The first step to sizing your solar system is to determine your average daily power consumption (kWh). This will help you in knowing how many panels you need to install in your home. Start by gathering your electricity usage (kilowatt-hours) based on your electricity bills from the utility company to determine your consumption for the last twelve months. From this, you can identify your peaks in electricity usage throughout the year.
Mostly, your energy usage spikes during the winter and summer months because of the heavy use of the heating and cooling systems. In the summer months, your grid-tied system tends to produce more electricity due to peak sun exposure. Below is an example of an electricity usage history.
From your annual power consumption bill, you can get your average daily electricity usage. Add up your power consumption for twelve months. In the example above, the annual consumption is 10700 kWh. Then divide the total number by 365 days to get the average daily power consumption: (10700 / 365) = 29.31 kWh, average daily consumption.
Your local peak sun hours depend on your location and climate in the area. You have to determine the peak hours of sunlight per day in order to get the most of your solar power. Identify peak sun hours in your geographical area to estimate how much energy the panels produce during the peak hours. You can
use the sun hours map chart to get the average peak hours for your city. Assuming you live in Arizona, your array will experiences 5.5 sun hours per day. Using annual average daily sun hours will help you roughly estimate your average daily power generation; however, it does not reflect the actual power generation potential of your system on sunny, summer days and cloudy, winter days. Instead, you can consider the winter daily average sun hour to be more conservative.
To obtain the power output of your PV system, take your daily power consumption and divide it by peak sun hours. In this case, let’s calculate how much energy your panels generate each hour. Start by multiplying your hourly power usage by 1000 to convert your power consumption into watts. Then divide it by the number of daily peak hours. 29.72 kWh * 1000 = 29720 watts.
Solar panels output = daily power consumption (kWh) / average peak sun hours Assuming you live in Arizona, which experiences 5.5 peak sun hours per day, your array’s output per hour is: 29720 watts / 5.5 sun hours = 5403.6 (rounded to 5404 watts). PV setups do experience system losses from the solar inverter, connected cables, and others that amount to 25% of the system’s total power; therefore, the actual size of the system is derived by adding 25% to the solar array’s output: 5404 watts * 1.25 = 6755 watts Based on your roof size, location, peak sun hours, and grid reliability, you can decide what percent of power consumption to cover by your panels. For the example above, we decided to cover 75% of the daily consumption; however, most homeowners consider 50% to 60% when they first install a PV system. 5404 watts * 75% = 4053 watts (desired output)
Lastly, divide the solar array’s output by energy rating for each individual panel. Since panels are rated based on individual consumption, most of the panels are in the range of between 275 and 380 watts. If you choose a 360W High-Efficiency LG Solar Panel, we can refer to the datasheet and figure out the Pmax under NOCT is 325W; therefore, the number of panels needed for your system will be: 4053 watts / 325 watts = 12.47 panels.
Since there are no partial panels, you can round up this number. So, you need fourteen 360-watt panels to meet 75% of your energy needs. As you observed here, using the power output under NOCT provides more realistic results than when considering STC. Using some basic information, we have done an approximation of our Solar PV System Sizing & Solar Panel Design Calculation.
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