An odd battery sizing question

[NeilTheCop]

I've looked everywhere for an answer to my question with no success, so I would beg your indulgence.
I live in south east New Mexico where we have almost constant sunshine so A/C is essential. To this end I have a Pioneer split system, no ducting just refrigerant lines running through the wall to the interior unit. I already have a small solar set up so we can have lights for a few days when the power goes down but it is a dream to run the A/C off solar as a sort of poetic justice.
My plan is to run my A/C from solar but only during the day when the sun is shining. On overcast days and at night if needed it will be switched to grid power.
Many sites say that it could be run directly from panels but I have my doubts and believe that I will need a battery bank to act as a sort of buffer rather than for any storage.
My A/C consumes on average between 750 and 900 watts and the specs say the maximum power draw when running flat out is 1200 watts at 120 volts. It is a DC inverter type, soft start so no startup surge current and will run happily on a modified square wave.
So my plan was to use about 2Kw of panels, FLA batteries with a maximum 75% discharge, bit I'm not sure if It will be 12 or 24 volt.
With all this in mind, how much battery capacity would I need?

[SunEagle]

I don't know how it is where you live but here in Florida is stays HOT even after the sun goes down so I still need to run my AC system when the sun don't shine..

You are correct you would have a hard time running any AC load directly from solar panels. You will need a battery buffer to maintain the voltage your loads need.

Oh discharging a battery more than 30% will shorten it's life and going more than 50% daily will certainly kill it so you will need a much bigger battery system along with about 5kW of solar panels.

[Mike90250]

if you are just using the batteries as a buffer, ordinary size Group 24 or 27 would be fine. Because of the rather high power it can consume, I'd use 24V, if you can get a cheap mod sine inverter for that.
900W @ 12V= 75A (a pair of 6V 200ah golf cart GC2 batteries in series = 12V 200ah
900W @ 24V = 38A Wire gauge for 37a is much easier to work with. ( a pair of group 24 deep cycle in series 24V 90ah
either battery bank would be good for about half an hour of use, then ya need to shut down, or you will deeply discharge the batteries and have short life

battery watt hour = volts x amps 12V x 200ah = 2400wh. 24V x 90ah = 2160wh 900W for 1 hour will take the batteries down almost 50% , 30 min is a much easier 75%

[Sunking]

Neil your plan needs more thought. If you insist on using solar to run the AC only means you have to spend some big bucks on a Controller and at least some sort of buffer. That may not catch your attention at first but there is one thing that has not occurred to you. Actually two things.

1. Efficiency, you are going to loose 30% of the power going through the battery and conversions.
2. Your AC cycles on/off during the day, and with a battery buffer all that power has no where to go when the AC is off, lost forever that you cannot get back. Like pouring your beer out between sips in your yard. You cannot get it back and lost forever costing you money.

You would be way ahead of the game using grid tied. Will take less wattage and no waste. When the AC cycles off the utility is your infinite battery and the panels keep producing power from sunset to sun rise going somewhere and being used. Cost less and achieves your goal with no waste.

[bcroe]

Because your split system runs internally on DC rectified from the line, in theory it would be
possible to feed it DC directly from the panels, causing the line drain to drastically decrease.
The line would pick up the load as the sun fades. To make this work requires some engineering,
which apparently has not yet been done.

I am in the process of installing mini split heat pumps and will be snooping their operation. Of
course nothing here is under warranty. Ask again in a few years. Bruce Roe

[NeilTheCop]

Thanks for all your input.
I did realize that the idea makes no economic sense even before I started but still decided to give it a go.
Thanks Mike90250 for the calculations and the use of golf cart batteries. Not much cash lost if I end up frying them so I'll pick 4 up on Saturday (Sam's Club has a $20 off each one in a 1 day sale)
I'll keep you updated on how it works out.

[Sandman333]

This is eerily similar to my pet project! I am building a ground mount array to shade the West wall of my house, with 1.2kW of solar panels. I will be running a portable AC unit off of them in the afternoon to get a bit of extra cooling. My electricity rate triples between 3PM and 6PM, so I have optimized the panel tilt and orientation for that period. I won't break even ever, because I too need batteries as a buffer, but it'll be fun to play with the parts. The biggest advantage of mine might end up being the shade on my West wall in the afternoon. Last summer I measured 100F on the INSIDE of the wall in the late afternoon (Phoenix).

[Mike90250]

This is eerily similar to my pet project! I am building a ground mount array to shade the West wall of my house, with 1.2kW of solar panels. I will be running a portable AC unit off of them in the afternoon to get a bit of extra cooling. My electricity rate triples between 3PM and 6PM, so I have optimized the panel tilt and orientation for that period. I won't break even ever, because I too need batteries as a buffer, but it'll be fun to play with the parts. The biggest advantage of mine might end up being the shade on my West wall in the afternoon. Last summer I measured 100F on the INSIDE of the wall in the late afternoon (Phoenix).

Why not use simple Grid Tie and skip the batteries ?

[Sunking]

Why not use simple Grid Tie and skip the batteries ?

I already pointed that out. Wants no part of that.

[SunEagle]

Why not use simple Grid Tie and skip the batteries ?

More than likely the OP believes he can save money using solar panels to directly run his AC unit. The sad part is that there are a lot of people that have "drank the cool aide" and feel they do not need to spend very much to install a solar pv system which gives them "free" energy. It is a very misconceived belief.

They really don't understand that to run air conditioning equipment directly from the DC output of solar panels requires the purchase of very specific equipment which probably costs much more than standard AC equipment and will take years to pay for itself when you figure in the total system cost.

[Sandman333]

Why not use simple Grid Tie and skip the batteries ?

Hi Mike,
Due to my utility company here forcing a different rate plan when you have grid-tied solar panels, my electricity bill would actually increase if I had grid-tied panels (and did not have a battery pack to carry my usage through the evenings and diminish load peaking). New solar installations have practically ceased here, but old ones are grandfathered into an older and more advantageous rate plan. I have considered going grid-tied with a zero-export system in place and not telling my electric company, but I would then be violating city code by installing such a system and not informing the electric company.

Just like Neil said about his project, this plan won't break even ever, but it is a fun project that is educational and doesn't cost much out of pocket.

[bcroe]

More than likely the OP believes he can save money using solar panels to directly run his AC unit. The sad part is that there are a lot of people that have "drank the cool aide" and feel they do not need to spend very much to install a solar pv system which gives them "free" energy. It is a very misconceived belief.

They really don't understand that to run air conditioning equipment directly from the DC output of solar panels requires the purchase of very specific equipment which probably costs much more than standard AC equipment and will take years to pay for itself when you figure in the total system cost.

I have seen ads for air conditioning equipment with solar panel input supplements, they looked like
the current (typ 14 SEER) stuff. Probably are the model using an internal inverter to drive a variable
speed compressor. In fact the variable is a bit more efficient, payoff may not be much different, but
reliability might be the bigger issue.

With the smaller mini splits, is available a doubling of SEER, am going that route now. Seems to me
its about time to work out the direct panel connection to this class of conditioner, commercially or at
least as a science project. Bruce Roe

[SunEagle]

I have seen ads for air conditioning equipment with solar panel input supplements, they looked like
the current (typ 14 SEER) stuff. Probably are the model using an internal inverter to drive a variable
speed compressor. In fact the variable is a bit more efficient, payoff may not be much different, but
reliability might be the bigger issue.

With the smaller mini splits, is available a doubling of SEER, am going that route now. Seems to me
its about time to work out the direct panel connection to this class of conditioner, commercially or at
least as a science project. Bruce Roe

I think you are correct. Someone needs to develop a cheap AC system that can work directly from a solar panel input similar to what a DC fan motor does. Although I still don't know how you can keep a compressor from having an issue if the sunlight doesn't stay constant while it is on.

[bcroe]

I think you are correct. Someone needs to develop a cheap AC system that can work directly from a solar panel input similar to what a DC fan motor does. Although I still don't know how you can keep a compressor from having an issue if the sunlight doesn't stay constant while it is on.

That is a good point. These might have a couple operating modes, one with a PoCo connection and
explanation (and perhaps an indicator) that operation will revert to conventional power when good sun
isn't available. This would let owners run on solar without a net metering agreement.

The other mode would be off grid (not recommended with grid available) entirely, with shutdown
without good sun. This has the additional requirement that EVERYTHING (fan motors, logic)
be powered from the main rectified DC.

How is cheap defined? A unit requiring half the energy but costing twice as much will probably save
money most places. I don't have a handle on relative costs of central vs decentralized units. Mini
splits are almost unheard of in these parts, the local service people tend to look askance at anything
that high tech. Buying them near wholesale out of state and doing a DIY install doesn't seem so
much. Just a few holes in the wall, no duct work. Bruce Roe

[Sunking]

Hi Mike,
Due to my utility company here forcing a different rate plan when you have grid-tied solar panels, my electricity bill would actually increase if I had grid-tied panels.

Solar users should pay more.

The point you are missing anything you take off grid is stil going to cost you 5 to 10 times more than buying power. You just set yourself up for a huge rate increase. Difference is you have to pay for it all up front in cash when you put the batteries in initially, and every few years with each battery replacement at even higher cost.