Unlike off-grid solar system, grid tied solar system is the most common solar systems installed in locations that have electricity available from the utility company. A grid tied system simply takes the power generated from the solar panels during the day, and uses it real time in your house.
If you have any extra power available, it sells that power back to the grid for a credit, and at night or on days when you don’t generate enough power, you use that credit to buy power back from the grid. Any more power needed is just bought as usual.
Nowaday, the solar companies are providing a very good service if you want to setup a grid tied solar system for your home. But if you are passionate about doing it by yourself, this article is for you.
We’re going to walk through how to select the equipment for your grid tied system, including the solar panels, racking, overcurrent protection, and inverter.
#Step 1: Determine your monthly usage
From your current monthly bill, you can figure out how much power you use a day. Based on your location, and the amount of sun you get, you can determine the size of the solar array needed. From there you figure out what racking, inverter, and breakers you need. Let’s go through each of these steps.
How much power do you use?
Take a look at your electric bill. It is generally billed by the month.
If we had electric heat, you might have seen higher bills in the winter instead. The nice thing about higher usage in the summer, is that that is when there is the most solar energy available as well. From this bill, add up all of the monthly kwh, and divide it by 365 to get a daily kwh average. We’ll use that daily kwh number to size the solar array.
#Step 2: Size the solar panel array
First we need to figure out how much sun you get on average. Insolation maps show the available sun hours for your area. This map of the United States gives you a good idea of the solar potential.
The darker the color, the better the sunshine. Obviously the southwest and Hawaii are the best for solar, but even locations not known for their sunshine, like New England and the Pacific northwest still have enough sunshine on average to make solar a very good solution.
Here’s a quick peak to see how different regions of the world compare.
For our calculations, we need a more accurate number than what I can see on the map. There are several online sources available to find more specifics for your area.
This chart shows for my area near Worcester Massachusetts. You can see the monthly versus annual average numbers.
For a grid tied solar system, since I’m just supplementing the electricity I buy, so I can buy less, I’m just going to use the average number. The ideal angle for installing solar is at latitude, but my roof isn’t that steep, and I’m just going to mount them flush without tilting them up, I’m going to use the Latitude minus 15 degrees row. The good news is, for my location, I’ll get the same amount of power as if I was at the “ideal” angle.
Because we don’t live in an ideal world, I also need to take into consideration less than ideal conditions. Generally, for a grid tied system, we calculate that we will lose about 23% due to losses in the system, from voltage drop in the wires to bird poo on the panels.
Now let’s do some math!
We take that daily average kwh from earlier, multiply it by 1000 to get watt hours, divide it by your annual average sun hours, to get 11,254W. We divide it by 77% to take into account the system losses, which gives us 14,615 W of solar to provide 100% of our electricity needs.
As we said earlier, most grid tied systems don’t try to make all of their power, just cut their existing bill. For this example, I’m going to cut that in half to provide half of my electricity with solar. So I need a solar array of about 7300 watts. Now let’s use this information to pick out the rest of the system.
#Step 3: Select Grid tied Solar Inverter
Grid tied solar inverters are sized based on the size of the solar array they are connected to. There is a certain window of number of panels in series and in parallel that will work with the inverter. When selecting the inverter, you’ll find that most inverter manufacturers these days have an online calculator called a “String Sizer” to help select the right power inverter for your panels.
We’ll walk through ABB’s string sizer to find the right power inverter and panel configuration.
I enter the temperatures that the panels will be seeing during daylight hours, and if I’m mounting them on a roof or on the ground. This matters because the solar panels’ voltage changes pretty dramatically based on temperature, the string sizer needs to be able to calculate the highest and lowest voltages it will see.
I’m also selecting the solar panels I’m going to use. I picked Kyocera’s 250W panels, they are a terrific panel at a very good price. Since I’m looking at around 7300 watts of solar, I picked the ABB Uno 7.6kW inverter.
I can see that depending on how many parallel strings I do, I can use series strings of anywhere from 4 to 14 long in series. However, these may not be the ideal string lengths, if there are any warnings, the string sizer will alert you in a note.
I picked 2 sets of 2 strings of 8, for a total of 8000W, the inverter is very happy with that size. It’s a little bigger than my 7300W that I calculated that I needed, so it will actually generate more than half my power.
Now I’ve got 32 Kyocera 250W panels, and an ABB Uno 7.6k Transformer less inverter.
So how will I mount them?
#Step 4: Select Solar Racking (also called Photovoltaic mounting systems)
Solar racking is used to fix solar panels on surfaces like roofs, building facades, or the ground. These mounting systems generally enable retrofitting of solar panels on roofs or as part of the structure of the building.
Luckily for those of us doing a lot of designs, IronRidge also has a time saving Design Assistant to help speed up the design work. They’ve got one for roof mounts, and one for ground mounts.
We’ll walk through the roof mount one. You enter what solar panels you are using, how many, and how they are laid out. I’m doing 2 rows of 16, flush against the roof.
For my area, the building code requires the system be designed to withstand 100mph winds and a snow load of 40psi. For 4′ spacing between mounting feet, which lines up with every other rafter, it tells me I can use the IronRidge XR100 rails.
Just a few more inputted details, like what color clamps to match the panels, And it outputs a bill of material, and the manufacturer’s suggested retail price. They do suggest a flashing for an asphalt shingled roof, if you have a different type of shingle, you may need a different flashing to prevent leaks.
#Step 5: Sizing OCP – Over Current Protection (Breaker and Fuse)
The last piece is over current protection, protecting your system in the event something goes wrong.
In a grid tied system, there are 2 locations we need to put in over current protection, on the DC side by the solar panels, and on the AC side in the Main breaker box.
The combiner box I chose for this system is a disconnecting combiner box. It allows you to turn off the power coming out of the panels right by the panels, in compliance with NEC 2014 Rapid Shutdown requirement.
Each string of panels gets its own fuse. The datasheet of the panel usually tells you what size fuses to use, for grid tied panels under 300 watts, it’s usually 15A. To calculate it, you take the solar panel’s Short Circuit Current, and multiply it by 1.56.
The combiner box wires the strings into parallel, and gives you a place to transition the wire into conduit. It’s also a good place to put a lightning arrestor.
The AC output of the inverter goes into a dual pole breaker in your home’s Main breaker box. To calculate the size breaker to get, you take the watts of the inverter.
#Step 6: Put it all together
Now let’s look at a schematic to see how this all schematic that shows how this all fits together.
We have 4 parallel strings of 8 panels in series, going to a combiner box with a 15A fuse for each string. The combined strings are sent in conduit to the string inverter.
The AC output of the inverter may be required by your electric company to go to a lockable AC disconnect by your meter, so that the linemen can turn off your system if needed. it then goes into a 40A breaker in your main breaker box, to your house.
Then any excess power goes out to your bidirectional meter, which will be spinning backwards or forwards, depending on if you are selling or buying power. From there, it goes out to the grid.