Step-by-Step Guide for Sizing Solar Panels for Your Home
Installing solar panels at home can be a great way to reduce your energy bills and reduce your carbon footprint. However, Sizing Solar Panels for Your Home can be confusing. In this blog post, we will go through step-by-step instructions on how to calculate the solar panel needs for a home installation.
Step 1: Determine Your Energy Needs
The first step in determining your solar panel needs is to determine how much electricity your home uses. You can do this by looking at your utility bills and calculating the average monthly energy usage in kilowatt-hours (kWh). Your energy bill may be given in terms of Units, but don’t be confused, it is the same thing.
1Unit = 1 kWh
You can also use an online energy calculator to determine your energy needs. This information will help you determine the size of the solar panel system you need.
Let’s say your energy consumption is 300 kWh ( 300 Units ) per month.
Step 2: Calculate Your Daily Energy Consumption
Next, you need to calculate your daily energy consumption. To do this, divide your monthly energy consumption by 30 (the number of days in a month). This will give you average daily energy consumption.
From the previous step, we know that the monthly energy consumption is 300kWh ( 300 Units )
So, your daily energy consumption is 10 kWh (300 kWh ÷ 30 days) which can also be read as 10,000 Wh ( 1kWh = 1000Wh )
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Step 3: Calculate Peak Sun Hours
Peak sun hours refer to the number of hours during the day when the sun is strong enough to generate electricity. In more technical terms, it is the number of hours per day during which the average solar irradiance (sunlight) is 1000 watts per square meter (W/m2) or 1 kilowatt per square meter (kW/m2).
One peak sun hour = 1000 W/m2 or 1kWh/m2 of sunlight
Image Credit: Solar Energy International
Example: If a given location receives a total of 6,650 Wh/m2 of solar radiation over the course of a day, then that location gets 6.65 peak sun hours. You can see the above picture for a clear understanding.
The following factors affect the number of peak sun hours:
1. Geographical Location: Solar panels installed at different locations, receive different amounts of sunlight. The panel installed near the equator receives maximum sunlight, as it is closer to the sun.
2. Time of Day: The amount of sunlight falling on the solar panel, varies throughout the day, based on the sun’s position in the sky. It receives a maximum at noon and a minimum during the morning and evening.
3. Season: Maximum amount of sunlight received during the summer and minimum amount during the winter.
Step 4: How to Calculate Total Peak Sun Hours
The solar irradiance map can show you the amount of solar energy your location receives on an average day during the worst month of the year.
To find out the amount of solar insolation in your area, you can use the Global Solar Atlas. Follow the following steps:
Step-1: Find Your Location
Search your location in the search bar (or click on your location on the map). For example, if you live in New Delhi, India, you can search “New Delhi” in the search bar.
Step-2: Find GHI of your Location
Locate “Global Horizontal Irradiation” (GHI) in the Site Info section. This is the number of total peak sun hours your location gets per year. In this example, I can see that New Delhi gets 1713.9 peak sun hours per year.
Step-3: Find Daily Peak Sun Hours
To find your average daily peak sun hours, change the results from “Per year” to “Per day”.
Step-4: The number is the peak sun hours
When I change from “Per year” to “Per day”, you can see that New Delhi gets an average of 4.696 peak sun hours per day.
When sizing a solar power system, peak sun hours should be calculated using Global Horizontal Irradiance (GHI), or Global Tilted Irradiance (GTI) at the tilt angle of the solar panels. Many people mistakenly calculate peak sun hours using Direct Normal Irradiance (DNI).
If you are situated in North America, you can use this reference chart, to get the number of peak sun hours.
Reference for Solar Insolation Map: NREL
Step 5: Calculate the Total Watt-peak rating needed
Multiply your daily energy consumption by 1,000 and divide by the peak sun hours. This will give you the minimum number of watts your solar panel system needs to generate to meet your daily energy needs.
From Step -2, the Daily Energy Consumption is 10,000 Wh
From Step- 4, the Peak Sun Hours is 4.696
So the Watt-peak required = Daily Energy Consumption ( Wh )/ Peak Sun Hours ( h)
Watt-peak = 10000 / 4.696 = 2,129.47 Watt
So 2129.47 Watt-peak ( use 2130 Watt ) rating of Solar PV panels is needed to operate the appliances of your home.
Step 6: Consider Solar Panel Inefficiency Factors
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 500W-rated panel would only output 400 watts.
In addition, solar panels are tested in STC ( Standard Test Conditions ) i.e cell temperature of 25°C and irradiance of 1000 W/m2 with an air mass of 1.5 (AM1.5)s. 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 the above factors, it’s wise to include some extra solar capacity so that you can reach your target generation after accounting for the inefficiencies of the system.
20-30% is a good amount of headroom to account for inefficiencies. Multiply your solar array size by 1.25 (125%) to account for this:
2130 x 1.3 = 2662.5 Watt solar array
Step 7: Calculate the Number Of Solar Panels Required
In the previous step, you have already calculated the Total Wattage of the Solar Array. Now you have to determine the number of solar panels that are required for your installation.
Currently, solar panels available in the market are in the range of 100 to 670 watts.
To determine the number of solar panels you need, divide the total Wattage of the Solar Array by the wattage of each panel.
In our example case, we need a system that needs to generate 2662.5 Watts per day and each panel is 670 watts, you will need 4 panels.
No Of Panels = Total Wattage Required / Rating of each Panel = 2662.5 / 670 = 3.97 ( use the next whole number 4 )
Step 8: Consider Other Factors
There are a few other factors you should consider when determining your solar panel needs. First, you should consider the angle and direction of your roof. Ideally, your solar panels should face south and have a tilt angle close to your location’s latitude angle. Second, you should consider the size of your roof and the available space for solar panels.
Conclusion
In conclusion, calculating the solar panel needs for a home installation requires some math, but it is not too difficult. By determining your energy needs, calculating your daily energy consumption, determining your peak sun hours, and considering other factors, you can determine the size and number of solar panels you need for your home. With the right solar panel system, you can save money on your energy bills and reduce your carbon footprint.
References
Global Horizontal Irradiation Map by the Global Solar Atlas is licensed under CC BY 4.0.
