
Detailed Instructions for the System Sizing Estimator. 
Step 1 is to calculate the daily WattHour usage of each item. This is done by multiplying the item wattage by the number of hours used each day. The wattage of a UL listed/approved appliance can usually be found near the AC power cord. Sometimes only the voltage (120) and amps (example 1.5) are given. No problem. Simply multiply 120 x 1.5 and you have watts, 180 in this example. P=E*I This is the power formula from Ohm's Law.


SPECIAL NOTE: In the case of refrigerators, freezers, and similar appliances, keep in mind that although they are on 24 hours per day, they actually cycle on and off and really only run about 1/3 of the time. The more times you open the door, the longer they run. In the Estimator, this is figured into the equation.


Step 2 is to add up the WattHour results for all of your appliances. This will give you the total daily WattHours required.

Step 3 is to assume that you want at least 3 days of operation before the batteries need to be recharged. So you multiply the total daily WattHours by 3. In practice, you will only have to be concerned about this in bad weather or winter. See Meters and Monitors for more about keeping an eye on things.

Step 4 is to find the total battery capacity required by multiplying the 3 day WattHour figure by 2. This way, if you run for 3 days without recharging, you will only discharge the batteries to about 50% capacity. You can greatly increase performance and battery life by not going below 50% charge. (except of course for emergencies) Get more information about this in the Storage Batteries tutorial.


SPECIAL NOTE: You can combine step 3 & 4 by simply multiplying the total daily WattHours (from step 2) by 6.


Step 5 will calculate the size of the battery bank in AmpHours. We use AmpHours because this is how batteries are rated. (Kind of how much fuel they can hold). This is figured by dividing the total battery capacity required (from step 4) by your system battery voltage, usually 12, 24, or 48 volts. Simply stated, the higher battery voltage you use, the smaller (and therefore cheaper) size copper wire can be used to connect the solar panels to the batteries. (The Wires and Cables tutorial has a chart for calculating wire sizes.) Here is an example of this calculation: The default values in the Estimator give you a total battery capacity of 21120/12 volts = 1760 AmpHours. Then divide the 1760 AmpHours by the 105 AmpHour rating of a typical 12 volt battery (1760/105 = about 17). In this example you would need about 17 batteries rated at 12 volts & 105 AmpHours each. More information is available in the Watts & Power tutorial.

Step 6 is to determine the number of solar panels you'll need. For this step you will divide your total daily WattHours by your solar panel wattage times the hours of sunshine. Example: 3520/(90*5)=8. The Estimator uses the value of 450. This assumed a 90 watt solar panel times 5 hours average daily sunshine for mid latitudes in the US. So, using the Estimator's default selections as an example, you get 3520 daily WattHours divided by 450 = 8 solar panels rounded up to the next panel. See Solar Radiation to find the number of average daily hours of sunshine for your area.


