Sizing Grid Tie Systems
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.
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.
- Find your monthly average electrical usage from your electricity bill. This will be listed in kWh.
- Find your daily average electric use. This will be your monthly average kWh divided by 30.
- Find your locations average peak sun hours per day. The above map has rough figures.
- 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
- 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.
- Multiply your result in #5 by 1000 to get watts of system.
- Divide the wattage of the module that you would like to use into this number to determine the number of modules you would need.