog said:

R-value isn't everything. I installed reflectix stapled to the underside of my attic rafters, so it has an air space on BOTH sides. My attic temps went from 139.9°F (thermometer sensor doesn't go any higher) to 120°F. That made a HUGE difference inside the house where we used to be able to feel the ceiling get hot by the end of the day. I added a solar attic vent fan and got the temps down another 10°F. We definitely need to add more attic insulation, but the reflectix made a huge difference in our comfort inside the house.

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Our 2.5 season camp is exposed loft with 2x8 rafters, would get up to about the same temps as one side is S-E facing. Placing you hand on the back side of the sheeting would almost hurt it got that hot.

3” closed cell spray foam was our solution, no venting to speak of, so it does kind of trap air once it gets warm, a roof peak vent or two is next on the list.

740GLE said:

Investing in white paint is always an option too!

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Unless it's winter, and I'm more worried about chilly NMC.

sunshine_eggo said:

Unless it's winter, and I'm more worried about chilly NMC.

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Wood stove, the original solar heater ?

I did an almost back to back comparison in overnight power consumption between a conventional bottom of the barrel 5000btu window unit and an 8000 btu midea inverter window unit in a very similar sq footage (maybe 135-140sq ft, marginal difference) and found both were similar at 1.6-2.0kwh overnight. Thats in San Antonio Texas last summer, nighttime lows are typically 80f and thats not even the whole night. So overnight use is not really a problem on a relatively small amount of battery even in a hot area.

I saw my 8k midea go barely over 1000w when set to max in a hot space. It has no startup inrush so you could honestly run it on a ‘true 1000w’ inverter if you never set it to max in a hot space, or a 1500w inverter to be safe. Minimal inverter requirement for an air conditioner!

So you dont need a ton of battery or inverter, you really just need a lot more solar. If you can consistently make ~2000w of solar production (not the peak rating of the array) during daytime hours into a ~5kwh battery, you can probably run the ac basically 24/7 although you may have to reduce settings during the evening to make it through the night with that setup.

Considering the tiny structure i WOULD actually recommend water cooling of a sort.. ive found that the cooling load after the sun goes down is out of all proportion to the actual heat GAIN of the structure.. basically i can pump heat out of the interior air all day but i never get so far ahead that im cooling the structure, which is a huge thermal mass that has accumulated a lot of heat, way above ambient temp. Once the sun goes down the outside air temp drops fairly rapidly but the structure is still exchanging its stored heat to the interior air, so the interior temp will continue to rise after sunset if i turn off the ac.

With a tiny structure you can actually ‘wash’ some of the heat down to the ground (and some to the air through evaporation) with a not-obscene amount of water. A 75ft hose kit with like 20 mister nozzles is all of like $30 online. Run that for ~30 minutes after sun sets and you will knock down some of that stored heat before it soaks into the interior air over the next several hours. Another thing you can easily do with such a small structure is just make a bunch of crossflow if you have windows across from each other, or a really strong attic/exhaust fan up top. Its going to be noisy and take longer but jusy exchanging a bunch of exterior air right after sunset will get rid of some of the structures heat as well.

Nowhere near as hot as TX but...
I put a kill a watt on my Frigidaire 6k btu window unit for 24 hours. 350 square feet room but with an open door to the house which is central air.
95⁰ outside. Decently insulated.
6.4kwh.

https://www.energystar.gov/productf...&sort_direction=desc&page_number=0&lastpage=4 The most efficient non-inverter window/room AC is on page 6.
The LG - LW6019ER LG quit making the 8K BTU version of this AC when Midea started selling the U-shaped ACs.

Perhaps a more helpful thing is that it is entirely viable to put a refrigerator in their and run it from solar and a small power system.

In an area with difficult weather, this can be a very useful feature.