Amount of PV panels seems high for proposed battery backup?

1 Posts
Nov 16, 2011 12:29 am
Amount of PV panels seems high for proposed battery backup?

Hey all,
So following the mess that all the storms hitting the North East have made of the power grid these last few months, my parents have asked me to plan out a backup energy system for them. While a simple gas/propane generator seemed to be the easiest way to go, I wanted to try my hand at designing a battery bank backup, tied to the grid for charging, and a few panels to provide charge when the grid goes down.

Because I didn't know the exact location of where the bank would be relative to the panels, I'm planning everything out at 48v to allow smaller/cheaper wires for the panels.

I started with a minimum of about 2.5 kw/day to provide power for lights, furnace pumps (2x 1/25 hp pumps for a gas furnace), and some other misc loads. Initial hope was to be able to run the well pump off of this amount of power as well, but as I did some research it seems their 220v well pump will probably suck too much power out of the bank for that to be feasible. In the market now for a 12v DC well pump or something else more efficient.

So at 2500 watts/day with 48v system voltage, we're looking at 52 AH/day.
Three days would be ideal storage, bringing us to 156 AH.
With a 50% DOD, I get 312 AH.
I'm assuming the bank will be in the basement, about 40F lowest temp. 312 * 1.3 for operating temp = 406 AH total.

I was thinking of going with the Interstate UL-16HC (415 AH), so that would require 8 batteries for 415AH/48V.

With that number I tried to calculate the required number of panels to provide a charge to the bank when the grid is down.
I used 10% of bank AH to calculate PV AH, so that would be about 41 AH.
I assumed around 57v required to charge the 48v bank, so 41*57 brings me to 2337 watts, and assuming a 77% efficiency, I'd require about 2875 PV watts.

Now, eight batteries and almost 3kW of PV on their own carry a hefty price tag, let alone the inverter/charge controller/etc.

I guess I wanted another opinion on my numbers, and any suggestions to what I can do to bring the cost down some? Initially I thought this would be a better/cleaner way to go, but that $500 5kW craigslist gas generator is seeming more and more appealing.

Thanks all for any help you can throw my way!
462 Posts
Nov 16, 2011 09:48 am
Re: Amount of PV panels seems high for proposed battery backup?

Yeah Mike it can get pretty expensive. Dealing with a customer now with the same situation. She has the setup you want only in 24 volts. A little cheaper that way especially when it comes to breakers and such.
 You could also consider using 12 volt batteries and a basic battery charger that you can get at any automotive store to maintain the batteries' charge til you need them. Then you can wire them up in any configuration you want.
 The well pump is an issue as far as power goes but you could purchase several smaller inverters for dedicated power neccessities.
 PV panels can be used also as you know and can maintain their charge for year round use.
 The generator can be used also to charge the batteries. '
 So lots of options and lots of cost options also. Good luck.

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