# backchannel: solar calculators

from this old post I found while Googling a different topic:

I have about 364 watt [daily use] total. I don't know how accurate that is becuase im guessing on hours used and wattage of leds but 364  is what i inputed.  Alt e gave me 300 watts panel, 310 ah battery, 25 amp charge controller. Renogy gave me 196 watt solar and 60 ah battery

# Battery sizing

364w/day would be about 30Ah usable capacity needed, 60Ah usable.

Option 1:  Compact:  Trojan type 27 AGM, 89Ah.  My interpolation of published data suggests s/he’d get about 1500 cycles at 30% DoD, about \$6/kWh.  Could sustain 30A discharge with no sweat.

Option 2:  Large but cheap:  2x Trojan T-105 flooded, 225Ah.  This would be 15% DoD, over 3000 cycles at a cost of about \$2/kAh.  Could sustain discharge of 45A in theory, but in practice I wouldn’t expect much more than the 30A the AGM offers. .

# charging requirements

• replacement:  we have to put 34.5A back into the bank (30A * 1.15 for charging inefficiency.

• current:  the AGM will require 17.8A minimum (C/5), about 223 actual charging watts.    The larger flooded bank will require 22.5A minimum (C/10), about 281.25w charging watts.

The easiest way to meet the current requirements is with an isolator.

Meeting current requirements with solar alone using an MPPT controller on the small AGM would take about 256w.  The flooded would require 323w.   So let’s assume we install an isolator that we run for 15mins each morning.

# solar requirements

15mins of charging will put 7.42A (C/3 * .25) back into the AGM.  It will put 11.25A (C/5 * .25) back into the flooded. Doubling the running time will not double the Amps replaced because the batts will come up to alternator voltage fairly quickly.

Since the poster was on Cheap RV Living forum we assume they are snowbirding, so let’s say they will be getting at least 2.75 hours of full sun equivalent at a minimum;  that’s Phoenix in December.

The AGM needs another 27.08A (34.5A - 7.42A), or 389w of panel to get the job done in winter.   Much less than that in other seasons.

The flooded bank would need 23.25A (34.5A - 11.25A), or 334w of panel to get the bank charged in winter.

The forum participants and Alt-E (which I respect) gave 300w, and I think that’s sane.  Could be rough in winter but maybe we tilt that 300w then to get back to the effective output of 400w.  The rest of the time 300w would be golden.

Updated: