📏 Measuring the Power

The power rating of solar panels is in “Watts” or “Wattage,” which is the unit used to measure power production. However, electric utility companies will use kWh, which stands for one thousand watts of electricity used per one hour.

In 2021, the average annual electricity consumption for a U.S. residential utility customer was 10,632 kilowatthours (kWh), with an average of about 886 kWh per month. These days, the latest and best solar panels for residential properties produce between 250 and 400 Watts of electricity per hour. 

So… is it worth it? How do you calculate one solar panel’s output per day? Month? How much would it take to meet your energy needs? 

1. Find the solar panel output per day

Work out how much electricity—measured in kilowatt hours (kWh)—your panels would produce each day by using this formula:

• Size of one solar panel (in square meters) x 1,000
• That figure x Efficiency of one solar panel (percentage as a decimal)
• That figure x Number of sun hours in your area each day
• Divide by 1,000

To estimate the number of sun hours in your area, use this calculator.

Example: 

Panel is 1 square meter in size

• 1 x 1000 = 1000

Panel is 20% efficient:

• 1,000 x 0.2 = 200

Your area gets 4.5 sun hours per day:

• 200 x 4.5 = 900

Divide by 1,000:

• 900 ÷ 1,000 = 0.9 kWh per day

2. Find the solar panel output per month 

• Multiply the daily figure by 30

0.9 kWh x 30 = 27 kWh

3. Find how many solar panels are needed to power 800 kWh per month

• Divide the power needed by the monthly figure

800 kWh / 27 = about 30 solar panels 

You might be wondering, and for good reason, what makes a solar panel efficient?

The following factors influence how much electricity your solar panels will generate:

• Capacity: The maximum amount of electricity the system can produce under ideal conditions (known as ‘peak sun’). 
• Efficiency: How much sunlight solar panels can turn into electricity. Most residential panels have an efficiency of around 20%.
• Materials: What a panel is made of can also affect its efficiency.
• Direction: South-facing roofs get the best results.
• Angle: Some panels move on their own to face the direction of the sun.
• Shade: The less shade the better, but solar panels can still collect sun energy even on cloudy days.
• Your location: The American Southwest and Northeastern Africa are two of the sunniest regions on Earth!