How to figure out the Solar Panels for your System
The first part of the question has to do with the inverter specifications and the second part has to do with the panel specifications.
So for the first part, the inverter which has an PV operation voltage range: 120-500Vdc.Â
This is important because it means you need enough solar panels on the low end to reach 120 Vdc, but on the high end the total panel voltage CANNOT exceed 500Vdc.
The way solar panels work is that when you connect them in series. the voltages add up but the current stays the same.
Every type of solar panel has a couple of different specifications for the voltage that it will produce under different conditions. The specifications are the 1) Open Circuit Voltage (Voc), 2) Maximum Power Voltage (Vpm), 3) The maximum cold voltage.
Knowing these values for the panels that you choose is important because they will determine how many panels you can use given the operating voltage range of your inverter.Which panels you chose will have a profound impact on the cost of your system. Older panels that still have good production can be found from individuals sellers on places like Craigslist or FB marketplas for as little as $20 a panel for 175w. The newer the panels the more expensive, but they will also have a higher wattage rating, for example up to 600w, 700w, or even 800w. Consequently, its not only a function of how much money you want to spend, but also how much space you have available. For example, you could have one (1) 600w panel for lets say $435, or 4 panels at 175w for $80.
Now lets get back to our example using those 175w panels that we got for $20 pop. In this case, these are panels manufactured by Sharp (NT-175U1) about 15 years ago, and have specifications of Voc = 44.4V and Vmp of 35.4V. In order to meet the low end of our operating volatage of 120Vdc, that means we would have to string together (120Vdc / 35.4 = 3.4) in series rounding up in whole numbers (because we can't have partial panels) at least 4 of them for 141.6V (ie. 35.4 X 4). But as Will states, to be on the safe side to guarantee that the inverter will turn on, it would be better to have at least 200V. So that means having at least 6 panels for 212.4Vdc (ie. 35.4 X 6). The cost is therefore 4 X $20 or $80 total.
To know the maximum number of panels we could use, we then have to look at the upper range of the specification or 500Vdc. In this case, however, instead of using the Vmp as we did for the turn on voltage, we need to use the Voc which you'll remember is higher at 44.4Vdc. So that gives us 500 / 44.4 = 11.26 which we round down to 11 X 44.4V = 488.4V.
However, there is an additional complicating factor that we need to account for. Namely, that when panels are "cold" they will have an even higher Voc then the stated specification with is measure at sort of an average operating temperature know as Standard Test Conditions (STC) which is at 25 degrees C.
So it needs at least 120Vdc to turn on, and it can take a maximum voltage of 500 Vdc. So if you want the inverter to start (turn on) reliable, you would probably, as Will says, need at least 200 vdc from your solar panels