Wish to run my Room Air-Conditioner on solar during day time

Am thinking of using solar power to run my room Air Conditioner during day time.

Have a 18000 BTU ( 1.5 tons ) split air-conditioner runs on 220V AC, 50Hz. At start it draws a 30Amps current for few milliseconds, and then it comes to 12 Amps for few seconds, and then after say 15 to 20 seconds its current requirement comes to 8 Amps.

My idea is to find some soft-starter or a VFD ( Variable Frequency Drive ) so that I am able to nullify the starting torque current of the air-conditioner. And after its installation, I hope then I just have to worry for 8 Amps regular current requirement only.

This 8 Amps of current needed makes power requirement of my A/C 1800 Watts.

If I install solar panels of 2500 Watts, and a MPPT charge controller , then I guess, that I be able to have sufficient power to run my A/C during day-time for 6 to 7 hours.

Now I come to question about battery costs. This cost is high, and I wish to by-pass this cost.

Upon installing a Soft-Starter / VFD to the air-conditioner. And having 2500 Watts panels, a MPPT controller, and an off-grid Inverter ( no batteries connected ). Would I be able to run the Air-Con during 5 to 6 hours a day ?

Please comment on the above.




  • BB.BB. Super Moderators, Administrators Posts: 29,805 admin
    Re: Wish to run my Room Air-Conditioner on solar during day time

    Welcome to the forum Omar.

    Guessing that you are from around Lahore Pakistan, and using the Solar Electricity Handbook to see "how many hours of sun" per day you get:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 73° angle (from vertical--or 17 degrees from horizontal for summer use):
    (Optimal summer settings)





    You have some good sun, but also summer monsoons(?) that limit solar collection...

    The problem is that off grid solar power (solar panels + charge controller + battery bank + AC inverter) is pretty expensive power... In the US, we typically pay around $0.10 to $0.30 per kWH and off grid solar power costs around $1-$2+ per kWH (cost of equipment, installation, new batteries every ~5 years or so, new controllers/inverters every 10+ years, etc.).

    Using a VFD to limit the starting surge of your A/C is a very good plan. In fact, see if you can find any "inverter" mini-split A/C (or heat pump if you can use winter heating too) systems. There was an older Sanyo unit that would run on low speed/power at 300 Watts (and no starting surge):

    Sanyo Mini-Split AC

    Sanyo was purchased by Panasonic and many of the (very nice) Sanyo products "went away". But Fujitsu, Panasonic, and others still make some nice mini-split inverter A/C systems still (I believe). Of course, you have to find something local to Pakistan (guessing that is where you are from).

    The batteries are always going to be a limiting and costly factor with off grid solar. If you can reduce the surge current to near "running" current, that will be a big help. It will still be an issue though running AC during the day from solar+charging the batteries.

    Lead Acid batteries actually "like" to be cycled (discharge by ~25% or so at night and recharge during the next day). Running an A/C system during the day and having a battery bank to charge is a "questionable" setup to figure out the "optimum" way of doing this.

    For the sake of giving you a starting point... Say you assume the days you need A/C, you have at least 5 hours of sun per day, run the A/C at 1,800 Watts, and have a "minimum" sized battery bank to run the A/C for 4 hours without sun (and discharge the battery bank to 50% of capacity--Or C/8 discharge rate).

    The size of solar array to "break even" would be:

    1,800 watts * 7 hours per day * 1/0.52 system eff * 1/5.0 hours of sun per day = 4,846 Watt array minimum

    A C/8 battery bank capacity would be:

    1,800 Watts * 7 hours * 1/0.85 inverter eff * 1/24 volt battery bank = 618 AH @ 24 volt battery bank (C/8 rating)

    If you get C/20 Rated batteries, roughly:

    618 AH * 1.2 Peukert factor (wild guess) = 742 AH @ 24 volt battery bank

    The above are probably conservative numbers--You might be able to reduce them some (by 20% or so)--But it is a starting point.

    Does the above (assuming +/- 20-30%) make economic sense to you?

    You could possibly cut the battery bank capacity by 1/2 -- But I am not sure how long it would last. And you will have to be sure to only run the system when the sun it up and bright. By something like 4pm, you would have to turn the A/C down to low (or even off) to prevent damaging the battery bank with too much current draw.

    There is another alternative--Grid Tied solar (solar panels + GT Inverter). The GT system will power your local AC loads and even "push" power back into the grid (and even turn your power meter backwards--if it is designed that to run "backwards").

    With GT Solar--The Utility Power looks exactly like a giant "AC Battery"--Your system literally behaves as if it is "recharging" your utility power. It is "cheap" (no battery, no battery losses), and reliable (no batteries or "extra" components).

    GT solar "net metering" is common in many places in the US, but may not be common (or even legal) in your region.

    It is difficult to "save money" with Off Grid/Battery Backed solar power unless you have very expensive utility power (remote location with long power lines) or unreliable power (afternoon power failures and you need battery power for a few hours most evenings).

    If you wish to save money, typically conservation is your best first step. Lots of insulation, awnings over your windows. Trees to shade the roof/walls. Double pane windows to keep heat out, etc. all are usually better investments vs just installing off grid solar power.

    All wild guesses at this point--Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jcheiljcheil Solar Expert Posts: 722 ✭✭✭
    Re: Wish to run my Room Air-Conditioner on solar during day time

    Like Bill said, you need more like 5000w of solar panels (minimum). And even then, it is only going to work in the daytime, full sun.
    It is NOT going to save you money trying to move it to solar (off-grid).
    Even running a small 5000BTU window AC requires quite a bit of solar and batteries. I know because I do that.

    But if you grid-tie, then perhaps in 7-10 years it will pay for itself and you will be able to run it all the time.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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