Sizing A Simple Solar Electric System

EXAMPLE: You want to run two 12 volt, 13 watt compact fluorescent bulbs each night, one for two hours and the other for four hours. The area you live in gets an average of five hours of sun per day.

A. How many panels do I need and what size should I get?

1) Figure how much power each bulb (also called a "load") uses each day in watts.
Multiply the wattage of each load by hours used per day = total watt/hours per day
13 watts x 2 hours + 13 watts x 4 hours = 78 watt/hours (78 watts used per day)

2) Add 20 % to your watt/hours to account for inefficiencies and emergency power usage.
78 watt/hours + 20% (multiply 78 by 1.2) = 93.6 watt/hours

3) Divide watt/hours by hours of sun to get watts per hour.
93.6 watt/hours / 5 hours of sun = 18.7 watts per hour of sun. You need 1 panel that puts out 20 watts per hour.

B. How big a battery bank do I need?

1) Decide how many days you want to be able to run your loads when there's no sun. Then you can determine how much power the battery needs to store (how big a battery bank you need) to be able run your loads during that period. An average is 3 days without sun. Since batteries are sized in amps, convert watt/hours (from above) to amp/hours.
Volts x amps = watts, therefore: watts / Volts = amps
93.6 watt/hours / 12 Volts (system voltage) = 7.8 amp/hours
7.8 amp/hours x 3 days without sun = 23.4 amp/hours for 3 days without sun

2) Add 20% for battery inefficiencies.
23.4 amp/hours + 20% (multiply 23.4 by 1.2) = 28.1 amp/hours used for 3 days without sun

3 ) Since you never want to use more than 40% of the battery's capacity (more than that will shorten it's life), divide by 40%.
28.1 amp/hours / 40% (divide 28.1 by 0.4) = 70.3 amp/hours
You need one 70 amp/hour battery to run your lights for three days without sun.

C. What size voltage regulator (charge controller) do I need to protect my batteries from being overcharged and also to prevent the batteries from losing power at night?

1) Read the amperage rating from the panel's specifications, then add the amps of the total number of panels wired in parallel. (Parallel wiring increases amperage; series wiring increases voltage.)
Your 20 watt panel's specifications are: watts--20, volts--15.1, amps--1.4
Since you have only 1 panel that generates 1.4 amps, you need a 2 amp voltage regulator.

More on watt-hours
Beware of Phantom Loads!