Choosing a system

HOW TO SIZE A SOLAR SYSTEM IN 6 STEPS

When sizing a solar system, follow these steps to find out exactly what will cover your energy needs.

Step 1: Determine Your Average Monthly kWh Usage

Statistics show that most people consume more electricity during the summer and winter, when the A/C or heat is running. If possible, collect your last 12 months of electric bills, then tally up your kWh usage and divide by 12 to get a monthly average.

Step 2: Calculate Your Daily kWh Usage

Next, divide your monthly kWh usage by 30 to estimate your average daily kWh usage.

Step 3: Estimate the Amount of Sunlight Your Solar Panels Will Receive

Sunlight availability affects how much energy your solar panels generate. Use NREL’s GHI maps to see how many sun hours you can expect to get in your location.

Take the daily kWh target from step 2 and divide it by the number of sun hours in your location. 

Step 4: Account for Inefficiencies

From there, we need to add a bit of overhead to account for inefficiencies and degradation rate of the panels.

The output of solar panels drops slightly each year, which is outlined by their performance warranty. If your solar panel’s performance warranty guarantees 80% performance after 25 years, then their degradation rate is calculated as 20%/25 years, or 0.8% production loss each year. By the end of its lifecycle, a 400W-rated panel would only output 320 watts.

In addition, solar panels are tested in ideal conditions ‒- a temperature controlled lab with nothing obstructing the panels. In the real world, solar panels often fall short of these lab-tested conditions, meaning they produce a bit less power than their wattage rating.

Because of these factors, it’s wise to budget extra solar capacity so that you can reach your target production figures after accounting for the inefficiencies of the system.

20% is a good amount of headroom to account for inefficiencies. 

Step 5: Full or Partial Offset?

Most grid-tie homeowners choose to offset 100% of their energy needs with solar. But it is also possible to start with a smaller system for partial offset, and then expand down the line as the budget allows for it.

If partial offset is your goal, you can account for that here. 

Step 6: Determine How Many Solar Panels You Need

Once you have your final array size, simply divide by the wattage of your desired solar panels to figure out how many panels you need.