i know how it feels :-(. here in south cotobato, the brownout right now is 2x a day - afternoon and evening with at least a 1 hour duration.

below is my notes on how to compute the required battery when i was doing a cost analysis of taking our pressure tank off-grid and via solar power to see if it makes sense to be totally off-grid or not.

1. assume unit consumes 750W (e.g. wattage of my water pump motor) for computation. check your aircon specs to determine its actual power consumption in watts.

2. determine number of hours the unit runs in a day - your estimate is 3 hours.
750W * 3 hours = 2250 Watt-Hours
multiply the result by 1.5 to factor in efficiency losses during the operation of the unit.
2250 * 1.5 = 3375 Watt-Hours

3. determine the number of days in which the battery is to be use without being charge. for solar power, the min scenario is 2 days without charging but if you'll charge your battery via AC, the 1 day should be OK as you mentioned that brownouts only last 3 hours - you have 21 hours to charge the battery again.

3375 Watt-Hours * 1 day = 3375 Watt-Hours = this will be the 50% battery capacity. you don't want to drain the battery too much as it will lead to a shorter life span of your battery.

4. multiply by 2 to get total capacity of the battery
3375 * 2 = 6750 Watt-Hours
6750 / 1000 = 6.75 KWH (kilo watt hour)

5. to convert watt-hour to ampere hour, you need to know the voltage of the battery that you will be using - 12V or 24V.
6740 / 24V = 281.25 AH the needed power to run the aircon for 3 hours and still have 50% capacity in the battery

so, if your looking at a battery capacity of 100AH you need at least 3 batteries (281.25 AH / 3) for your project ... cheers

Boo!

thanks again bro you've been very helpful to members here. After some reading I also came up with 3 100AH batteries thus I am now confident after your confirmation that my computation is correct

no problemo 🙂 ... just take note that the 3 batteries i computed above is for a 24V battery. if you will be using a 12V battery, then that would be 6750 / 12V = 562.5 or 6 batteries ... cheers

Boo!

, pre, i just saw my other old notes with inductive motors (e.g. water pump). what was not taken into account in the capacity computation before was the amount of energy required to initially start the motor from stand still - the 'oomph' required to drive the motor to spin. for a water pump, as in what i have now, the specs requires 54 amps which is about 12KW or 12000W (54A*220V=11.88KW) to initially start the motor. although it's only needed for a few seconds during start-up, if it's not available, then the motor will not run.
after that, the water pump motor will require 3.7A or 814W (not 750W) to run. in my case, the water pump motor needs to run 2.60 mins (2 minutes and 36 seconds) each time to refill the pressure tank with water at 30 psi (from 20 psi to 30 psi).

same with your case, you need to add 12000W (for 57A, if higher, just recompute) to 3375W to ensure motor will run at startup, total now is 15357W = 15.36KW

a 24V system will require two 12V battery (e.g. 100AH) in a series connection. this setup will only give 2.4KW (24V*100AH=2400W) so you need to setup 6 sets in a parallel connection (6*2=24 batteries) to get 14.4KW which is enough to handle the 12KW needed during motor startup.

a 48V system will require four 12V battery in a series connection. this setup will only give 4.8KW (48V*100AH=4800W) so you need to setup 3 sets in parallel connection (4*3=12 batteries) to get 14.4KW.

this was one reason why i'm still reviewing the idea of using a solar energy for our house for the water pump + lights (the need to have 12KW ready just for the motor is too costly). it also confirms what i have read in the internet before, everyone was saying that it's too costly to have electrical items with high energy demand continuously (e.g. air-con) to be included in the system ...

for your case, i think it's more practical, in terms of cost, to just buy a small generator with enough capacity to support 1 aircon ... cheers

Boo!

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