MrM1: How long will my system run an A/C

SystemSystem Posts: 57 admin
This discussion was created from comments split from: Solar system configuration for newbie.

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  • MrM1MrM1 Posts: 273Registered Users ✭✭✭
    edited May 16 #2
    Just as a simple baseline,  would the "system" that I have listed in my sig run his needs / loads?  For how long?

    - 2.5 Kw rated array
    - 3200 watt inverter with a 4000 watt surge
    - 4 435Ah L16 batteries in series 24v
    - MN Classic 150 CC

    I am thinking of adding the smallest BTU / highest SEER mini split I can find for just one room in NE Florida and wondering how this would preform and what changes I might need to make to have the mini split run during the heat of the day.


    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • BB.BB. Posts: 27,782Super Moderators admin
    Obviously, it depends greatly on your load and sun... Not worrying about how fast you discharge your battery bank (C/20 rate vs C/8 hour discharge rate)...

    Just based on your battery bank (no sun/over night), your 435 AH @ 24 volt battery bank supply 300 Watts (mini-split AC on low) or 900 Watts (on med-high) 100% of the time (mini splits usually run with a VFD internally--Variable Frequency Drive--And draw a fixed load depending on settings and room temperature):
    • 24 volts * 435 AH * 0.50 maximum discharge (for longer battery life) * 0.85 AC inverter eff * 1/300 Watt load = 14.8 hours
    • 24 volts * 435 AH * 0.50 maximum discharge (for longer battery life) * 0.85 AC inverter eff * 1/900 Watt load = 4.93 hours

    Some of the folks that run Mini-Splits here recommend that you run the unit during the day on low--Rather than running (for example) when home from work on high to cool the home down.

    Next, based on your solar power system. Say fixed array near Jacksonville Florida:

    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Jacksonville
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 75° angle from vertical:
    (Optimal summer settings)
    Jan Feb Mar Apr May Jun
    3.51
     
    4.15
     
    5.03
     
    5.91
     
    6.06
     
    5.42
     
    Jul Aug Sep Oct Nov Dec
    5.57
     
    5.22
     
    4.61
     
    4.41
     
    3.83
     
    3.37
     
    Assume ~5.03 hour of sun minimum for "summer" cooling season (March through August) (boy, do you have a lot of clouds/marine layer/summer thunderstorms?):
    • 2,500 Watt array * 0.52 off grid AC system efficiency * 5.03 hours of sun (breakeven March) = 6,539 Watt*Hours per day
    • 6,539 WH per day / 300 Watt load = 21.8 hours per day
    • 6,539 WH per day / 300 Watt load =7.3 hours per day

    The above is fairly conservative--If you run the A/C system during the day (don't discharge the battery bank), you can get (roughly) 1/0.80 more hours per day (1.25x) because you are not cycling the battery bank (typical around 80% worst case efficiency for flooded cell lead acid battery banks).

    • 7.3 hours per day / 0.80 not cycling battery bank = 9 hours per day

    On the other hand, it is difficult to run 100% of harvested energy from an off grid system... So, you might (on average) run 75% of daily harvested energy (unless you really manage your power usage closely).

    • 7.3 hours per day (optimum harvest) * 0.75 actual usage = 5.8 hours per day (average March day)

    Anyway--Some estimates and ways to look at the question--Your actual mileage may vary.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MrM1MrM1 Posts: 273Registered Users ✭✭✭
    Perfect Bill.  Thanks so much for your time.  Is your 6,539 Wh per day from the array and average for the summer season or for the year? 

    I know on full or mostly sunny days, just charging the battery bank and then using a dump load to heat water for 3 hours at 1000 watts per hour (240v)... I can see 12 kWhs generated from the array.  And on a day like today, in full clouds and intermittent rain I have seen 4.6kWhs generated so far by 1:25pm EST doing nothing but charging batteries - the bank was at 67% SOC to start and is now up to 93%.  (And the system was not running at full available power because there just was not the demand).

    It seems my panels do good in shade and partial sun.

    I was asking this question because a friend of mine asked me this yesterday about mini splits and I want to do it soon too.  But also, because the OP of the other thread seemed  to have gotten a little miffed at the responses,  and so If my system could do what he wanted,  I could shot him a PM of the link to the system build and price I paid.  Then at least he would have some kind of idea of what it might actually take to power his AC and medical equipment.  (although he did get some pretty good info ... just did not want to hear it)
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • BB.BB. Posts: 27,782Super Moderators admin
    6,539 Wh per day is based on >5.03 hours of sun per day for ~March thru August (see chart). Realistically solar harvest numbers are useful to +/- 10% or so accuracy. Many variables that are not worth spending too much time on.
    i
    Shading is usually a killer (even partial shading). I followed your link to the Midnite discussion and see that you seem to be harvesting pretty well even with some (not too bad) shading on your site.

    We have some pretty nice threads about Mini-Splits... Put this search string into Google/DuckDuckGo, etc... and browse them:

    site:solar-electric.com mini-split air conditioning

    I don't have A/C or Off Grid system, so I can only help get the discussion started.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MrM1MrM1 Posts: 273Registered Users ✭✭✭
    Thanks.  For me right now its academic,  but something I want to do in the near future.  And I have followed a few of the recent threads on mini splits here ... interesting.

    And yes,  I have been pretty pleased with the harvest of this array and the REC TwinPeak II panel design so far.  the .02 day was when I did not power up the system until 5pm because I was using the inverter / charger on grid power to charge for testing purposes.   Yesterday (3.7kWh) it rained. No 12kWhs this month ... but I have not pushed the loads that hard yet either.  In April I saw a 12.6kWh day, but it has been spring and it has been more cloudy/hazy/hotter.


    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • PhotowhitPhotowhit Posts: 4,670Solar Expert ✭✭✭✭
    MrM1 said:
    Perfect Bill.  Thanks so much for your time.  Is your 6,539 Wh per day from the array and average for the summer season or for the year?  
    Part of Bills through put includes the losses in charging a battery bank (15-20%) if you are using the AC as a load when the batteries have reach float you will have more energy to use.

    I've run a small window unit off of 4 golf cart batteries (215 amp hours at 24 volts) with a small array @1000 watts to start, it took most of the day to charge the battery bank after running it down to @50% over night. I added about 600 watts to the array so that I could run the air conditioner more during the day after the batteries reached absorb for an hour or so (pretty hot charge coming in) and the system could finish charging the batteries and reach float. Providing for afternoon cooling and reduce the load for the evening cooling. I did not run this 24hours a day, but it was pretty near thing. they batteries which were a year old when I started lasted 4 more years running this way. There were minimal additional loads other than a fridge which I ran the 2nd and 3rd year and disconnected the 4th as the battery bank was exhibiting signs of reduced capacity.

    I can hunt up some discussions about this if you like. This system was installed about 12 years ago now and I can't recall everything to well with my feeble mind. The cabin was 10x16 and very well insulated (6" in walls) it also was built in the shade.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • MrM1MrM1 Posts: 273Registered Users ✭✭✭
    Thanks for the feedback. 

    So you ran the window unit all night? 
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
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