Question

Estimate the size of a PV array in Watts for a 12 V DC stand alone system with battery storage for a mountain cabin near Ruidoso with the following energy demand:

5 DC lamps, each at 10 W operating for 5 hours/day

1 DC television at 12 W operating for 4 hours/day

1 DC refrigerator at 50 W operating for 12 hours/day

Assume array tilted to provide a critical winter design solar resource of 4 PSH. Show work and units.

Answer 1

1. determine power consumption demands

total appliances use = (5 lamps* 10W* 5 hours)+(1* 12 w * 4 hours)+ (1* 50w *12 hours)

= 898 wh/day

total PV panels energy needed = 898* 1.3

= 1167.4 wh/ay

2. size the PV panel

total Wp of PV panel capacity needed = (total wh/day )/3.4

= 1167.4/3.4

= 343.35 Wp

number of PV panels needed = Wp/ power of each PV

assuming 12Vdc , 110 Wp PV module

= 343.35/110

= 3.121 modules

actual requirement = 4 modules

3. battery sizing

total appliances use = 898 wh/day

nominal battery voltage = 12 V

days autonomy = 3days

battery capacity = (total wh/day)* autonomy/(0.85*06*nominal of battery)

Divide the total Watt-hours per day used by 0.85 for battery loss.

Divide the answer by 0.6 for depth of discharge.

= (898 * 3)/( 0.85*.6*12)

=440.196 Ah

So the battery should be rated 12 V 500 Ah for 3 day autonomy.