Battery sizing questions

offgrid me

I have been getting a lot of conflicting info about what size battery bank I should purchase for my off grid system. The range of battery sizes I have been told I should use is all over the place from 400ah all the way up to 1000ah. I cant decide whether I should go cheap and buy a "learning" set or whole hog and get a battery that should last me 15 years but at substantial cost.
My system info
48 v, xantrex 6048 inverter charger xantrex 80 600 cc
5kwh daily use (kill o watt verified, actually 4.6)
4.2k of panel
lat 45* Maine
Full time residence
Generator backup
I know im panel heavy but I got a good deal and hope to charge the batteries in the short winter days
Thanks Ned

Cariboocoot

Re: Battery sizing questions

Ned;

The math is simple:

4.6 kW hours per day? Use 4.8. Divide by 48 Volts (nominal system Voltage). You get 100 Amp hours needed. At 50% DOD that's a minimum 200 Amp hour battery. At 25% that would be 400 Amp hours.

The XW 6048 wants 600 Amp hours minimum if you're doing grid-tie.

Your 4.2 kW of panel would support 539 to 1078 Amp hours of battery.

That's probably where you're getting all the different numbers from.

I'd go with the starter set: use "golf cart" type 6 Volt units. They're usually around 220 Amp hours. You can use eight for 220 @ 48 Volt or sixteen for 440 @ 48 Volt. Even with the latter, the potential 54 Amps of charge current will be plenty and semi-cloudy days will still produce some fair charging.

You can use the PV Watts program http://www.nrel.gov/rredc/pvwatts/ to check your potential harvest year-round.

offgrid me

Re: Battery sizing questions

Coot
Thanks for the response. Ive run pv watts about 30 times and have set the angle of my panels so that even in the worst month of the year it says I should be able to generate over 8kwh per day. Why would some very experienced sales people with over 30 yrs experience tell me that I need at least 700ah of battery and everything I have read on line is wrong. They were also telling me that my dc from battery to ac conversion would only be 80% at best(xantrex lists 92%). I guess you really cant trust anyone that is trying to sell you something. T105s here I come:p

Cariboocoot

Re: Battery sizing questions

offgrid me wrote: »

Coot
Thanks for the response. Ive run pv watts about 30 times and have set the angle of my panels so that even in the worst month of the year it says I should be able to generate over 8kwh per day. Why would some very experienced sales people with over 30 yrs experience tell me that I need at least 700ah of battery and everything I have read on line is wrong. They were also telling me that my dc from battery to ac conversion would only be 80% at best(xantrex lists 92%). I guess you really cant trust anyone that is trying to sell you something. T105s here I come:p

DC to AC conversion efficiency is open to interpretation. If you want to get picky about it you'd be factoring in the losses for your particular wiring, size and length, in addition to everything else. The inverter runs at 92% - but at what input Voltage and output load? It varies.

If you just look at the rule-of-thumb calculation notoriously know around here as The Icarus Formula your 4200 Watts of panels * hours of "equivalent good sun" (usually 4) / 2 (over-all system efficiency of 50%) you'd get something like 8.4 kW hours per day. That's pretty close to the PV Watts prediction, eh?

Thing is, all systems have ranges and tolerances and losses. How you juggle the numbers can make a big difference in things like battery bank size or daily harvest or usable AC Watt hours. What charge rate do you go for? 5%? 10%? How deeply do you discharge the batteries? 50%? 25%? It all makes a difference.

What salesmen are experienced at mostly is selling stuff. :roll: