How to calculate and size solar power?

So you have calculated how many AH and Betteries you need from How to calculate battery size? now let's calculate the solar power in panels and watt. 

From the wattage you need to run all your appliances. Determine how many hours each day they will run. For instance, your refrigerator can run approximately 1/3 of the time during a 24 hour cycle, or 8 hours a day. The light that draws 75 watts may run 3 hours a day. Once you have determined how many watts you use a day from each appliance, add them up to give you the daily watt-hour results.

Example 6:

  • Refrigerator 1600 watts
  • Lights 400 watts
  • Misc 400 watts
  • ———–
  • Total 2400 watts daily watt-hours

Divide the daily watt-hours by the wattage of your solar panels, times the number of hours of sun. If you have a 150-watt panel and 5 hours of sun per day, you would produce 750 watts per day per panel. Now divide the daily watt-hours by the wattage produced by one solar panel. Using the 2400 daily watt-hour figure from the above example, you would divide the 2400 by 750 and need 3.2 panels. I you can, round this off to the higher number.

For this system you would need four 150-watt solar panels. If you were using 300-watt panels, you would need only two panels.

Example 7:

Let's say  you have a 100 watt load that needs to be operated for approximately 10 hours, in that case the total power required could be estimated simply by multiplying the load with hours

100 Watts x 10 hours = 1,000 Watt hours.

This becomes the absolute power necessary from the panel.

Next, we need to determine the approximate dimensions of the solar panel for satisfying the above estimated load requirement. If we assume a roughly ten hour daily optimal sunshine, the specifications for the solar panel could be simply and quickly calculated as explained in the following expression:

1,000 Watt hours / 10 hours sunlight = 100 Watt solar panel.

However, you may notice that mostly during the summer seasons you may normally get around 10 hours of reasonable amount of sunshine, but the winter season may produce roughly around 4-5 hours of effective sunshine.

Contemplating the above scenario, you too might agree and recommend considering the worst possible sunshine hour into calculation so that even on the weakest of sunshines your load keeps running optimally.

Therefore taking into account the 4 to 5 hours sunshine per day consideration, we calculate the true power for the solar panel which would enable your load to keep running throughout the year .

1,000 Watt hours / 5 hours sunlight = 200 Watt solar panel.


Next: How to size Solar Charge controller & Inverter?