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

desertsun · September 2018

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!

Estragon · September 2018

It may help if you list details of the 3kw system you have already, your approximate location, and maybe some background/expectations for going off-grid.

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.

BB. · September 2018

Just to be clear... Is this an 8kWatt AC inverter and 8kWatt of solar panels? What is the AH and voltage rating of the proposed battery bank?

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
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.92	5.55	6.45	6.83	6.74	6.57
Jul	Aug	Sep	Oct	Nov	Dec
6.08	5.78	6.03	5.82	5.24	4.74

Say, 5.55 hours minimum sun (warm/hot weather):

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

Photowhit · September 2018

I would say it's likely that you would need that large a system...

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...

Dave Angelini · September 2018

Read some of the threads here in the energy mini-split section, or e-mail me. That is ridiculous size split for offgrid, unless it is really huge house with 8+ people.

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.

desertsun · September 2018

Thanks for the responses. We'll be in the Lake Havasu City area (desert, no trees or shading) so similar solar hours to Phoenix. https://solarenergylocal.com/states/arizona/lake-havasu-city/
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...

Photowhit · September 2018

desertsun said:

4000w per day running a/c

Where did this number come from?
I can use almost triple that running a small window unit.

desertsun said:

The house currently has no functioning ac unit. (1700 sq ft)
We were told upgrading the current system is not possible (they looked into it) plus the batteries are old but still functioning.

So do they want to quote you for a new complete system, or will they reused the panels?

desertsun said:

If we use a maximum of 7000w per day (working from home)

What do you do that uses that much energy? Does it create heat?

desertsun said:
Yes, the quote was for a 8kw inverter and 24 panels @320w  12 batteries (unknown specs)

desertsun said:

4.4 kw inverter

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.

mcgivor · September 2018

Having a single 36 000 BTU is perhaps not the best way to go, multiple smaller units would allow multiple zones, rooms could be set independently or even turned off, unless of course the large one allows multiple evaporators. The other advantage of multiple units is redundancy one failure won't affect everything. Just some thoughts.

desertsun · September 2018

Photowhit said:
Where did this number come from?
I can use almost triple that running a small window unit.
500 Watts * 8 hours = 4,000 WH per day
The hvac guy told us the mini split will use half the wattage of a conventional ac.
I do get conflicting info all the time though so I know this number could be inaccurate.

So do they want to quote you for a new complete system, or will they reused the panels?
The quote is for an additional 8kw system to tie into / add to the existing system.

What do you do that uses that much energy? Does it create heat?
The energy calculation was based on if we were to run all appliances for their required hours within a 24 hour period, so not likely to actually use that much. Two computers running 10-12 hours a day is a requirement though.

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.
Yes that is the inverter that we have. Thanks for the info!

mcgivor · September 2018

For the same redundancy reasons as having multiple units, building a dedicated system for air conditioning would allows uninterrupted supply until repair can be made should there be a fault, like lightning may cause.

desertsun · September 2018

mcgivor said:

For the same redundancy reasons as having multiple units, building a dedicated system for air conditioning would allows uninterrupted supply until repair can be made should there be a fault, like lightning may cause.

I agree.
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.

Photowhit · September 2018

desertsun said:

Photowhit said:
Where did this number come from?
I can use almost triple that running a small window unit.
500 Watts * 8 hours = 4,000 WH per day
The hvac guy told us the mini split will use half the wattage of a conventional ac.
I do get conflicting info all the time though so I know this number could be inaccurate.

Dave has said his likely smaller mini splits might use;

Dave Angelini said:

...we use about 5KWH over night...

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.

mike95490 · September 2018

And before installing solar and air cond, check out venting the attic and upgrading insulation before installing AC. Here at 1,400' we have 95F days, and can keep the downstairs below 70F by purely passive cooling, opening windows at night, closing them daytime and using the shaded porches to keep direct sun off the windows.

If you have low humidity, swamp coolers can also get you 15 degrees of cool for a lot less power.
(we do not have swamp cooler)

Dave Angelini · September 2018

Window opening at night rarely works in the desert summer...... It does not work here in the Sierra. Not a big enough differential. Evaporative cooling also fails when the humidity goes up in the monsoon season.

showme · September 2018

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

Dave Angelini · September 2018

Hi showme
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!

jonr · September 2018

AC should be sized for approximately the worse case load (not much larger). So on the hottest days, it will run close to 100% of the time, at least until the sun goes down and it cools off. A "Manual J" is the proper way to predict load- don't guess.

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.

oil pan 4 · September 2018

They make more money selling you a new system. I don't expect them to make any effort what so ever to do something that make them less money.
They will probably also offer to haul away your old system for free.
You are going to have to diy.

Hill_Country · September 2018

@desertsun ,this thread is very interesting and timely. We are in the design stage of doing a very similar setup (off-grid), except with a smaller mini-split system in Northwest Arkansas (so significantly fewer sun hours than the Lake Havasu City area). We plan on having a manual J calculation performed, and, like other posters have suggested, investing in insulation and air sealing. Please note, the sig line for my system is our current system in Texas...not what we're planning on doing in NW Arkansas

So, without derailing the OP's question about the appropriateness of an 8KW system to run a 36,000 BTU mini-split in the Lake Havasu City area...I was wondering if any folks have experience with, or know of, the Chiltrix system that @jonr references in thread #17 above? It seems intriguing but there is almost no user reviews or experience with this system that I can find...

Dave Angelini · September 2018

Anything more complicated that a split for Offgrid is way out of the realm of DIY. Even if you hire someone, it seems like a source of alot of work when you probably would rather be doing something else. The 36Kbtu was probably from an HVAC professional who did not understand anything about living offgrid.

oil pan 4 · September 2018

I was saying DIY the solar, not the A/C.

Dave Angelini · September 2018

You can do it all yourself and still be safe. I specialize in helping folks do this in over 150 offgrid homes. Sometimes DIY is the only way when you are remote

desertsun · August 31

UPDATE 2019:

After living off grid for almost one year now, I can positively say that adding an 8kw system to our 3kw system would have been insane. In fact, even adding a 4kw array to our current system would have been a waste. We are still living on a 3kw system and running enough air conditioning to survive. For us, adding 1kw or 1.5 kw of panels to our array would be perfect (for a total 4kw array).

Instead of the mini-split, we got a 14,000btu LG dual inverter window air conditioner (soft starts and adjusts wattage based on compressor speed). I highly recommend it for off grid and it's much easier to install than a mini-split and can be moved to a different spot, if needed. At night, we run a bedroom a/c window unit off a small generator if it's too hot to open the windows.

Just wanted to provide an update in case it will help someone else in a similar situation.

Photowhit · August 31

desertsun said:

UPDATE 2019:
Instead of the mini-split, we got a 14,000btu LG dual inverter window air conditioner (soft starts and adjusts wattage based on compressor speed). I highly recommend it for off grid and it's much easier to install than a mini-split and can be moved to a different spot, if needed. At night, we run a bedroom a/c window unit off a small generator if it's too hot to open the windows.

Sounds Great! We have a thread going on the LG dual compressor air conditioners. I had been waiting on the 9500btu dual compressor, but they introduced it so late in the summer, I figured I'd look for a end of summer sale!

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!

spacebass · September 1

BB. said:

Just to be clear... Is this an 8kWatt AC inverter and 8kWatt of solar panels? What is the AH and voltage rating of the proposed battery bank?

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
Average Solar Insolation figures
Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
(For best year-round performance)
Jan Feb Mar Apr May Jun
4.92
5.55
6.45
6.83
6.74
6.57

Jul Aug Sep Oct Nov Dec
6.08
5.78
6.03
5.82
5.24
4.74

Say, 5.55 hours minimum sun (warm/hot weather):
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

Hey Bill I think You must be a computer ♥️

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

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PhoenixAverage Solar Insolation figures

PhoenixAverage Solar Insolation figures

Phoenix
Average Solar Insolation figures

Phoenix
Average Solar Insolation figures