How to Size a Solar System: Step-by-Step
Key takeaways
The average home needs between 15 and 19 solar panels to cover its daily electric usage.
You can calculate the number of solar panels you will need with your energy usage, the amount of sunlight you get, and the wattage of the solar panels you choose.
The formula for calculating how many solar panels you need = (Monthly energy usage ÷ Monthly peak sun hours) ÷ Solar panel output
The exact amount of solar panels needed for your home can vary with the characteristics of your roof, environmental factors, your local climate, your budget, your personal energy needs, and the size of your home.
Estimating of Your Energy Usage
Before you begin to size a solar system, follow these steps to determine your home’s average electricity consumption and PV needs:
1. Calculate Your kWh Usage
- Gather the kilowatt-hours (kWh) usage from your electric bill. You’ll want to have full 12 months of usage to be able to look at peaks and valleys in usage over a year. Energy consumption spikes in the summer and winter with heavy use of your A/C and heating units.
- Determine your average monthly kWh usage. Add up your kWh usage for 12 months and divide by 12 to figure out your average monthly consumption. Your grid-tied system will tend to overproduce in the summer with peak sun exposure.
- Figure your daily kWh usage. Divide by 30 to determine your daily kWh usage.
2. Look Up Your Peak Sun Hours
Average peak sun hours vary greatly depending on your location and local climate. You’ll want to determine how may peak hours of sunlight you’ll get so you can make the most of the solar power:
- Look up your peak sun hours, through a sun hours chart to determine the number of hours per day the sun produces peak sunlight.
- Find the nearest city to you and write down the daily average of peak sun hours.
3. Calculate the Size of Your Solar System
To figure out how to size your solar system, take your daily kWh energy requirement and divide it by your peak sun hours to get the kW output. Then divide the kW output by your panel’s efficiency to get the estimated number of solar panels you’ll need for your system.
(Daily kWh ÷ average sun hours) x 1.15 efficiency factor = DC solar system size
For example, if you live in New Mexico, you average six peak sunlight hours per day. You’ll need 6.2 kW DC according to the formula:
(33 kWh ÷ 6.1 sun hours) x 1.15 efficiency factor = 6.2 kW DC solar system size required
Using the example above with a 6.2 kW DC system, you can multiply this number by 1,000 to confirm that you need 6,200 watts of solar panels.
6.2 kW x 1000 (convert to watts) = 6200 watts solar panel required
4. Calculate Solar System Output
Once you know how much area you have for solar panels, and what angles and direction you will be working with, use a PV watts calculator to figure out how much power your system will put out on a monthly basis:
Enter the address and hit the orange arrow to the right.
Once you are on the System Info page, enter the DC system size from the previous section.
Choose a standard module.
For array type, select “fixed” for roof mounts, or “open” for ground mounts.
Leave the system losses at around 15%.
Enter the slope of your roof in degrees, and the azimuth. Azimuth is the degrees relating to north and south, with north being zero and south being 180.
5: Calculate how many solar panels you need
Finally, you can divide the system size by the power output of a solar panel to find out how many solar panels you need. The higher a solar panel’s power output, the fewer panels you need to install.
