Sizing Your Grid-Tie System
Determining the size of your grid-tie system is much easier than an off-grid system as the utility company has kept all of the records for you. There are several ways that you can build a solar system of this type. You can start by finding out how many kWh you use on average per month, and creating a system that would cover all of your electricity bill (see our load evaluation worksheet). Once you have completed this, you may be surprised at how large a system this would be, or that you just don’t have the roof or ground space to support a system like this. There may be ways to cut down on your electric use, or change out appliances within the home (see the off-grid section previously in this guide for references on making your home more energy efficient). If your utility has a tiered billing structure where you are billed a rate for baseline usage, and then higher rates for a percentage of power used above that baseline, you may be able to build a solar system that eliminates the overages and higher rates which equates to a greater savings per the investment. The following steps will help you determine the size of system you will need.
1. Find your monthly average electrical usage from your electricity bill. This will be listed in kWh.
2. Find your daily average electric use. This will be your monthly average kWh divided by 30.
3. Find your locations average peak sun hours per day. The above map has rough figures.
4. Calculate the system size (AC kW) to cover 100% of your electric bill by dividing your daily average electric use in #2 by the figure in #3
5. Divide your result in #4 by .77 to get a derate factor for the components in the system. This takes into consideration temperature, wire loss, component inefficiencies, etc.
6. Multiply your result in #5 by 1000 to get watts of system.
7. Divide the wattage of the module that you would like to use into this number to determine the number of modules you would need.