Do we really need an 8kw system just to run an AC unit?!

My boyfriend and I are moving off grid soon and have just been given a quote for adding an 8kw solar system just to run one 3 ton (36,000 btu) ductless minisplit a/c with heat pump. That seems way too extreme to me. It seems like maybe they were counting on us running the unit on high 24/7 but air conditioning only runs about 8 hours a day (from my understanding). Is this true for a mini split? I have tried to run the numbers but I am still learning about solar. We currently already have a 3kw (48v) system in place. We can not upgrade that system but can only add another system. Here are the specs:
Planning to get Daikin mini split https://www.amazon.com/Daikin-220V-Split-Inverter-Conditioner/dp/B00UCEZ0Y0
Installing a 230v outlet outside for the unit.
Have 3kw system already.
We expect our usage to be 7000 watts a day before adding air conditioning/heat pump wattage.
Any and all opinions would be greatly appreciated!
Planning to get Daikin mini split https://www.amazon.com/Daikin-220V-Split-Inverter-Conditioner/dp/B00UCEZ0Y0
Installing a 230v outlet outside for the unit.
Have 3kw system already.
We expect our usage to be 7000 watts a day before adding air conditioning/heat pump wattage.
Any and all opinions would be greatly appreciated!
Comments
7kwh/day is on the high side for off-grid, but not totally unreasonable. 3kw solar may be on the low side, depending on location etc.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Energy usage is highly personal... Using US averages, a 36,000 BTU A/C may use around ~500 Watts (just a guess, I am not an A/C HVAC expert by any means).
If you start cool and run 8 hours per day (i.e., don't start with a hot house and run 2+ hours on full cool to bring the temperature down)...
- 500 Watts * 8 hours = 4,000 WH per day
If you are in the Phoniex area (again, guessing on my part), the average hours of sun per day for a fixed array would be:http://www.solarelectricityhandbook.com/solar-irradiance.html
Phoenix
Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:Average Solar Insolation figures
(For best year-round performance)
- 4,000 WH per day * 1/0.52 off grid AC system eff * 1/5.55 hours per day = 1,386 Watt array minimum
And the battery bank would be (assuming 2 days of storage and 50% maximum discharge for longer battery life):- 4,000 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/48 volt battery bank = 392 AH @d t 48 volt battery bank
Nominally you need to charge your battery bank at 5% to 13% rate of charge, and for full time off grid, 10%+ rate of charging is recommended. A 10% rate of charge with the above battery bank would require a solar array of:- 392 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 3,004 Watt array minimum "nominal" size
The size of an AC inverter for a 392 AH @ 48 volt battery bank would be somewhere 2,000 to 4,000 Watt rated (don't oversize the inverter if you don't need to. Maximum "suggested" size of solar array would be ~4,000 Watts for this battery bank.If you wanted 8,000 Watt*Hours (8 kWH) per day, the above calculations would be 2x larger.
-Bill
36,000 btu is quite a large air conditioner, from Daikin's web site (trying to find specs), looks like the single head units stop at 24,000 btu.
I don't understand why you couldn't add to your existing system? You might need to run multiple Charge controllers. You would likely need a larger battery bank, It is hard to 'grow' a system.
I understand not adding new batteries to old, if you have a working system now and want to add more air conditioning, you might be able to add array/panels to your existing system. This would allow you to have more air conditioning during the day without drawing from your battery bank and allowing you to charge normally. You could use the battery bank you have until it dies then add a larger battery bank...
When we cool 24/7 in 100F, we use about 5KWH over night but can produce 50KWH + during the hot day, if we needed to.
Many clients doing the same thing.
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The house currently has no functioning ac unit. (1740 sq ft)
We were told upgrading the current system is not possible (they looked into it) plus the batteries are old but still functioning.
Yes, the quote was for a 8kw inverter and 24 panels @320w 12 batteries (unknown specs)
The current 3kw system is 4.4 kw inverter 12 panels @250w 48v 400 Ah batteries
So it seems that an 11kw total (3kw system + 8kw system) is way too high for our needs? If we use a maximum of 7000w per day (working from home) + 4000w per day running a/c = 11000w maximum usage in a 24 hour period. Wouldn't a 7KW system (current 3KW + 4KW installed) be more than enough?
Hopefully this is correct, I'm still learning...
I can use almost triple that running a small window unit.
So do they want to quote you for a new complete system, or will they reused the panels? What do you do that uses that much energy? Does it create heat?
This sure sounds like a Magnum MS4448PAE system, not sure there are other 4.4 kw inverters. These can be run in parallel adding another MS4448PAE to your system if you really need it, and you might if you use high wattage items in your work. I don't know if they can 'hot sync' so you could just turn one off at the end of your work day to save the tare energy draw, but I think you could.
I think they want to install a whole new system rather than work with yours, might see if the original installer is still around.
@Dave Angelini has experience with similar systems with yours. Might give as much information on the insulation value and interior space and do some on line quotes for suggested air conditioner sizing.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
I never liked that ac unit idea but it was suggest by an "expert". We haven't committed to anything yet so I'm definitely open to other ideas. I'm also considering just keeping the solar as it is and using a propane generator to supplement the extra power. Not sure if that is a good idea for long term though.
This would be during the cool part of the day during the shortest days of the year... So maybe 2.5 - 3x that for his, likely smaller unit.
Best of luck to you, this is really out of my wheelhouse, but it sounds like you have been given a 'best for them' system rather than what you need. It might be worth talking with a couple HVAC companies and see what they think your home needs.
(we do not have swamp cooler)
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
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We're getting ready to build our earth home (earth covered), and although we may not need it, we decided on a small minisplit just in case, or for humidity control. This one runs on 900w of panels during sun hours, and is 12,000btu. We're in the Missouri Ozarks, not the desert. But it may help. http://www.hotspotenergy.com/solar-air-conditioner/HotSpot-ACDC12C.pdf
"Knowing is not enough, we must apply. Willing is not enough, we must do." Goethe
NOS Trace Xantrex power panel, 3 SW5548's (2 NOS in boxes, one used), Xantrex DC/disconnect, AC disconnect/conversion, (3) C40's, 2 NOS, one used) 8k LG Neon 320w, 1k Evergreen 120w, Bergey XL.1 1.5k genny, AirX 400 genny, 48v Interstate Workman batts (frozen/toast), and one nice Ozark hilltop to set them all.
We were going to do the earth home in Arizona but our plans changed. At the time there was no such thing as the mini-split.
The Hotspot is not a bad choice but there are better. One of the sort of misleading parts of there spec is how they calculate SEER.
They inflate the number by comparing it to utility cost which is wrong in my mind. Anyone offgrid does not have utility cost and so the number seems useless.
Mini-splits from the mitsu, fujitsu, LG and other big names are all near 30 SEER for a 12,000 btu or 1 ton. This SEER number is useful as it is an industry standard. Even so, there are plenty of ways to get the most out of this with strategies. Drop me an e-mail ?
If you are going to have an inverter for other household needs it seems like overkill for the Hotspot. Did you find any pricing on it?
When I looked about 5 years ago it was up there, I seem to remember. Good Luck to you!
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A good mini-split will move about 4x heat/power as it uses. Eg, ~2647W for a 36,000 btu/hr unit(s). This can drain batteries quickly.
If you really need 36,000 btu, then 8Kw of panels is about right.
Consider better insulation and air sealing and/or only cooling a couple of well insulated/sealed rooms. A 9,000 btu (Fujitsu 9rls3) can easily do this.
You could consider a chiller based system (eg, Chilltrix, needs about 2Kw of panels) that stores cooling in a water tank (only while the sun shines, so very little battery use). It will be more complicated and require space for a tank, but can be much less expensive than using batteries for solar AC.
I am available for custom hardware/firmware development
They will probably also offer to haul away your old system for free.
You are going to have to diy.
Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.
Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.
http://members.sti.net/offgridsolar/
E-mail [email protected]
Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.
Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.
http://members.sti.net/offgridsolar/
E-mail [email protected]
Adding to your array, particularly if you are finding your not getting your batteries topped off, seems prudent. Particularly if you could do a South West facing addition to the array to give you some 'virtual tracking'.
Thank you for the update!