Battery bank for swimming pool

2 Posts
Nov 13, 2014 09:41 pm
Battery bank for swimming pool

Is this calculation right?

i have a pool with a 1hp pump - or 743W, say 750W

Following the method on this site:

- pump only: 750W, working 10H full time: 7500 Wh/day
- 2 days back up: 15000 W
- Depth of Discharge: 40%:       15,000 Wh/40% = 37500 Wh
- Temperature factor: 1      37500Wh
- System Voltage: 24V deep cycle batteries >>    37500AVh/24V = 1563 Ah  (top up to 16750 Ah for calc)

say w/o inverter, just an oscillator, 24V and 250Ah batteries:

250 Ah: 1750Ah/ 250Ah = 5 such batteries in parallel
24 V: 220V / 24 = 10 strings in serials

That is 50 batteries for a 750W pump?  My calculation must be wrong somewhere.

And how to calculate the bank if an inverter is to be used?

Marc C
47 Posts
Nov 18, 2014 01:53 pm
Re: Battery bank for swimming pool

You almost had it right, just made a slight mistake at the end.  First, let's include the inverter, you divide the watt hours by the inverter efficiency.  Let's guess 94%.  37500WH / .94 = 39,893WH / 24v Battery bank = 1662ah.

Your mistake came with calculating the voltage.  You are  now figuring a 24V 1662ah battery bank, so if you figure using 12V 250ah batteries, that's 1662ah / 250ah batteries = 6.6 parallel strings, round up to 7.  24V battery bank / 12V batteries = 2 in series.  7 strings of 2 = 14 batteries. 

However, I recommend you don't do 7 strings in parallel, that can lead to uneven charge/discharge cycles, shortening the life of the bank, so if you did a 48V system, 39,893Wh / 48V = 831ah 48V battery bank.  831ah / 250ah batteries = 3.3 strings, round up to 4.    48V battery bank / 12V batteries = 4 in series.  4 x 4 = 16 batteries.

Hope that helped.

altE store
Solar Queen
2 Posts
Nov 18, 2014 07:42 pm
Re: Battery bank for swimming pool

Thank you.

The reason why my calculation was w/o inverter is that in Spain they avoid using those and simply add an oscillator.
So I did the calculations to check where the benefit could be in doing so. Other than the % loss because of the inverter I do not see an immediate advantage indeed.

A good day to you,
47 Posts
Nov 19, 2014 10:04 am
Re: Battery bank for swimming pool

Ah, that explains the 220V in the battery calculation. Then yes, you were right, 220V / 24V batteries = 9.1, round up to 10 batteries in series.

But if you are using 220V as the battery bank voltage, you would do
37,500 wh / 220V = 170ah / 250ah battery = .68, round up to 1 string of 10 24V 250ah batteries.


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