Solar Home Grid-Tie System Sizing Part 1: Using a Utility Bill

Grid-Tied (a/k/a “grid interconnected”) solar systems are the most common and simple types of solar electric systems homeowners install. These complete solar power systems are connected to the utility grid and generate electricity while the sun is shining and the grid is running. If the system produces more energy than your home uses, then the system sends (sells) the excess energy back to the grid, which would offset any electricity you buy from the utility at night (check with your utility provider for details, as these policies vary). 

If you consume more energy than your system is able to generate, then the utility would supply your additional energy demand as usual. Grid-tied solar panel power systems are great for reducing your energy bill.

Sizing a system can seem confusing at first, mostly due to the flexibility of sizing a grid-tie system. We can base a system on a certain amount of production to off-set current usage, to fit into a budget, or to just take advantage of the roof-space available to install the solar panels on. 

Figuring out the size of system to offset current usage is pretty straight forward and simple though, so we will start the series off with this. Here is an easy method to help you figure out the correct solar panel array size for your home.

Step 1: Find your monthly average electricity usage from your electric bill.This will be in kilowatt-hours (kWh). Due to air conditioning, heating and other seasonal usage, it is a good idea to look at several bills. You can add the typical summer, fall, winter and spring bills and divide by four to find the average monthly usage.

Step 2: Find your daily average electricity use. Divide the monthly average number of kWh use by 30 (days).

Step 3: Find your location's average peak sun hours per day. Find what is known as an insolation map, or average sun hour list. For example, Colorado is around 5 peak sun hours. Alternatively, a PV Watts is a great online tool from NREL that will provide great insight for month by month and average solar radiation for your specific location. (https://pvwatts.nrel.gov/)

Step 4: Calculate the system size to provide 100% of your electricity. Divide your daily average electricity use by average sun hours per day. For example, if the daily average electricity usage is 30 kWh, and the site is in Colorado, system size would be: 30 kWh / 5 h = 6 kWh. It's a good idea to multiply this number by 1.15 in order to account for factors such as inverter efficiency, dirt or pollutants that may accumulate on panels, etc., if you want to cover all of your use. (One of our solar home packages in the 7 kW range would be great for this!)

Take a look at some of our system options for grid-tie packages here, or contact our sales team for more information, we are eager to help out and provide you with everything you need to go solar!