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Basic Guide to Sizing a Solar Energy System

Basic Guide to Sizing a Solar Energy System


Sizing a solar energy application:

Solar energy applications can be sized by answering some questions about your power requirements and conditions. Some of the key numbers, etc. are in bold and links are provided as well.

1. First, you need to determine the load (the watt-hours used by the device you are trying to power) that you will be drawing off of the system. Is it an AC (120V household power) or DC (usually 12V DC) load? Most devices have a label that provides you with the voltage and current requirements for the product. It will usually be shown in V (volts), A (Amps), Watts (V X A) or mA (one thousandth of an amp, one milliamp = .001 amps). VA is the same as Watts (Volts X Amps).

2. How many hours per day do you want to power this load? The most number of hours in a day you want to power this load.

3. Will you need to power this load year-round (winter months included) or only summer months? This effects sun-hours per day that the panel will receive.

4. Where are you located? (see link to US solar maps below).

Example system:
2 – 12V 11 Watt CFL Bulbs for 4 hours/day.
2 X 11W = 22W X 4 hrs = 88 watt-hours/day.
88 watt-hours per day required at 12V DC.
Now, we’ll calculate amp-hrs needed per day by dividing 88 watt-hours by 12V = 7.33 amp-hrs/day needed.
We’ll add 10% for charging losses and we get about 8 amp-hrs/day needed.
For a detailed explanation of why we use the solar panels rated amps (current) rather than using the rated watts see our article Solar Panel Ratings.

No DC to AC inverter needed (converts 12V DC to 120V AC).

Using our Solar Insolation Maps determine your minimum sun-hours per day in the months of the year with lowest sun hours (Jan-Dec in North America) in your area.

We’ll use Arizona as a location and call it a year-round application. They have 5 hours of average sun in Dec-Jan.

Now we'll divide the amp-hours needed by the hours of sun.
8 amp-hours/ 5 hours of min sun = 1.6 min. amps for solar panel to cover load in winter months.
40 Watt solar panel = 2.35 amps:
This would cover your load with room to spare. Remember, this is sized so that you have plenty of power in the months when their is the least sun. Therefore, the system will produce extra energy during the spring, summer and fall months. All solar panels

4. Batteries storage – How many days do you want to be able to go with no sun? Example: 3 days of back-up power required. 3 days X 8 amp-hrs/day = 24 amp-hrs in 3 days. Max battery discharge at 50% after 3 days = 48 amp-hr battery required min. Our 55AH Battery  would be our closest size.

5. Choosing a charge controller – The solar charge controller's primary purpose is to prevent your battery from over-charging. Select a solar charge controller that is rated at the "open circuit current" of your solar panel. For example, the specs for the 40W solar panel show 2.57 short circuit current, so minimum solar controller rating would be 2.57 amps.

Any of our charge controllers would work with this system including the small 4 amp controllers.

If you intend to expand with more solar panels down the road, purchase a larger controller now so you have room to grow.

If you have questions, email us at


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